Here we cover some elementary concepts and study a few common chemical reactions. We are mostly interested in learning a simple explanation why the reactions happen at all, and how they happen when they do.

Atoms are stable, ordered collections of more fundamental particles (see the article on Quantum Mechanics for details): neutrons (0 charge, mass = 1 atomic mass unit = 1 AMU), electrons (-1 charge, mass = 1/1836 AMU) and protons (+1 charge, mass = 1 AMU). In an atom, the protons and neutrons are densely packed in the very small center of the atom, called the nucleus, while the electrons fly rapidly around the nucleus in the electronic orbitals or clouds. The electron cloud takes up most of the volume of an atom. Most of the mass of the electron is contained in the nucleus.

An atom is the smallest possible unit of a chemical element, a fundamental, individual substance. All stable matter in the universe consists of chemical elements and combinations of elements called chemical compounds.

The number of protons (in the nucleus) distinguishes atoms of the various chemical elements from each other. In fact, every element has an atomic number that is exactly the number of protons in one atom of that element. The atomic number is the single most important property of a chemical element, since the atomic number gives the identity of the element.

The second most important property of an element is its atomic mass. Naively, adding the number of protons, neutrons and electrons each times the mass of the particle (given above) computes this number. In fact, there is a slight discrepancy – the real atomic mass as measured by very precise experiments is always slightly lower than the sum of its parts. This mass deficit is caused by conversion of some of the mass of the atom to binding energy. The safest way to get the atomic mass for an element is read it off a periodic table of the elements. This is the familiar table found on the wall of almost every chemistry classroom in the world. Very nice copies (8.5 x 11 inches – notebook paper size) are readily available for a low price.

Most chemistry textbooks feature a periodic table printed on the inside front cover of the book.

Atoms must be electronically neutral in their pure state. Since like charges repulse each other with great force, and any charge on an atom would be repeated on every atom, a pure sample of an element with any charge at all would explode violently. In order for an atom to be neutral (zero charge), every proton (+1) must be matched with an electron (-1). Thus, there is an atom with 1 electron (and one proton), hydrogen (H), one with two, helium (He), three, lithium (Li), four, beryllium (Be), and so on up to about element 92, uranium (U). (Beyond element 92, the nuclei of the heavy elements are unstable, and so these heavy elements are not found naturally.)

Elements with certain "magic" numbers of electrons (2, 10, 18, 36, 54 and 86) are so stable, the elements do not react chemically. These elements are called the noble (or inert) gases. All other elements react with other elements to form compounds. In the special case where the reaction produces fixed numbers of the same types of atoms in exactly the same configuration, we say the elements have formed a molecule. Molecules are the simplest possible samples of new substances called molecular compounds. Usually, molecules form so that the atoms within them share electrons and thereby attain a magic number of electrons, with enhanced stability similar to the noble gases.

Although the orbitals that are occupied by the electrons in an atom are quite complicated to describe exactly, we can think of them as concentric shells around the nucleus. Each successive shell starting from the nucleus and working outward can contain only so many electrons. Once a shell is filled, additional electrons must occupy shells further out from the nucleus. The first shell can take only two electrons, the second 8, the third 8, the fourth 18, the fifth 18, and the sixth 32. Shells seven and above contain elements which are unstable. We have a simple rule for the reactions we study: apart from hydrogen, which is trying to get two electrons and so look like helium, all elements are trying to achieve eight electrons in their outer shell. Needless to say, this rule has many exceptions, but so many reactions obey it that we will consider it our cardinal rule for reactions.

Atomic hydrogen is rarely found, but molecular hydrogen is common. Explain why.

The most abundant molecular substance on the surface of the earth is water, H₂O. Explain why.

There is certainly a lot of oxygen near the surface of the earth, since the atmosphere is 21% oxygen. However, there is little free hydrogen. The atmosphere contains some, but not nearly as much as it does oxygen. On the other hand, interstellar space contains mostly free hydrogen, albeit very rarified. Where did all of earth’s hydrogen go? The simple answer is that most of it reacted with oxygen to form water.

Let’s review our fundamental rule for reactions. All elements, apart from the noble gases, are chemically reactive, because they are more stable electronically when they share electrons with other atoms. When this happens, the outer shell of electrons in each atom forms a magic number. Another example will be helpful here.

One atom of carbon reacts with four hydrogen atoms to form methane, CH₄. Show how this reaction works.

A nitrogen atom reacts with three hydrogen atoms to form ammonia, NH₃. Show how this reaction happens.

In short, describing chemical reactions as we have done reduces to a game: complete the shell. We will do additional examples in the next article to really nail this concept down. After the game of complete the shell has been mastered, we will start considering some of the many exceptions to this game and explore some of the consequences of violating our fundamental rule.