General Chemistry Atoms, Molecules, and Ions

Hi, welcome back to Educator.com.0000

Today's lesson in general chemistry is on atoms, molecules, and ions.0003

We are going to go ahead and look at the lesson overview right now.0009

We are going to get a brief introduction into atomic structure which is more of a historical background.0012

Following this, we will get into what we mean by isotopes.0019

We will get introduced to the periodic table in this lesson followed by naming compounds.0022

Basically ionic compounds, those that are made from metals and nonmetals.0028

Polyatomic ions, hydrates, and also what we mean by molecular compounds.0036

We are then going to get associated with the concept of the mole0042

and how that can be used to determine what is called a percentage composition0047

which is going to ultimately lead to the finale of this lesson0053

which is on determining what is known as the empirical and molecular formulas.0057

By the beginning of the twentieth century, physicists had already known the following.0065

Number one, that opposite charges attracted and that like charges repelled.0071

Number two, a subatomic particle existed that was negatively charged which had been called the electron; commonly abbreviated e^-.0075

One of the first accepted models of the atom was called the plum pudding model.0086

What the plum pudding model was the following.0090

It was essentially a sphere of positive charge.0092

Embedded in the sphere of positive charge were the subatomic particles, the electrons.0099

These were completely stationary if you will.0107

But there was no accounting for any other type of subatomic particle.0113

Once again, this is called the plum pudding model.0119

A sphere of positive charge that contained electrons scattered throughout.0122

We now get into one of the earliest theories that was accepted to explain for atomic structure.0129

This was called John Dalton's atomic theory.0138

John Dalton's atomic theory contained the following premises.0141

Number one was that matter was composed of individual units called atoms.0146

Number two, that these atoms were the smallest unit possible and that they were indivisible.0171

That is we could not break them down any further; atoms were indivisible.0177

Number three, the third premise of John Dalton's atomic theory was that atoms of one element were all identical to each other.0188

Number four, atoms of one element are going to be different than atoms of another element.0213

That is what we reference as chemical identity.0218

Atoms of one element different than atoms of another element.0224

Finally the fifth and final premise was that when elements combine, they can combine in small whole number ratios to form compounds.0239

Atoms combine in small whole number ratios to form compounds.0250

This fifth premise here is going to be looked at again.0268

That is what we call the law of multiple proportions.0273

I am going to go ahead and put an asterisk next to0284

each of the premises which we actually find not be so true today.0286

Atoms were indivisible; that is not true because we can divide an atom in a process called nuclear fission.0292

So premise two is not completely accurate.0298

The next one is atoms of one element are all identical to each other.0302

We are going to talk about this--is that not all atoms of the same element are identical.0307

Because atoms can exist in different forms which we call isotopes.0312

That is going to be coming on later on.0317

Following John Dalton's atomic theory was one of the first major experiments on atomic structure.0323

This is what we call Ernest Rutherford's gold foil experiment.0331

What Ernest Rutherford did was the following.0334

He shot a beam of high energy particles that were positively charged.0337

These positively charged particles were what we call alpha rays.0343

This was shot at a very thin piece of gold foil.0348

What was the expected result?--the expected result was the following.0355

That all of the alpha rays should have gone straight through.0360

Why?--because the positive sphere was mostly empty space.0366

And that the positive ray should have been attracted to the electrons that are embedded in the sphere.0370

This is expected result due to plum pudding.0377

However the actual result was the following.0387

From the gold foil, yes the majority of the rays did indeed go straight through.0390

However what was determined was that some rays actually got deflected at various angles.0397

You would actually have a couple rays get deflected completely backward.0405

This was highly unusual because this was not predicted by the plum pudding model.0411

Instead what came about was the following implication; an implication was the following.0416

That matter and atoms contained a dense center of positive charge.0424

Because it is positive charge that was making the alpha rays deflect.0441

This positive charge is what we today refer to as the nucleus.0449

The nucleus is at the center of all atoms and is positively charged.0456

To resummarize the implications of the gold foil experiment was that all atoms0464

have a dense center that is positively charged which we call the nucleus.0469

Inside the nucleus, two subatomic particles exist.0474

The proton is positively charged; the neutron has zero charge.0477

Number three, the nucleus is responsible for the mass of the atom; that is what gives an atom weight.0482

Finally electrons, which were already known to have existed, exist outside the nucleus and have negligible weight.0489

Let's go ahead and summarize these subatomic particles--neutron, proton, and electron.0499

Please make a note that these relative masses and the relative charges are indeed in relation to each other.0510

A neutron is going to be assigned a mass of 1 and a charge of 0.0520

A proton is assigned a mass of 1 and a charge of +1.0524

An electron has a negligible mass, a mass of 0, and a charge of -1.0529

Of course, we symbolize the neutron lower-case n, proton p, and the electron is going to be e.0535

One fallacy in John Dalton's atomic theory was that atoms of the same element are all identical to each other.0547

But we are going to learn that atoms of the same element can exist in different forms; these are called isotopes.0555

Isotopes are atoms of the same element; they differ only in their number of neutrons.0563

Number of protons is going to be equal to the number of electrons; this is because of charged neutrality.0570

But the only difference is the number of neutrons; number of n differs.0577

We are going to introduce something called isotope format.0586

For example, carbon has two isotopes, carbon-12 and carbon-13.0589

Anytime you see the number immediately following the hyphen, this is what we call the mass number of the isotope.0597

The mass number of the isotope is simply equal to the number of neutrons plus the number of protons.0607

When we write an isotope in a specific format, it is always going to have the following design.0618

Element symbol, X; A on top and Z on the bottom as a subscript.0628

X is simply your element symbol; A is your mass number; Z is simply your atomic number.0634

The atomic number is strictly equal to the number of protons of the element.0648

A mathematical relation follows that if you take the difference of A minus Z, you are going to get the following.0659

The number of neutrons plus the number of protons and then minus the number of protons; as you can see protons cancels.0668

When you take the difference of A minus Z, you simply get the number of neutrons.0677

For example, if you take 12, 6 and carbon.0683

We have 12 is equal to the number of neutrons plus the number of protons.0689

6 is on the bottom; that is the number of protons which is also equal to the number of electrons.0698

Therefore A minus Z is equal to 6; that is equal to the number of neutrons.0704

So carbon-12 has six protons, six neutrons, and six electrons.0712

However if you do the same analysis for carbon-13, we are going to get six protons, seven neutrons, and six electrons.0721

As you can see, isotopes strictly differ by the number of neutrons present inside the nucleus.0730

Now we get introduced to the periodic table.0739

The periodic table is essentially organized into different rows and columns.0742

Each column is also referred to as a group; each row is referred to as a period.0749

Typically there is always a staircase that is embedded in all of our periodic tables.0766

Anything to the left side of the staircase is what we call a metal.0776

Anything to the right side of the staircase is what we call a nonmetal.0785

When we look at the periodic table, you see always the following.0797

You see a number on top; the number of top is what we call the element number.0802

The element number is actually the atomic number, the number of protons.0810

The number on the bottom is the atomic mass of the element.0816

Finally right in the middle is your element symbol.0823

We are going to talk now more about different areas of the periodic table.0830

We talked about metals; metals have the following characteristics.0839

They appear lustrous which is shiny; they are good conductors of heat and of electricity.0843

They are malleable which means we can form dinged sheets out of them.0850

And they are ductile which means we can pull them into fine wires.0856

Nonmetals are going to be the opposite.0866

They are going to be electrically and thermally insulated which is what we mean by poor conductivity.0869

In addition, they are brittle which means that if we take a hammer and hit them, they are going to crumble.0878

Instead are not too malleable; so will crumble.0885

But right in the middle of the border between metals and nonmetals, that is what we find semimetals.0891

Semimetals are also known as metalloids.0900

They border that staircase that we drew previously; border the staircase.0904

Because they are called semimetals or metalloids, they are going to have properties of both.0915

They tend to be good conductors of electricity but poor conductors of heat; they tend to be malleable.0923

The typical examples are of course silicon and germanium.0928

We see these two being used in a lot of electrical devices and what we call semiconductors.0938

What we are going to analyze next are some of the major columns of the periodic table.0960

Group 1 is what we call the alkali metals such as lithium and sodium and potassium, etc.0966

Group 1 metals tend to have the following characteristics.0974

That is they are moisture sensitive which means they have a tendency to react with water.0977

They can be explosive when they react with water.0982

Group 2 are what we call the alkaline earth metals such as calcium and magnesium and barium.0986

These are harder than group 1 and less moisture sensitive.0995

Group 6 are the halogens which are fluorine, chlorine, bromine, and iodine.1001

All of these occur naturally in nature diatomically which means they occur in pairs.1007

When someone says fluorine, it is really F₂, Cl₂, Br₂, and I₂; the halogens are incredibly reactive.1015

Finally group number 8, the rightmost column of the periodic table are what we call the noble gases such as helium, argon, and neon.1028

Unlike the group 7s, these are going to be monoatomic; that is they occur just as individual units.1039

They are called noble gas species because they are relatively inert and have a smaller tendency to react.1045

Now that we have been introduced to the periodic tables and the elements that comprise them, let's now talk about different types of compounds.1057

The first type of compound we are going to study are called ionic compounds.1066

Ionic compounds are formed between a metal and a nonmetal or they contain what are known as polyatomic ions.1069

Because of the strong electrostatic attractions that exist between opposite charges, ionic compounds tend to have very high melting points.1078

Below are a couple of these polyatomic ions; polyatomic could be the following.1087

Something like CO₃^2- which is called carbonate or SO₄^2- which is called sulfate.1094

NO₃^1- which is called nitrate; you see why these are called polyatomic ions.1107

Because it is several atoms coming together to form one entity; those are what we call polyatomic.1111

That is different than monoatomic ions of course which are just formed from one unit.1120

Something like Cl^-, O^2-, Al³⁺, etc.; polyatomic versus monoatomic.1124

It turns out that we can actually use the periodic table to predict the ionic charge for main group elements.1135

Main group elements are going to be group 1 and group 2, then group 3 to 6, and then 7 and 8.1141

It turns out that all of the group 1's tend to form a 1+ charge.1160

All of the group 2's tend to form a 2+ charge; group 3's tend to form a 3+ charge.1167

We are going to skip 4 and 5 because they tend not to form ionic compounds to such a large degree.1174

Column 6, we are going to get a 2- charge; column 7 is going to be a 1- charge.1183

Group 8, we say no charge because group 8 are the noble gases and they tend not to react with other species.1188

You noticed also very importantly that the positive charges, which are what are called cations, these are going to be formed by metals.1196

The negative charges are what are called anions; these are all nonmetals.1208

I am sorry; I am leaving out group 5.1219

Group 5 is of course... this is going to be a 3- charge.1222

So you have your cations and then your anions.1226

Once again cations are going to be formed by metals and anions are going to formed by nonmetals.1229

Now we have to learn how to name ionic compounds, such as NaCl, such as BaF₂.1237

We need to now go over the systematic procedure for doing so.1247

You are going to name the cation first followed by the anion and the anion will end in ?ide.1253

Something like NaCl; the cation is simply going to be sodium.1259

Cl is usually chlorine; but now we are going to become chloride.1267

So the correct name of NaCl is simply sodium chloride; very simple.1275

You do not use prefixes mono, di, tri, etc, to denote subscripts in a formula.1284

Something like for example BaF₂; let's go ahead and name it.1289

That is going to be barium followed by fluoride; you do not say difluoride.1294

No, no difluoride, for example; again we ignore the prefixes.1302

For certain transition metals, you are going to use a Roman numeral in parentheses to indicate the metal charge.1310

If we want Fe²⁺, we are going to denote it as iron(II); Fe³⁺ cation is going to be Fe(III).1317

Again you should definitely ask your instructor as to which transition metals he or she1331

would like you to know for sure where you have to use a parentheses to indicate charge.1336

Finally you can also have a hydrate; a hydrate is when you have water attached.1343

For example, CoCl₂·5H₂O, we are going to use the prefixes followed by the word hydrate.1350

Five using the Greek prefix would be penta; we would just say that this is the pentahydrate compound of CoCl₂.1360

Let's go ahead and put everything together and do a couple of examples.1372

Al₂O₃, FeCl₃, CuI₂·3H₂O; we are going to name the cation first.1378

Al₂O₃, that is going to be aluminum followed by oxygen becoming oxide.1394

FeCl₃, Fe is one of those we need to know.1402

Iron, something in parentheses, and then chlorine becomes chloride.1406

Now we have to come up with the charge; each chlorine is a 1- charge.1411

There is three of them, giving me a 3- overall.1416

For iron, iron is going to be 3+ here, giving us iron(III) chloride in the end.1422

Don't forget for the ionic compounds, the net charge should equal zero to maintain charge neutrality.1431

Right here we are going to have copper.1442

Copper is one of those where you need a Roman numeral for, followed by iodide.1444

Then we have three waters which becomes trihydrate.1451

Here iodine is a 1- charge; there is two of them giving me a -2 overall.1457

Copper must be a +2 to balance it; I am going to indicate that in the parentheses as Roman numeral.1464

Now we have to do the other way.1474

Before I gave you the formula and you were asked to provide the name.1476

Now let's go ahead and write the formula given the name.1480

Let's go ahead and look at the following examples; barium phosphide; ammonium phosphate.1485

Barium phosphide; barium is Ba; phosphide is phosphorus which is P.1511

What I would like to do is I would like to put the charges up in the top just for my own purposes.1518

Barium is 2+; phosphorus is 3-; what I always tell my students to do is then I use the crossing rule.1524

Basically if you cross the charges, they become the subscripts for each of the other elements.1530

Ba is going to get subscript of 3; phosphorus is going to get a subscript of 2.1537

You can convince yourself to make sure that the charges cancel.1543

Three of the bariums times 2+ plus two of the phosphorus times 3-, yes they do cancel to zero.1547

Next one is ammonium phosphate; ammonium is NH₄; phosphate is PO₄.1556

Ammonium is a 1+ charge; phosphate is a 3- charge; let's go ahead and cross them again.1564

When we cross them, we are going to get (NH₄)₃PO₄ with a 1.1572

Let's go ahead and make sure that they all work out.1582

Three of the ammoniums times a +1 charge plus one of the phosphates and a 3- charge; yes it does cancel to zero.1585

That is some nice sample problems on naming ionic compounds.1596

The other type of compound we are going to learn about now is called a molecular compound.1603

Molecular compounds are not a metal and nonmetal but instead they are composed from two nonmetals.1608

Something like CO₂; something like CO; and of course H₂O itself.1613

To name a molecular compound, the rules are going to be very similar with one main exception.1620

We are going to name the first element first by its entire name; then the second element has a ?ide suffix.1627

This time we do pay attention to the subscripts.1636

We are going to use the Greek prefixes, mono, di, tri, tetra, etc, to determine what the subscript is.1639

However you omit mono for the first nonmetal.1647

Something like CO₂, name the first element first which is carbon.1651

The second element is oxygen becoming oxide.1656

There is two of them which is how we come up with carbon dioxide.1660

Something like N₂O₅; there is two nitrogens which is di.1665

The first element gets the full name; so dinitrogen; then five oxides or just pentoxide.1670

So actually naming a molecular compound is always easier than naming an ionic compound.1679

Why?--because you don't have to worry about charge.1685

Now onto what we call the concept of the mole.1691

When you go to the supermarket and you purchase eggs, they are always in either a half dozen or a dozen.1697

It is very hard and very seldom that you are going to purchase just an individual egg.1704

It is convenience; it is convenience for us to refer to eggs as a dozen.1708

For chemistry, we have the equivalent of our dozen.1716

The mole is our chemists' dozen for convenience.1721

When you go into lab, you don't measure just the weight of for example one atom.1730

You don't deal with an individual atom; it is not practical.1735

What the mole is all about is that it goes from the microscale to a macroscale; it is more practical.1739

Basically our dozen which is what we call the mole... the mole is abbreviated simply m-o-l.1755

It is equal to the following: 6.022 times 10²³ units of whatever you are measuring.1763

This is equivalent of saying one dozen is equal to twelve units of whatever you are measuring.1774

Again our dozen, the chemists' dozen is what we call the mole.1782

This mole is probably... this number you have to memorize I am pretty sure.1786

6.022 times 10²³ is what we call Avogadro's number; Avogadro's number.1792

The mole we are going to find is a central unit; it can connect the following quantities.1803

The mole, we can equal and calculate individual units; the mole we can also get grams.1809