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Lecture Comments (36)

1 answer

Last reply by: Dr Carleen Eaton
Thu Sep 29, 2016 6:06 PM

Post by Stephen Hunsberger on September 27 at 03:28:10 PM

I am a teacher trying to use your series as a supplement to my honors biology class. Are there any worksheets or practice problems associated with your presentations?  All I'm seeing are lectures.  Please help!

0 answers

Post by Apolonia Gardner on November 24, 2015


I am a high school senior about to send off my applications for college. I am stuck on one thing – my intended major. Biology and chemistry have been my favorite courses throughout high school, and I would like to get a college degree that will enable me to perform research with viruses. My lifetime goal is to find a cure for a disease. From your experience, what undergraduate major should I shoot for? Biochemistry? Microbiology? Molecular Biology? Immunology? Chemical Biology? Organic Chemistry? Pharmaceutical Science? Any guidance is appreciated.

0 answers

Post by Mohamed Al Mohannadi on October 22, 2015

Carbon.. The atom of life.. Has 6 protons, 6 electrons and 6 neutrons? WOW this should evolve a religious debate (666)

0 answers

Post by James Weaver on July 23, 2015

Would this course be good for general biology?

1 answer

Last reply by: Tom Glow
Sat Dec 27, 2014 3:03 PM

Post by David Gonzalez on July 16, 2014

Where and how are isotopes discovered? And why don't isotopes have their own place on the periodic table? Thanks.

1 answer

Last reply by: Dr Carleen Eaton
Wed Jan 8, 2014 7:26 PM

Post by robina saeed on January 2, 2014

Hi Dr. Eaton,
Is this a complete course or just a review? thanks

0 answers

Post by jaime samano on August 16, 2013

I have trouble on a homework question. It asks what element is the electron configuration of 2,8,8,18,17. How do I sole this?

0 answers

Post by soe bryan on February 17, 2013

what is an example of Van Der Waals forces?

3 answers

Last reply by: Kenosha Fox
Mon Dec 15, 2014 3:11 PM

Post by Ikze Cho on January 14, 2013

is drinking pure H2O dangerous?

1 answer

Last reply by: Dr Carleen Eaton
Thu Dec 6, 2012 6:02 PM

Post by Jonathan Aguero on December 4, 2012

how is ionic bonding used in DNA

1 answer

Last reply by: Dr Carleen Eaton
Mon Nov 12, 2012 6:26 PM

Post by Lisa Lim on October 28, 2012

I don't understand the part where Dr. Eaton says, "the electron pretty much stays within these shells and they (electrons) only pass through them (?) on the way to a different shell." Who is "them"? I'm confused.

3 answers

Last reply by: stephen legge
Tue Mar 12, 2013 12:16 PM

Post by Aniket Dhawan on October 16, 2012

Professor is it possible that I could get a worksheet to do based on this lecture.

Otherwise you were very good.


0 answers

Post by Andrea Gulyas on July 23, 2012

Thank you very much!
EI can hardly wait for the rest of the lectures!

1 answer

Last reply by: Yejia Chen
Fri Apr 5, 2013 3:04 AM

Post by Swetha Atluri on June 3, 2012

In the Ionic Bonds section (32:35), how is the problem of the valence shells solved? It's solved in the Sodium, but it isn't solved in the Chlorine yet. After the electron from Sodium is transferred to Chlorine, there are only 8 electrons in the 3rd shell for Chlorine. Aren't there supposed to be 18 electrons in the third shell for it to have a full valence shell?

1 answer

Last reply by: Dalar Karimian
Wed Aug 7, 2013 7:13 PM

Post by nhuy nguyen on March 25, 2012

are we able to download the lecture? because I don't see where to download.

1 answer

Last reply by: Dr Carleen Eaton
Thu Feb 23, 2012 11:36 AM

Post by Louise Finlayson on February 14, 2012

What a great lecture series - Thank you so much made things so much more understandable :)

1 answer

Last reply by: Dr Carleen Eaton
Sat Feb 4, 2012 4:30 PM

Post by Jialan Wang on January 27, 2012

what is atr-x syndrome?

0 answers

Post by Raj Patel on November 21, 2011

buy, lots of stuff being covered in the first unit. still pretty understandable.

0 answers

Post by Daniel Delaney on August 17, 2011

This takes a loooong time to download & google chrome doesn't help. Too bad because I like Dr. Eaton.

1 answer

Last reply by: Dr Carleen Eaton
Mon Feb 7, 2011 5:45 PM

Post by Jay Patel on February 1, 2011

Slight slip of the tongue at 3:50. She meant to say, this is two atoms of *hydrogen* bonded to one atom of water

Elements, Compounds, and Chemical Bonds

  • Elements are composed of atoms. Atoms are made up of protons, neutrons and electrons.
  • Electrons can be found at different energy levels. The particular energy levels that electrons spend most their time at are described as electron shells. The valence shell is the outermost electron shell in an atom.
  • Covalent bonds are formed when atoms share electron pairs.
  • Polar bonds are the result of an electron pair being more strongly attracted to one atom in the bond than another. Nonpolar bonds occur when an electron pair is equally attracted to both atoms forming the bond.
  • An ionic bond is the attraction between positively charged ions, called cations, and negatively charged ions, called anions.
  • A hydrogen bond is formed when a hydrogen atom covalently bound to one electronegative atom is also attracted to another electronegative atom.
  • Molecules undergo reactions by forming or breaking bonds. The initial substances involved in the reaction are called reactants; the set of substances resulting from the reaction are the products.
  • A mole (mol) = 6.02 x 1023. The molecular mass is the mass that contains one mol of the substance. The molarity of a solution is its concentration in moles of solute per liter of solution.

Elements, Compounds, and Chemical Bonds

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Elements 0:09
    • Elements
    • Matter
    • Naturally Occurring Elements
    • Atomic Number and Atomic Mass
  • Compounds 3:06
    • Molecule
    • Compounds
    • Examples
  • Atoms 4:53
    • Atoms
    • Protons, Neutrons, and Electrons
    • Isotopes
  • Energy Levels of Electrons 13:01
    • Electron Shells
    • Valence Shell
    • Example: Electron Shells and Potential Energy
  • Covalent Bonds 19:52
    • Covalent Bonds
    • Examples
  • Polar and Nonpolar Covalent Bonds 23:54
    • Polar Bond
    • Nonpolar Bonds
    • Examples
  • Ionic Bonds 29:04
    • Ionic Bond, Cations, Anions
    • Example: NaCl
  • Hydrogen Bond 33:18
    • Hydrogen Bond
  • Chemical Reactions 35:36
    • Example: Reactants, Products and Chemical Reactions
  • Molecular Mass and Molar Concentration 38:45
    • Avogadro's Number and Mol
    • Examples: Molecular Mass and Molarity
  • Example 1: Proton, Neutrons and Electrons 47:05
  • Example 2: Reactants and Products 49:35
  • Example 3: Bonding 52:39
  • Example 4: Mass 53:59

Transcription: Elements, Compounds, and Chemical Bonds

Welcome to Educator's AP Biology course.0000

I am Dr. Carleen Eaton, and we will be starting out the course with the discussion of elements, compounds and chemical bonds.0004

Although this is a biology course, we are starting out with some chemistry basics because life involves many chemical reactions.0011

Chemical reactions occur constantly within the cells and are essential to life.0020

For example, the conversion of carbon dioxide and water to glucose using sunlight as an energy source by0024

plants is a chemical reaction, and this is known as photosynthesis.0031

Animals use chemical reactions to break down food sources such as carbohydrates, fats and proteins to release their energy to fuel cellular processes.0035

Starting from the beginning with the discussion of elements.0046

Elements are substances that cannot be broken down into simpler substances through chemical reactions.0049

Matter is anything that takes up space. Elements are a form of matter.0056

There are 12 elements on the periodic table.0065

Here, as shown one square, an example of an element from the periodic table, and this is oxygen.0070

So, there are 12 elements. Of these, 92 are naturally occurring.0075

The remaining elements can be made or created in a particle accelerator. However, only 92 are naturally occurring.0090

Although there are 9 naturally occurring elements, in biology, we are going to be focusing on mainly 4 elements.0098

Living organisms are composed mainly of oxygen, carbon, hydrogen and nitrogen.0105

There are small amounts of other elements found in living organisms, for example calcium, which is found in our bones.0119

Phosphorus and magnesium are also found in small amounts.0127

There are other elements called trace elements, and these are also essential to life; but they are found in very small or what is called "trace amounts".0131

An example of this would be iron. Iron is found in red blood cells, and it is essential in allowing us to carry oxygen using our red blood cells.0140

Iodine is another example. The thyroid gland needs iodine to function correctly, and without enough iodine, people develop a condition called goitre.0150

Here, looking at this example of oxygen on a periodic table, you will notice a couple of numbers.0159

The first is the atomic number, and we will get into exactly what this means.0166

But for right now, I just want to give you an idea of how to read the periodic table of elements.0171

O is a symbol for oxygen. Here is the full name, and then, down here, is the atomic mass.0176

A molecule is composed of two or more atoms bonded together in a specific arrangement.0189

Compounds are composed of different elements that are combined in a fixed ratio.0196

Let's look at an example of a molecule. O2 is two oxygen atoms bonded together.0203

These are molecules. However, since these are the same type of atom, this is not a compound.0210

An example of a compound would be H2O or water.0219

This is two atoms of water bonded to 1 atom of oxygen, and this is a molecule of water; and it is also a compound.0224

What you will notice here is that compounds are composed of different elements, and they are combined in a fixed ratio.0244

This is very important because if I look at water/H2O, and then, I look at H2O2/hydrogen peroxide, I will notice0249

that these are both composed of hydrogen and oxygen, however, they have very different properties.0264

Water is the solvent of life. You drink it.0268

Your body is made largely of water, whereas hydrogen peroxide has entirely different properties0273

altogether, and it is good for treating wounds for example but very different substance than water.0279

So, it is not just the atoms that are in a compound that matter. It is the ratio and the order that they are combined in.0287

Elements are composed of atoms, and these are the smallest unit that an element can be broken down to that0298

still retains the characteristic structure and property of the element.0304

Now, remember, elements are the substances that cannot be broken down into smaller particles through chemical reactions.0309

But it is possible to break down an element further, just not through chemical means.0316

And if you did break down an element further, what you could find are the different subatomic particles that comprise an atom.0319

So, let's look at what an atom is composed of. It is composed of protons, neutrons and electrons.0331

The central part of the atom is called the nucleus, and the nucleus contains protons and neutrons packed together tightly.0344

Protons are positively charged. Neutrons, as the name suggests, are neutral.0356

And orbiting around the nucleus, you will find the electrons, and these are negatively charged.0367

These electrons are held in their orbitals by their tractions to the positively charged nucleus.0381

The protons are positively charged. Electrons are negatively charged, and this attraction keeps the electrons orbiting around the nucleus.0388

Each element has a characteristic number of protons, and the atomic number I mentioned at the top of the periodic table is the number of protons.0395

So, the atomic number equals the number of protons.0403

When we talk about an atom, we are usually talking about it in its neutral form, and that neutral form would have the same number of protons and electrons.0411

If there is 4 protons that are positively charged and 4 electrons that are negatively charged, they will cancel each other out.0422

An example would be carbon. Let's go get carbon.0430

Carbon has an atomic number of 6.0435

A second number associated with an element is its mass number. OK, atomic number is number of protons.0442

The mass number is the number of protons plus the number of neutrons. The mass number of carbon is 12.0449

Now, if carbon is neutral, that means that I can determine that I have 6 protons from the atomic number, and since it is neutral, I also have 6 electrons.0464

And then, all I have to do to figure out the number of neutrons is to take this mass number,0476

subtract the atomic number, and that will give me the number of neutrons.0484

Here, the mass number is 12-6. Therefore, there are 6 neutrons.0490

Sometimes, you will see an element written as such.0502

You will see the symbol like C for carbon and then, a number at the top and a number at the bottom.0506

This is the mass number. This is the atomic number.0512

The unit of mass that we use to express atomic or molecular weights is called the unified atomic mass unit0525

or just atomic mass unit, and this is abbreviated with a U.0533

It is also the same as a Dalton- 1 Dalton. A Dalton is abbreviated Da.0546

Now, a proton or neutron has a mass of 1.7 x 10-24 grams, so that equals the mass of proton or 1 neutron.0556

If you add up the mass of the protons and neutrons, you have what is very close to the mass of the atom.0576

And the reason for this is that electrons are much, much, much smaller than protons or neutrons.0585

And their mass is, therefore, negligible. Frequently, we, therefore, talk about the atomic mass as being approximately equal to the mass number.0590

So, again, remember that the mass number is number of protons plus neutrons.0602

Since that mass of electrons is negligible, the atomic mass or the mass of the atom is approximately equal to the mass number.0609

When you look on the periodic table of elements, you will see a slightly different number, which is at the bottom, and that is more of an exact atomic mass.0628

It is a weighted average of masses of the various isotopes of the atoms, of the element, and if you just round that off, you will get the mass number.0636

OK, concept of isotopes, let's look at carbon as an example and look at the various isotopes of carbon.0646

Isotopes are forms of an element that have different numbers of neutrons.0655

Remember that for an element to be the same element, it has to have this characteristic number of protons. For carbon, there are 6 protons.0663

When we talk about this form of carbon right here that I have been discussing, carbon 12, it has 6 protons and 6 neutrons.0673

However, you can talk about isotopes of carbon. One is carbon 13.0680

I have a 1 up here for my mass number, and that means, if I took that 13, subtracted the number of protons, I would end up with 7 neutrons.0685

So, carbon 13 is a form of carbon and is much more rare than carbon 12, and it actually has 7 neutrons.0695

There is another isotope called carbon 14, and this is a radioisotope. It is a radioisotope.0703

So, if I took the mass number here - I said 1 - and I subtracted atomic number 6,0713

I would come up with 8 neutrons, and carbon 1 is actually a radioactive form of carbon.0720

This is a radioisotope. The difference between radioisotopes and other types of isotopes is that radioisotopes are unstable.0725

They actually decay spontaneously, and they give off energy in the process.0735

The nucleus decays, and the number of protons or neutrons can change.0739

The result is that it can actually decay and turn into a different element as it is decaying.0742

These are very important in medicine. For example iodine 13 or I-131 is a radioisotope.0748

I mentioned that iodine is important to the thyroid.0760

Well, if somebody has thyroid disease or thyroid cancer, we can actually infuse a radioisotope into their body.0762

It will localize to their thyroid, and it will help decrease the activity of that tissue or destroy malignant thyroid tissues.0770

So, radioisotopes do have important uses in biology and medicine.0779

To understand chemical bonding and chemical reactions, you need to understand more about electrons and particularly about the energy levels of electrons.0788

Electrons are found at different energy levels, and the levels that they spend their time in are called electron shells.0798

The outermost shell is extremely important in chemical bonds, and it is called the valence shell.0806

Let's talk about the concept of energy, a little more in potential energy.0813

Recall that the nucleus is positively charged, whereas electrons are negatively charged, so electrons are attracted to that positively charged nucleus.0818

The farther the electrons are from the nucleus, the greater, what we call, potential energy they have.0829

Potential energy, think of it as stored energy, or it is energy that is there and that could be released/has the potential to be released.0832

Think about if you were using a bow and arrow and you pulled back the bow.0842

You are using energy from your muscles. You are transferring it into the bow, and you are pulling it back.0847

Now, that bow has potential energy. It has stored energy.0851

When you let go of the bow, the arrow shoots out. The energy is transferred to shoot that arrow.0855

Now, the greater the distance you pull back the bow, the harder you pull on it,0859

the greater the potential energy stored in the bow, and it is the same idea with electrons.0866

The farther away they are from the nucleus, the farther the electron shell is that they are in, the greater the potential energy.0873

To move to a shell that is more distant from the nucleus, an electron would have to use energy.0877

To get closer, it would release energy.0890

Electrons pretty much stay within these shells, and they only pass through them on their way to a different shell.0898

Electrons fill up these lower energy shells first, and then, they go to the higher ones.0901

The first shell can hold two electrons, whereas the second shell can hold 8 electrons, and each shell has a characteristic number of electrons that it can hold.0910

Again, the outermost shell is called the valence shell, and a lot of the chemical behavior of an atom is driven by a need to fill this valence shell.0915

If the outermost shell is full, if the valence shell is full, then you have what is called an inert element.0936

Inert elements do not readily interact with other atoms. They do not have a need to.0948

Their Valence shells are already full.0957

OK, inert elements have a full valence shell, for example, neon. Neon has 1 electrons.0962

It has 2 in the first shell, and it has 8 in the second.0967

This is its valence shell. If there was more than 10, it would have to go to the third shell, but since this shell is full,0980

it is what we call an inert element, and it is not highly motivated to interact with other atoms.0988

Within valence shells, the electrons are usually found in what is called orbitals.0995

The shell gives the average distance from the nucleus where electron is found, but that is just an average.1004

Most of the time, electrons spend in particular regions called orbitals.1013

Orbitals can have various different shapes, and orbitals are listed by a number and a letter.1018

For example, this sphere is a 1s orbital. This tells me that it is in the first shell, and this tells me the shape, and so s, I just think of spherical.1025

It is a globe or round shape. Each orbital holds two electrons.1032

The first shell has only the 1s orbital that holds two electrons. That is full.1045

You need to go to the next shell. The next shell has a 2s orbital.1059

So, it is in the second shell, and it has a spherical shape. In addition, the second shell also has 3p orbitals.1065

p orbitals have this dumbbell shape, and I have only shown two here for simplicity; but in the second shell, there is one 2s orbital, and there are three 2p orbitals.1070

Each of these holds two electrons, so 2 here and 2 in each of the three orbitals. This holds the total of 8 electrons.1079

The behavior of an atom is often driven by a need, again, to fill the valence shell.1096

Let's look at an example of fluorine. Fluorine has 9 electrons.1107

That means that this 1s orbital is going to be full.1115

Here, we see these altogether. Remember, there is actually this third orbital, here, this third p orbital.1127

Draw that in, just not shown in order for simplicity, but looking at these electron orbitals with fluorine.1132

This first shell would have two electrons in it that is full, then, fluorine needs to go up to the second shell.1141

The 2s orbital here has two electrons in it, so, I used up two in the first that left me with the seven.1149

I have used up two more. I have five.1161

One of the P orbitals can hold two. Another p orbital can hold two, and this last p orbital only has one.1169

So, there is a single electron missing to complete the valence shell, and bonding has to do with completing that valence shell.1171

One thing you will also notice about molecules is they can differentiate, and the shape of a molecule1180

has to deal with the position of the orbitals; and the position and shape of the orbitals can change when bonding occurs.1187

Let's go ahead and talk about bonding. Covalent bonds are formed when atoms share electron pairs.1191

Remember that the atom wants to fill its valence shell.1198

Let's look at hydrogen. Hydrogen has one electron.1204

It has just got a single electron in the 1s orbital. That is its valence shell.1209

The first shell is the valence shell in this case, and it holds one electron. It needs one electron to fill the valence shell.1214

One way for the hydrogen to get to fill that shell would be to share an electron pair, and that is what covalent bonding is.1219

In the simplest case, hydrogen could share an electron pair with another hydrogen, so the second hydrogen is also lacking of full valence shell.1239

Therefore, if they each shared an electron pair, if they shared one electron pair, they would have full valence shells.1248

Another way to write this that helps to clarify is called a Lewis dot structure.1264

Both of the methods I have shown here show a covalent bond.1274

This is a single bond, and it means that one electron pair is being shared by the two molecules, by the two atoms.1283

So, this forms a molecule of hydrogen/H2 via sharing of one electron pair in order to complete the valence shell.1289

Atoms can share more than one pair of electron. An example would be oxygen.1301

Looking at oxygen a little bit more closely, it has 8 electrons.1311

The second shell has 6 electrons, but to have a full valence shell, it needs two more electrons.1317

Therefore, sharing just one pair of electrons would not fill the valence shell. It needs to share two pairs.1329

So, oxygen, therefore, can do that a couple of different ways.1339

Let's look at the example of CO2. That would be a carbon double-bonded to one oxygen and then, on the other side, double-bonded to a second oxygen.1347

This means, or if you looked at the Lewis dot structure, what is happening is a double bond is formed.1354

A double bond is formed when two electron pairs are shared.1366

This is sharing two electron pairs because oxygen needs to share two electron pairs in order to complete its valence orbital, and what is going on with carbon?1380

Well, remember that carbon has 6 electrons. That means the first shell contains 2 electrons, and the second shell has 4 electrons.1386

So, it needs electrons to complete its valence shell. Looking at what is going on with carbon, it is sharing 1, 2, 3, 4 electron pairs.1400

The oxygen is a full valence shell sharing 2. This oxygen has a full oxygen sharing 2, and this carbon has a full shell sharing 4 electron pairs.1411

A covalent bond is formed when the atoms share electron pairs. There are different types of covalent bonds.1422

There are polar, and there are non-polar covalent bonds. Some atoms share the electron pairs equally, whereas others do not.1434

In certain situations, an electron pair is more strongly attracted to one atom of the bond than it is to the other.1446

That would form a polar bond. When there is an equal attraction between the electron pair to the two atoms, that is called a non-polar bond.1454

For example, I talked about H2/hydrogen molecule formed by a single bond between two hydrogen atoms.1462

Since these atoms are both the same, the electron pair is going to spend equal time near both of the atoms.1472

This would be a non-polar bond. Let's contrast that with water/H2O.1481

Oxygen is what is called electronegative. It is very electronegative, and what electronegativity refers to is the attraction that a particular atom holds for electrons.1488

Electrons are more attracted to a highly electronegative atom, so the more electronegative an atom is, the more strongly electrons are attracted to it.1495

The oxygen here is more electronegative than the hydrogens, therefore, in oxygen or in water/H2O, here is some electron pairs being shared.1510

There is a pair shared here, and a pair shared here, but these electrons tend to spend more time near the oxygen atom.1535

They do not share equally their time between the two atoms. They spend more time close to the oxygen atom.1548

What that does is results in what is called a partial negative charge for the oxygen.1556

δ-, this delta symbol and then, a minus next to it means partial negative charge.1564

Since the electrons are hanging out with the oxygen more, and electrons are negative, this is going to end up with a partial negative charge.1572

Now, the opposite is going to happen with the hydrogen. It develops a partial positive charge because the electrons are1582

by the oxygen more, slightly more negatively charged oxygen, slightly more positively or δ+ over here by the hydrogen.1589

Because of the shape of water, it has this V shape.1595

This side of the molecule ends up overall slightly negative, and the hydrogen side ends up slightly positive.1609

So, this is what we call a polar molecule. H2/hydrogen molecule is non-polar.1612

Now, let's look at CO2. Again, we have these electronegative oxygens, and the electrons are more drawn to these1618

oxygens than they are to the carbon, but because this is a linear molecule, these two bounce each other out.1626

Oxygen is pulling this way. Oxygen is pulling it that way, but you do not end up with one side of the molecule that is overall1634

electronegative or overall partially negative versus partially positive charge because of this linear shape.1640

So, it is not just the atoms. It is also the configuration of the atoms that determines whether or not a molecule is polar or non-polar.1646

And in the next lecture, we are going to talk about water.1653

Water is extremely important to living beings, and the fact that water is polar gives it many important properties.1662

Another name for polar molecules like water is hydrophilic. Polar molecules are hydrophilic.1665

This means they are water-loving. They are attracted to water.1672

They dissolve in water. You might have heard the expression "like dissolves like".1684

So, polar molecules dissolve in other polar substances. Non-polar are hydrophilic or water-fearing - excuse me - hydrophobic.1689

Non-polar molecules are hydrophobic. Polar molecules are hydrophilic.1693

So, non-polar molecules do not dissolve well in water- like dissolves like. Non-polar molecules like fats dissolved in other non-polar molecules.1708

That is why oil and water do not mix.1713

If you are making a recipe, and you need to put some oil into a mixture that contains water, you will see that the oil separates out.1724

It does not dissolve in water because the oil is hydrophobic, and water is polar or hydrophilic.1726

Hydrophobic fat or lipid or oil and a hydrophilic solution such as water will not mix.1733

Alright, a second type of bond is an ionic bond.1741

Remember that in covalent bonding, the atoms share an electron pair.1753

Another way to fill the valence shell is not by sharing electron pair, but by losing or gaining an electron.1756

An ionic bond refers to the attraction between positively charged ions, which are called cations, and negatively charged ions, which are known as anions.1763

This is best understood through example, so let's look at NaCl or sodium chloride.1769

Sodium has 11 electrons. That means the first shell will contain two electrons.1780

The second shell will contain 8 electrons. This gives 10, and the third shell will have a single electron.1787

Obviously, the valence shell, the third shell, is not full.1798

Chloride has 2 electrons, the first shell, again, electrons, second shell, 8 electrons, third shell, 7 electrons.1806

This shell holds 8 electrons, so to fill its valence shell, chlorine needs one electron.1811

This problem is solved when sodium transfers an electron to chlorine to form a chloride ion.1826

OK, sodium transfers an electron to chlorine. Let's think about what would happen.1837

If sodium transfers an electron, it is going to be down to 1 electrons, but it still has 11 protons.1847

That means it is going to end up positively charged because it has one more proton than an electron.1856

So, we say this as a +1 positive charge, or we just show it as a plus.1864

In ion we could have it 2+ or 3+ positive charge. This has just a single, so it is a plus.1869

It transferred one electron to chlorine. Chlorine, in turn, now has 2 electrons.1874

It has one more electron than proton, which is going to give it a negative charge- chlorine with a negative charge.1881

It received one electron. This is, now, called a sodium ion, and this is called a chloride ion.1894

Sodium is positively charged, so it is a cation. Chloride is negatively charged, so it is an anion.1901

This step is not actually the ionic bond. It is necessary in order to get to the ionic bond.1912

But the ionic bond refers to the attraction between this positively, negatively charged ions.1922

Sodium does not just transfer the electron and then, float away.1928

In fact, these two stay associated with each other as NaCl, which is an ionic compound or salts.1939

Notice that this is not a molecule. Molecules are held together by covalent bonds.1944

It is a compound, though. It is an ionic compound.1952

These are also called salts, or this is table salt. Notice that the problem of the valence shells has been solved.1957

Since sodium lost its electron, once it loses it, there is no electrons in the third shell.1960

The second shell becomes a valence shell, and it is full.1969

Chlorine has gained an electron, and now, has 8 electrons in its valence shell; so that shell is full.1975

These two have achieved completing their valence shells, but it is not by sharing an electron pair.1979

It is by losing or gaining an electron, and then, do the positive and negative charges, they stay associated with each other.1986

Salts often form crystals, and this is because of the interactions via these ionic bonds.1991

The strongest type of bond is a covalent bond, but these other types of bonds are very important as well; and they are often called weak bonds.1999

One type of weak bond is a hydrogen bond. Now, well, a single hydrogen bond might not do a lot.2010

Many of these together can achieve something and actually have strength.2018

So, even though bonds such as hydrogen bonds are weak bonds, they are very important.2024

Let's look at water as an example of hydrogen bonding.2030

A hydrogen bond is formed when a hydrogen atom that is covalently bound to an electronegative atom is attracted to another electronegative atom.2035

Revisiting the idea of water, oxygen, hydrogen, recall that this is a polar bond that the oxygen atom is partially negatively charged,2039

while the hydrogen atoms have a partial positive charge because the electron pair favors the oxygen atom.2049

Because this hydrogen is partially positively charged, and the oxygen is partly negatively charged, there is an attraction between2066

the hydrogen and oxygen, not just the oxygen it is bonded to, but oxygens on other water molecules2075

These dotted lines indicate hydrogen bonds. Here is the covalent bonds between hydrogen and oxygen.2084

And then, this hydrogen is attracted to this nearby water, the oxygen atom.2094

This hydrogen is attracted to this nearby oxygen atom, and so a lot of complex hydrogen bonding goes on with water.2100

As you can see here, hydrogen bonds can hold different molecules together, and they can also hold parts of large molecules together.2106

For example, proteins fold into complex shapes of conformations, and those shapes2118

are often held in place by hydrogen bonding between one region of the protein molecule and another region.2125

This hydrogen bonding is responsible for a lot of the important properties of water that we are going to talk about in the next lecture.2131

Now that we have discussed bonding, we are going to go on to talk about chemical reactions, and molecules undergo reactions by forming or breaking bonds.2138

Let's look at a typical chemical reaction: CH4, which is actually methane, combines with two molecules of water, so 2 O2.2147

And those undergo a chemical reaction to form carbon dioxide/CO2 +2 molecules of water.2156

The initial substances involved on this side are the reactants. Those end up forming products.2169

The substances that you end up with are called the products.2177

Notice with this reaction, what we have is a balanced equation.2190

This equation is balanced meaning that the atoms that I have in the left, if I count those up2193

and I count up the atoms I have on the right, I have the same number and the same type.2198

No atoms were lost or gained.2203

Let's look on the left side and the right side. Let's look at carbon.2209

I have one carbon on the left. On the right, I have one carbon, hydrogen.2212

On the left, I have 4 hydrogens. On the right, I have 2 H2s, so that is 2 x 2.2216

That gives me 4 hydrogens. I also have oxygen.2228

On the left, I have x 2 oxygens. I have 4 oxygens.2236

Here, I have O2. That is two oxygens, and then, I have x O, which is two more oxygens, give me a total of four oxygens.2240

So, we say that this equation is balanced. No atoms were lost or gained.2245

They are just combined differently.2254

Reactions can proceed in both directions, so this reaction can go in reverse.2258

You could combine CO2 and two molecules of water to get back two oxygen molecules plus a methane molecule.2261

If the ratio is not changing, we say that the reaction is in equilibrium, if ratio of the molecules is unchanging.2268

Now, that is not to say that I am going to have exactly equal amount of methane and CO2.2277

It means the ratio is not changing. In fact, often, a reaction favors one direction.2299

So, maybe this reaction favors formation of CO2 and water.2304

In that case, I might have 10 molecules of CO2 for every molecule of methane, but if the ratio is not changing, I still maintain that ratio.2309

We say that the reaction is in equilibrium. I am not gaining more and more CO2 relative to methane, then, the reaction is in equilibrium.2315

Now, when we deal with chemistry and chemicals, we are dealing with very, very small amounts.2327

And it would not really be practical to just use grams when we are talking about molecules and chemicals and atoms.2339

Since the mass is so small, chemists have developed a different system with which to measure molecular masses and concentrations of chemicals and solutions.2345

A mole is simply a number. It is 6.02 x 1023.2352

If you say you have a mole of something, you are saying you have 6.02 x 1023.2364

This is no different than saying you have a dozen of something.2370

If you say "I have a dozen apples", you are saying "I have 12 apples", the same way you could say "I have a mole of apples", "I have 6.02 x 1023 apples".2375

You could say you have a mole of glucose, 6.02 x 1023 glucose molecules.2379

This number, 6.0 x 1023, is known as Avogadro's number, and it is named after an Italian physicist.2390

Let's talk about molecular mass and moles. If you take a sample of an element that is equal to its atomic mass, you will have one mole of that element.2397

Take a sample of an element equal to the element's atomic mass. You will have one mole of the element, for example carbon.2415

Carbon has an atomic mass equals 12.2432

Therefore, if I take 1 grams of carbon equals 1 mole of carbon, that means that in 12 grams of carbon, you will have 6.02 x 1023 carbon atoms.2468

Now, let's say you are not just dealing with a molecule - excuse me - an element. Let's say you are dealing with a molecule.2476

I know for an element, that I just have to look at the atomic mass, take that amount in grams, and I have a mole.2498

In order to find a mole of a molecule, I need to find the molecular mass of the molecule.2506

Molecular mass equals the sum of the masses of the atoms in the molecule.2508

Simple example, water/H2O, here, I have two hydrogen atoms and one oxygen atom.2512

Hydrogen has a mass of 1. Oxygen has a mass of 16.2540

I am going to take one hydrogen atom with a mass of 1 plus the second hydrogen ion with a mass of 1 plus an oxygen atom with a mass of 16, and I am going to get 18.2548

This is the molecular mass. If I take a sample of 2 grams of water, this is going to equal mole of water or 6.02 x 1023 molecules of water.2559

Something else to understand is that let's say I had a dozen water molecules. Let's talk about a dozen instead of a mole.2575

I can say "oh, I have a dozen water molecules", and then you ask me how many hydrogens I have.2594

Well, if I broke up the water, the dozen water molecules, it ends up just hydrogens and oxygens.2601

Each water molecule has two hydrogen atoms, so actually, it would have 2 hydrogens and 12 oxygens in that water, in that dozen.2606

So, if I said I have a dozen water molecules, that means I have a dozen water molecules.2612

But if I broke them up, I would actually have two dozen hydrogen atoms and one dozen oxygen atoms.2626

In that same way, if you have a mole of water and you broke it up, you would end up with 2 moles of hydrogen atoms and 1 mole of oxygen atoms.2631

This has to do with mass, and when you are working with dry or just certain chemicals, it is very helpful; but a lot of times, we are working with solutions.2639

And when we work with solutions, we talk about molarity.2660

A 1 molar solution - and we will often write this as 1 mole - equals 1 mole of a substance dissolved in 1 liter of solvent, and it can be written as 1M as shown.2670

For example, let's say I wanted a 1 molar solution of glucose. Glucose has the molecular formula C6H12O6.2678

I need to figure out the molecular mass. OK, so I have glucose, and I want a 1 molar solution.2709

The molecular mass of glucose is going to be the mass of carbon, which is 12 times I have 6 carbon atoms2727

plus the mass of hydrogen, which is 1 x 12 hydrogens plus the mass of oxygen, which is 16 x 6 oxygen atoms.2754

And if you add these up, you will get 180. Therefore, the molecular mass of glucose is 180.2766

In order to get a molar solution of glucose, dissolve 180 grams of glucose in 1 liter of water.2779

OK, molarity, to make a 1 molar solution, figure out the molecular mass.2788

That will tell you how much of something you need, how many grams you need to get 1 mole.2813

So, I have 6.0 x 1023 glucose molecules and 180 grams of glucose, and I am going to dissolve that in 1 liter of water; and I will have a 1 molar solution.2821

Alright, let's try out some examples now.2827

Example 1: the atomic number of fluorine is 9, and its atomic mass is 19. How many protons, neutrons and electrons does it have, is it inert, why or why not?2841

Remember that the atomic number equals the number of protons.2844

The atomic mass, we often use interchangeably with mass number, although they are not exactly equal.2858

Because the mass in electrons is negligible, we often use them interchangeably, so the atomic mass equals 19.2868

In this case, we are using that as a mass number as well, and therefore, if I know that that is equal2876

to the number of neutrons plus the protons, then, I simply take 2 - 9 - so the atomic number is 9 - to give me 10 neutrons.2885

Alright, let's keep track of what we have. We have the atomic number.2897

Protons equal the atomic number equal 9. Neutrons, I am just taking the mass number, which is the number of neutrons and protons.2914

And I am subtracting the number of protons from that, and that leaves me with 10 neutrons.2919

In its neutral form, the number of protons and electrons will be equal, so I am going to have 9, 9 protons, 10 neutrons, 9 electrons. Is this inert?2934

Well, remember that an inert element has a full valence shell, so let's look at the electron situation.2938

I have 9 electrons. In the first shell, I have two electrons.2949

The second shell is going to have seven electrons. This is the valence shell.2957

This shell holds 8 electrons. Therefore, this is short 1 electron.2963

This does not have a full valence shell, so it is not inert because its valence shell is not full.2970

Second example, we have a chemical equation here.2981

Example 2: 6 carbon dioxides plus 6 water molecules is actually glucose plus oxygen.2991

This equation shows the formation of glucose, what coefficients needed to be placed in front of the products in order to balance the equation.2996

A balanced equation means you are not going to lose anything in terms of atoms or gain any atoms between reactants and products.3005

Reactants for carbon, I have 6 carbon molecules, products, 6 oxygen. 6 x 2 is 12, plus 1 is 13.3012

Here on the right or...excuse me, correction. This is 6 x 2 is 12, and then, 6 x 1 oxygen in each of the 6 water molecules is 6.3026

So, that is going to be actually oxygen is going to be x 2 is 12 + 6 more to give me 18.3044

On the right, I only have 6 + 2, and we have 8; so I have got a problem right here. This part is not balanced.3058

Hydrogen, x 2 is 12. On the right, I have 12, so I need to fix the oxygen, and you can actually use fractions.3070

You could use fractions on the left to fix it, make this number of oxygen smaller, but it would actually be easier to just use a whole number on the right.3078

Since the carbons are correct, and the hydrogens are correct, I do not want to mess with this, with the glucose.3092

I just want to focus on the oxygen where the problem is, so I am going to try some different coefficients.3100

I need quite a few more, so I am going to start out with 4. I am going to try putting a 4 in front of this and see what happens.3106

Now, I have 6 oxygens + 4 x 2, which is 8, 8 + 6 = 14.3112

That is not enough, so let's try a coefficient of 5. I have six oxygens here plus x 2 is 10.3119

That is 16. Remember, I want 18, so that is not big enough.3128

Let's try again. x 2 is 12, plus the 6 I have here, is 18.3141

Therefore, a coefficient of 6 in front of this O2 will balance this equation because now, I would still have my 6 carbons on both sides.3145

I solved my 1 hydrogens, and now, I have 2 oxygen on the left and then, 18 oxygen atoms on the right, so this is now a balanced equation.3153

Example 3: KCl/potassium chloride is a salt that disassociates into potassium and chloride ions. What type of bond holds KCl together?3165

Describe this type of bond and how it is formed. Well, this is a salt and this dissociates to form ions, therefore, it is an ionic bond.3175

This is formed by the attraction between the negatively charged chloride and a positively charged potassium ion.3187

What had to happen for this to occur is that potassium transferred 1 electron to chlorine.3198

The result is that there was one with more proton than electron left on potassium, and you ended up with potassium ion.3215

Chlorine got an extra electron and became the negatively charged chloride ion, and these two are attracted to each other due to their opposite charges.3228

So, this is an ionic bond.3239

Example 4: sucrose or table sugar is given by their formula C12H22O11.3251

What mass of sucrose in grams will need to be added to 1 liter of water to make a 1 molar solution of sucrose?3256

Remember that a 1 molar solution is 1 mole of a substance dissolved in 1 liter.3264

Remember that in order to figure out how much of a substance you would need to get a mole, you need to figure out the molecular mass.3271

So, I need to figure out the molecular mass of sucrose, of C12H22O11.3303

The atomic mass of carbon is approximately equal to 12 just counting electrons, so it is the carbon atomic mass.3311

Hydrogen mass is 1, and the mass of oxygen is 16.3324

Therefore, I am going to have 12 carbons x the mass of 12 + 22 hydrogens x the mass of 1 + 11 oxygens times the mass of 16.3342

And if you figure this out, 12 x 12 as it comes out to 144 + 22. 11 x 16 is actually 176, so that is 342 grams.3350

Therefore, I would need to add 342 grams of sucrose to 1 liter of water to form a 1 molar solution.3364

Thanks for visiting, and I will see you next time.3377