For our purposes, the universe is made of two things: matter and energy. "Matter" is material that has mass and occupies space, and can be anything from the pieces that make up atoms to most of a giant star.

A mixture is heterogeneous: if you put it under a microscope you can see more than one type of material. Most beach sand, for example, is a mix of small pieces of rock and small pieces of sea shell.

Pure substances are not mixtures: they are homogeneous, so if you stick them under a microscope there is only one type of material.

All pure substances are either elements or compounds. An element is a pure substance that cannot be broken down further by normal physical or chemical means. Examples of this are copper (a shiney, brownish metal found in pennies), nitrogen (a colorless gas which makes up much of the air we breath), and sulfur (which is a bright yellow solid).

A compound, on the other hand, is a pure substance that can be broken down further. Some examples of compounds are carbon dioxide (a colorless gas found in the air we exhale), hydrogen sulfide (a colorless gas that smells like rotten eggs), zinc oxide (a white powder found in some sun blocks that we use at the beach), and propane (which we burn in camp stoves and probably our gas-powered hotplates upstairs).

Another example of a compound is table sugar (called "sucrose" by chemists). In pure form it is a white crystalline material. However, when heated, it breaks down into carbon and water. The carbon cannot be broken down further, so it is an element. The water is a compound, however, because it can be broken down into hydrogen and oxygen.

To return to elements: Elements are classified using the periodic table. (See page 245.) While this table looks fairly unremarkable, its definition (or discovery) was a great breakthrough. In fact, here we can even see "theory in action": when the table was first devised in the mid-1800’s, there were holes in the table. These gaps helped early chemists know where to look for then-undiscovered elements, which were later found.

Basic properties: the elements are arranged according the number of protons in the nucleus of the atom. Hydrogen has one proton, helium has two, lithium has three, etc. In fact, the "proton count" what makes determines what an element is.

Isotopes, on the other hand, have the same number of protons, but different numbers of neutrons. In regard to chemistry, different isotopes of the same element behave almost identically.

Elements at the top of the table are lighter than the ones lower down, because their proton and neutron counts increase and the atoms are more dense.

If we move from left to right, we find that elements on the left side of the table are metals. The ones on the right are non-metals. (Metals generally are shiny, conduct heat and electricity well, and are ductile—i.e, they can be drawn into wire.) Most elements are actually turn out to be metals, so they crowd the left hand side of the table.

Between the metals (the left and center section of the table) and the non-metals (the right hand side) are a group of "in-between" elements that don’t really fit either category. These are the semiconductors (aka "metalloids"), which include silicon, germanium, and arsenic, plus less familiar elements like selenium, tellurium, antimony, and boron.

Elements in a single column on the periodic table tend to have similar properties, to the point that a column is called a "family." The first column is the "alkali metals": Na, K, etc. These are highly reactive, react violently with water to produce an alkaline (basic) solution.

The second column contains the "alkaline earths" (including Ca and Mg). These metals are also reactive, but less so than the sodium column.

The "VIIA" column contains special elements known as halogens. They are like the alkali metals in that they are extremely reactive (and poisonous in pure form!). These include F, Cl, Br, and I. They tend to be used in disinfectants (as in water and swimming pool chlorination, and in Chlorox bleach) and also in headlamps.

At the far right hand side are the "noble gases": He, Ne, Ar, Kr, Xe, Rn. These gases are very non-reactive and are chemically inert.

Why do elements in a single column have similar properties? The answer is that most of the time their outer electron shells are similar. Very generally, atoms "want" to have 8 electrons in their outer shells. This is called the rule of octets. (The exception here is H and He, which only "want" 2.) H, Na, K, Rb, etc. all have a single electron in their outer shell. This makes them very reactive, because they "want" to get rid of their outer-most electron. F, Cl, Br etc., on the other hand, really "want" one more electron to complete their outer shell, so they also are very reactive. The noble gases actually have 8 electrons in their outer shells, so they are stable and non-reactive.

The elements in between (like C, or Al, or Sn) would like to have 8, but since they are in between with moderately full shells, their "desire" to combine is present but less urgent. However, when elements do react to form compounds, their "goal" is to have 8 electrons in their outer shell, so they can be like the noble gasses. Ex: HF, LiBr, CO₂, SiO₂. (Note Figure 10.18.)

(With heavier elements, things get more complicated. The electron shells become close enough that the upper shell can sort of borrow electrons from lower shells. This allows some metals in the middle of the table to act like they have either two or three electrons to share with their non-metallic partners. For example, iron oxide can have two different forms: black (FeO) or rust (Fe₂O₃). The black form is magnetic, while the rust form is not.)

So now we have atoms and molecules, and elements and compounds. So what? The answer is: we can produce new chemical compounds by making or breaking chemical bonds. These rearrangements are called "chemical reactions." This is pretty much sums up the core of the entire science of chemistry!

Note that this is not some arcane activity that only takes place in the back of some government laboratory. Chemical reactions are going on constantly around us. In fact, they are what keep us alive, as the molecules of the food we eat are rearranged to release energy and thus keep us alive. This fundamental equation goes like this:

In English, this means that one molecule of glucose plus 6 molecules of oxygen are combined and rearranged to produce 6 molecules of carbon dioxide, 6 molecules of water, and energy.

There are many different types of bonds. The main ones we are going to talk about here are ionic and covalent bonds.

Ionic bonds: Think back to our "octet" rule, and to the alkali metals and the halogens. The alkali metals can’t wait to lose a single electron. The halogens are dying to gain an electron. Mix these two together, add a small spark, and there will a big explosion and an ionic compound will form. (Example: sodium (Na) + chlorine (Cl) è NaCl (sodium chloride, or table salt!))

The electrons are instantly transferred from one atom to the other. Each atom now has an octet. However, the charges are imbalanced: the sodium atom has a positive charge, because it has lost an electron needed to balance the protons in the nucleus. Similarly, the chlorine atom now has an octet, but has one too many electrons. Each is now an electrically charged molecule called an ion. They have solved their octet problem desires, but created an imbalance.

The solution? The two ions stick together and balance out the charges. The result, if enough are present, is a salt crystal.

This can happen with other elements, too. Calcium would like to lose 2 electrons, so it can bond with two chlorine molecules, each wanting one electron.

Ca + 2Cl è CaCl₂ + energy

We can think of each electron as a hand. A sodium atom has one hand, and a chlorine molecule also has one. A calcium has two. What makes the bonds here "ionic" is that the chlorine atoms like to hog the electrons, not really sharing very much with the sodium or calcium atoms. The chlorine atoms have "full custody" of the electrons, and the calcium has only occasional visitation rights.

What happens if the atoms are more "easy going" and share their electrons more equally? The result is a covalent bond. Carbon is the covalent bond par excellance. It has four electrons to share, and is willing to share more or less equally with either other carbons or other types of atoms. An example is olive oil:

Another (more simple) one is propane:

C₃H₈

In each case, most of the bonds share their electrons more or less equally, making them non-polar covalent bonds. Most of the atoms have "joint custody of the electrons".

There is a type of bond that stands in between the ionic bond and the "egalitarian" non-polar covalent bond. This is the polar covalent bond. Here the electrons are shared between the molecules more than with the ionic bond, but less than with the non-polar covalent bond. The best example of this is water (H₂O): the oxygen has primary custody of the electrons, but the hydrogen atoms still have partial visitation rights. (See Figure 11.11 on p. 266).

We have now been introduced to the elements of chemical bonds. The next step is to get a handle on chemical formulas.

An empirical formula is a simple whole-number ratio of atoms in a compound. This is most useful for ionic compounds because, technically, a salt crystal could be seen as one giant molecule, a lattice of Na and Cl ions!

Covalent compounds, whether polar or not, generally exist as separate individual molecules. For these a molecular formula is best, which gives the actual constituents of a complete molecule. Thus we would write glucose as C₆H₁₂O₆, not as CH₂O.

Solutions are formed when a homogeneous mixture of ions or molecules is formed when two or more compounds are mixed. The process is called dissolving. The most common solutions that we notice are probably things like salt in water or sugar in water. However, there are all kinds of solutions: a metal alloy, for example, is a solution of two metals. "White gold" (aka electrum) is a solid, frozen solution of gold and silver. Brass is zinc dissolved in copper. Oxygen dissolves into water, which allows fish to breath. Air itself is a solution of nitrogen (78%), oxygen (21%), and other gases (about 1 %). (See table 13.1, p. 301 for more examples.)

When we speak of solutions, the solvent is generally the component present in the larger amount. The solute is present is the smaller amount. Example: in sea water, water is the solvent, and salt is the solute.

Solubility can be vary greatly. Table salt (NaCl) and table sugar are both fairly soluble in water. However, other compounds are not very soluble at all. CaCO₃ (calcium carbonate) is almost insoluble in pure water—which is good, because this what sea shells are made of. If we raise the temperature of our water, significantly more sugar will dissolve into the water, but only a little more NaCl. If we add acid, the CaCO₃ will dissolve, but not more NaCl.

But what makes things dissolve in water at all? The answer is in large part hydrogen bonding. To understand this, we need to develop a better appreciation of water.

Water is probably the most familiar chemical compound for most of us. Humans are about 60% water by weight. Raw meat: 75%. Fruits and Vegetables: up to 95%!

Water is so common that we tend to take it for granted. Chemically, however, it is quite unusual. Hydrogen bonds have a BIG impact on how water behaves--the water molecules attract each other, causing its boiling point to much higher than one might expect. (One might guess -110 degrees F., rather than 212!) See p. 579 in McGee.

Hydrogen bonding is caused by the negatively-charged oxygen side of water lining up with the electrically-positive hydrogen side of the next water molecule. (See p. 305)

Hydrogen bonding also allows for it to hold a lot of "latent heat"--the heat energy that water can absorb before boiling.

Hydrogen bonding is also responsible for water’s high degree of specific heat—this is why it takes ten times as much heat energy to raise an ounce of water 1 degree F. as it does to raise and ounce of iron the same amount.

These characteristics of water help explain why things keep on cooking when removed from the oven, and also why we should put ice on burns immediately.

More positively, the characteristics of water help us live in varying temperature environments--the water in our bodies acts as a thermal buffer to keep us our internal temperature from varying too much, at least under normal circumstances. This thermal buffering is also very important to the life of the planet; the oceans play a big role in moderating climactic temperatures.

Finally, hydrogen bonding results in a very curious aspect of water--frozen water is less dense than liquid water. (Water is actually most dense around 4 degrees C.) Thus, ice floats, rather than sinks, and beer or soda bottles can explode when left in the freezer. Also, it explains why our oceans are mostly liquid, rather than filling up with ice!

Water is also intimately involved with acid and base chemistry. First, water itself is both an acid and a base. An acid can act as a hydrogen donor, while a base can act as a hydrogen acceptor. (See p. 312.)

Very simply, a surplus of hydrogen ions (actually, H₃0⁺, or hydronium ions), makes something acidic. A surplus of OH^-, or "hydroxyl ions", makes something basic.

We measure this with the pH scale. This is a measure of the amount of hydronium ions present. IMPORTANT: pH 7 is neutral. More acid means a lower pH, while more alkaline/basic means a higher pH.

IMPORTANT: One increment on the pH scale is equal to a factor of 10. A solution with a pH of 5 is ten times more acidic as a solution with a pH of 6. A solution with a pH of 4 is 100 times more acidic than a solution with a pH of 6.

Very important: things that are acid usually taste SOUR. Things that are basic tend to taste BITTER.

The electrically polar nature of water allows it to act as a "universal solvent"--many things dissolve in it well.

As a result, pure water is almost never found in nature, or even in a laboratory. In nature, water often contains dissolved carbonate and/or sulfates of magnesium and calcium, making the water "hard." (This can also affect the color and texture of vegetables, and leave a scale on pots and in pipes. It also prevents soap from lathering.)

Soft water, on the other hand, contains salts of sodium and/or potassium, making water "soft."

Distilling and/or filtering water can get rid of a lot of these impurities, but even then carbon dioxide can dissolve in the water, making it slightly acidic.

Water boils at 212 degrees F.--as long as you are at sea level. The boiling point actually drops one degree F for every 500 feet gain in elevation.

If things are dissolved in water, the boiling (and freezing) points change. The boiling point is raised, and the freezing point is lowered. This is how antifreeze works (and why vodka doesn't freeze in the freezer). It is also how why we salt icy roads, and freeze ice cream using rock salt and ice.