Basic Chemistry

We begin with a few definitions: • Element – A material with certain invariable characteristics that cannot be chemically divided into a material with different characteristics. This is the basic unit of chemistry. Each element has a unique symbol.

•Atom – The smallest possible piece of an element. These can be subdivided only by nuclear reactions that change the element and therefore the chemical nature of the material. Atoms can combine with other atoms in both chemical reactions (through chemical bonding) or nuclear reactions (through fusion). The latter changes the nature of the element. The former creates a new chemical material with different characteristics, but that can be reversed chemically.

•Compound – A material with certain characteristics created by chemically bonding atoms. The atoms may all be of the same type (gold, silver, mercury, etc.) or they may be of different types (carbon dioxide, dihydrogen oxide, sodium chloride, etc.). The compound has different, emergent characteristics than the constituent elements, but as mentioned, those elements can be re-extracted from it by chemical means.

•Molecule – The smallest possible piece of a compound. The component atoms and their proportions are given in the compound’s symbol and usually in the name, unless there is a more common recognizable name for it. For example, dihydrogen oxide is H20, more commonly called water.

•Chemical Bond – A mechanism whereby atoms are held together into molecules. Chemical bonds are electrical in nature as evidenced by the fact that the electrical properties of atoms are different when bonded vs not bonded, and the fact that electricity can break the bonds and separate the atoms from each other.

Among other things the periodic table gives us the names of the elements and some of their characteristics. We will look at a couple of examples.

The symbol for hydrogen is “H” and that for oxygen is “O”. Most elements have a two-letter symbol but some of the earliest known ones use only a single letter. Some of the letters come from the recognizable English name of the element, some (as we’ll see) come from older Latin names, used by the Romans and by alchemists in the Middle Ages.

The atomic number for hydrogen is 1 and that for oxygen is 8. Atomic number increases stepwise (1, 2, 3, …) on the periodic table as you read the elements like text on a normal page – left to right one line at a time. We’ll see what the atomic number means shortly.

The atomic mass for hydrogen is a little over 1 and that for oxygen is slightly under 16. Atomic number also increases as atomic number does and is usually roughly twice what the atomic number is. We’ll see what the atomic number means shortly and why it’s value does not increase as regularly as that of atomic number.

A full model of the atom of any element illustrates several things about the element, specifically its mass and its chemical reaction possibilities. Such a model is called a Bohr model. Let’s begin with the mass. There are a number of ways that we can calculate or measure the mass of an atom of any element, in or our of a chemical compound. It turns out that the mass is the same in either case. That is, the mass of the atom is unaffected by chemical reactions. In a Bohr model then the mass is diagrammed in the center of the atom, symbolically “out of reach” of ordinary chemistry. This central region is called the nucleus and any (non-chemical) reaction that does alter the mass is called a nuclear reaction. Ordinarily they also change the chemical nature of the atom, except in one type.

Notice that the atomic mass of H is given as 1.0079. There is no atom of H that actually has this much mass. Instead most atoms have a mass of a certain amount that is called “1”. It’s not one gram or one ounce or one nanopicogram. It is generally given without any units but implicitly it is “1 ordinary hydrogen’s mass”. A much smaller number of atoms that are chemically H have been found with a mass of 2 – that is, twice the mass of a typical atom. This is “deuterium” or 2H. Similarly, an even smaller number have a mass 3x the normal, and these are “tritium” or 3H. The average mass of all the H sampled on Earth is (xH+y2H+z3H)/3 = 1.0076 where x,y, and z are the numbers of each mass type in a statistically significant sample of Earth’s H.

The same applies to oxygen. Most atoms of O have a mass of 16, twice the atomic number, but a few are a unit or two less and a few are a unit or two more. Remember what a unit is – the mass of a normal H atom.

If we re-word this we can understand the mass this way: most oxygen atoms weigh 16 times what an ordinary H weighs, but a small number are a little lighter (by exactly one or two hydrogen masses) and a few a little heavier (by +1 or +2 H masses). No oxygen weighs 15.999 times what H does. They weigh 14, 15, 16, 17 or 18, with 16 by far the most common.

As we go to heavier and heavier atoms the masses will creep above the 2x atomic number value, in some cases by quite a bit. However, for any atom the mass will be an even multiple of 1 hydrogen mass. No atom has ever been found that does so and theory predicts that none ever will. It’s a good theory. It’s so close to certainly right that we can get away with calling it a fact and nobody will blame us.

The atomic number of an element is therefore related somehow to the mass, but it is also related to the chemical behavior of the element. Remember that atomic number increases regularly on the periodic table.

H is the first element on the table – in the upper left-hand corner. It’s atomic number is 1 and the typical mass of the atom is 1. H also reacts with other elements in a way that we can visualize it having one unit or reactivity. Let’s think about what this means. When H combines with O it does so in the ratio 2:1. The compound is H2O to reflect this. This is why we got 2x the H out of hydrolizing water as we did O. H combines with other elements in the same ratio, H2S for example. In other cases the ratio is different – HCl and HF arte hydrochloric and hydrofluoric acid respectively, and the H and the other atom are always in 1:1 ratio. Other ratios are possible, but they can always be reduced to a small whole-number ratio. (Note that H2O2 (hydrogen peroxide – an unstable substance that can be created but that rarely occurs outside of very special environments like a chemistry lab) has a ratio of 1:1. That is, 2:2 reduces to 1:1). Therefore H can serve not just as a unit of mass, but as a unit of chemical reactivity.

Because chemical reactivity (or bonding) is electrical in nature we use electrical charge units to describe it. For historical reasons – the way people came to understand this, in other words – the convention is to use a negative charge to “carry” the reaction.

The Bohr model of H therefore has a single (unit) negative charge on the outside of the atom, where it is “easily accessible” for chemical reactions, which are much easier to accomplish than nuclear reactions.

A Bohr model for ordinary H is very simple, and looks like this:

The red “dot” or “ball” on the perimeter of the model symbolizes the single (-) charge that H uses to enter into reactions with other atoms. We usually treat it as a (virtually massless) particle but sometimes as a package of energy. Either way it is called an electron. We can abbreviate it e-. The blue “dot”/”ball” represents the mass particle of the atom. Because H atoms in this state are electrically neutral this particle is assigned a charge of +1 to balance the -1 charge of the electron. It is called a proton (“pro” for positive, like voting “pro” a thing). It can be abbreviated either P or +.

A Bohr model for deuterium is a bit more complex, and looks like this:

Remember that deuterium is also called “2H” and has a mass double that of normal H. The Bohr model symbolizes this by adding another mass particle to the nucleus of the atom. However, this mass particle cannot be another proton because then the atom would be electrically unbalanced. Deuterium atoms are not electrically unbalanced. They are exactly like normal H except they weigh twice as much. Therefore this particle must have mass but no charge. We call is a neutron and abbreviate it “N”. Notice that the chemical properties of this atom are just like those of normal H because there is still a single e- to do the reacting. What would tritium’s (3H) Bohr model look like?

A Bohr model for helium (atomic number 2) is yet a bit more complex, and looks like this:

Chemically He is completely unlike H. It also has (usually) four times the mass per atom as H. The Bohr model therefore has four mass particles in the nucleus – two P and two N (thought there may be more or fewer than two N). This will serve as a pattern for the rest of the periodic table. At each step the atom gains an additional P and around two additional N, effectively adding two units of mass for each unit of (+) charge added to the nucleus. Adding the P, of course, means that another e- must be added to balance the charge. He has 2 P and 2 e- so the net charge is 0. (+2)+(-2) = 0. When an atom enters into a chemical bond though it becomes charged. We call such a charged atom an ion. If it is positively charged we call it a cation, if negatively charged an anion. The Bohr model represents that change in charge as some action that adds or removes electrons from the atom.

Helium, however, never becomes charged during chemical reactions. That is, it never combines with other atoms to form compounds! There is no such thing as a “heliide” or a “heliate”. He is a noble gas because it only exists as single atoms. The Bohr model incorporates this behavior of He by having the two electrons around it in a single “shell” – a spherical region in which they are envisioned to orbit the nucleus. There is another aspect to this that you might be interested in (or you might ignore). The 2 electrons, being fairly near the nucleus, may be all that will fit into that amount of space without excessively repelling each other because of their similar charge. There are other noble gasses and we will get to those pretty soon, but the information we have so far is almost enough to let us think about how atoms combine and about the structure of the periodic table. Let’s take it one more step.

Lithium has (usually) six mass particles – three P and three N. To balance the three P it also has three e-, but rather than sticking the third in the same shell as the others (where it doesn’t fit) it is placed in another shell, farther out. There is some geometric justification for this – atoms with more shells are typically larger than atoms with fewer, as evidenced by (among other things) how tightly they must be packed into a compound to account for the compound’s density. This also symbolizes something else about Li as well. With one electron in the outer shell (known as a valence electron in the valence shell). Li behaves chemically the same as H, which also has one e- in its outer shell. I will leave it to you to work out what the Bohr model an atom of Beryllium would look like.

As long as we are only interested in how two atoms interact chemically we do not need to think about the nucleus at all, so a simplified Bohr model simply represents it as a small dot in the center of the atom. Once we are interested in the physical properties of the resulting compound we then need to consider the nucleus more carefully.

Same # of shells

Same # of e- in the outermost (valence) shell.

1 2 3 4 5 6 7 (2 or) 8 e- in valence shell

The periods of the periodic table are the rows. Each row ends with a noble gas – an element (like helium) that is chemically inert. We can think of these as having “complete” or “filled” valence e- shells. All the other elements in the period (row) are chemically active. The groups of the periodic table are the columns. The same number of electrons in the outer shell within a group means that they will behave chemically the same, in the sense that they will combine wioth other elements in the same ratio. For example, HCl, LiCl, NaCl are all chemical compounds, as are H2O, Li2O, and Na2O, as are MgF2, and MgCl2. Examine each of these example compounds and study which group its elements belong to until you see the pattern.

Octet Rule: Except for the first shell (which can only fit 2 e- in the available space because of their mutual repulsion) electron shells fill when there are 8 e- in them. Filled outer shells mean that the element is chemically stable, or inert, and will not become involved in any chemical reaction. Elements in group 18 automatically have a filled outer shell 2, 10, 18 etc… Other elements can attain a similar unreactive state by gaining or losing electrons from their outer shells, or by sharing them. If Lithium (Li -- #3) dumps its one electron it also dumps its outer shell. (A shell doesn’t exist except when there’s an e- in it). If fluorine (F -- #9) gains an e- its outer shell has 8 and is full. We will look into this again when we study minerals.

18 elements per period.

We also need to examine this part of the table very briefly.

2 or 8 elements per period.

32 elements per period.

After period 3 of the table the number of elements per increases. Groups 3-12 are called the “transition metals” or “transition elements” and are chemically complex -- they can combine with other elements in variable ratios, as if they have different numbers of e- in their outer shell. This is symbolized in the Bohr model by assigning them “subshells”, so even though there can be 18 e- in the outer shell of, say, titanium (Ti) or zinc (Zn), they will use at most, 7 of them in combining with other elements, just like the elements in periods 1-3.

+10

+14

(8+10=18 8+10+14=32)

The groups at either end of the table, excluding the transitional metals, can all be treated as if they have a pattern of valence electrons like that of periods 2 and 3 – i.e., they behave as if there are up to 8 valence e- and they do not combine variably with other atoms like the transition elements do. In group 17, for example, bromine (Br), iodine (I), and Astatine (At) all behave as if they have 7 valence e-, just like F and Cl do. Similarly, all the elements in group 18 are noble gasses, as if they have 8 e- in their valence shells. In the same way, elements of groups 1 and 2 (excepting Fr and Ra at the bottom) behave as if the have 2e- in their valence shell, just like H, Li, and Na or Be and Mg do. Your book omits the lower two elements of these groups (Fr and Ra) from further consideration but radium (Ra) is known to combine consistently in the manner of group 2 metals and francium (Fr) is assumed to. Both are highly radioactive and Fr has a very short life-span (its half-life is about 22 seconds), so how it really would behave chemically if it stuck around long enough to behave at all is purely a theoretical matter. Presumably it would be like the other group 1 elements because that is where it sits in the table.

The last elements on the table, everything from #93 on, can also be safely ignored. They do not occur in nature, at least on Earth, and have exceptionally short half-lives. They are produced only during nuclear reactions and quickly decay again. They may exist during supernovae for similarly short intervals. Uranium (U -- #92) is the heaviest naturally occurring element.

So, to understand basic chemical processes, without too many complications, we can use the elements at the far ends of the periodic table. Because we don’t need to know about the nuclear structure to understand chemical behavior we can dispense with it altogether, and because only the valence e- in the outer shell participate in chemical reactions we can also disregard those. The electron-dot models below give us all the information we need to consider how elements combine: the atom involved (by its symbol) and the number of valence e- (by the dots).

. . : O . :

. . : O . : . .

: C . :

Let’s look at a couple of examples of how we can use dot diagrams to understand chemical compounds. Water (H20) and cabron dioxide (CO2) are well known compounds. Why are the ratios of elements in them 2:1 and 1:2 and not some other ratio? First look back to the preceding page and verify that the dot diagrams for these atoms are correct.

Oxygen is a group 16 element, meaning it has 6 valence e-. To have a full valence shell it needs 8, or two more than it actually has. Hydrogen is a group 1 element with one e- in its outer shell. Because that “outer shell” is the only one – the one with only 2 e- possible – H can do one of two things in a chemical reaction. Usually it empties that shell by allowing its e- to be incorporated into another atoms valence shell, but sometimes it takes an e- instead and fills its own shell. In water it does the former. One H is not enough though to fill the valence shell of one O; this brings it up to only 7 valence e-. (The H e- is shown in red in the diagrams below). It takes the second H atom contributing its e- to bring O up to 8 electrons in its outer shell. In a very metaphorical sense, O can now believe and act like it is a noble gas.

H .

. . : O . :

H .

H .

. . : O . :

For CO2 let’s use slightly different symbolism. Instead of bringing the atoms together physically we will simply indicate with arrows which valence e- are moved from one atom to the other. Note that we can put the dots for an element wherever they are convenient for doing this. There is no need to always place them in the same position as long as the number of them is correct. In this case C splits its 4 valence e- between two O atoms, giving them 2 each. This brings the valence shell of O to 8 e- making it a filled or noble gas shell. What is not so apparent from the dot diagram is that this also fills the outer shell of the C, not by filling it up but by making it disappear. The shell is not a real thing and doesn’t exist independent of the electrons in it. So when the 4 e- leave the C atom this backs it up, in a manner of speaking, to its next shell down. In the case of C this will be the first shell and it will have 2 e- in it, making it full.

Just to be complete we should remember that H and O and C and O can (and do) combine in other ways, as carbon monoxide (CO) and hydrogen peroxide (H2O2) for example. However, these compounds are not stable under ordinary conditions and combine with other materials as soon as conditions are right for that to happen. When you put hydrogen peroxide on something organic it will bubble furiously. When it stops the extra oxygen atom is gone and the remaining liquid is ordinary water. You should practice with a couple of compounds on your own. See if you can figure out the correct formula for sodium oxide (sodium is Na – Latin “natrium”) and for calcium chloride (Ca and Cl).