2.2: An atom’s electrons determine whether (and how) the atom will bond with other atoms.

Most substances are not pure elements but are compounds made of atoms from different elements that are joined by bonds. It is an atom’s electrons that determine whether (and how) it bonds with other atoms. Electrons move so quickly that it is impossible to determine, at any given moment, exactly where an electron is. Electrons are not just moving about haphazardly, though. Speeding around the nucleus, they tend to stay within a prescribed area called an “electron shell.” An atom may have several shells, each shell occupied by its own set of electrons. Within a shell, the electrons stay far apart because their negative charges repel one another. (Electron shells aren’t actual physical tracks but, rather, are simplified placeholders for the different possible energy levels of the electrons.)

The first electron shell is closest to the nucleus and can hold two electrons (FIGURE 2-6). If an atom has more than two electrons, as most atoms do, the other electrons are arranged in other shells. The second shell is a bit farther away from the nucleus and can hold as many as eight electrons. There can be up to seven shells in total, holding varying numbers of electrons.

Atoms become less reactive and more stable when their outermost shell is filled to capacity. Atoms with a completely filled outer shell behave like loners, neither reacting nor combining with other atoms. On the other hand, when atoms have outer shells with vacancies, they are likely to interact with other atoms, giving, taking, or sharing electrons to achieve that desirable state: a full outer shell of electrons. In fact, based on the number of electron vacancies in the outermost shell of an atom, it’s possible to predict how likely that atom will be to bond, and even which other atoms its likely bonding partners will be.

Let’s take a brief look at one element—carbon. Carbon’s electron configuration gives it considerable versatility when it comes to bonding with other atoms and making important compounds. Carbon has 6 electrons overall: 2 in the first electron shell and 4 in the second electron shell. Because the second electron shell has a capacity of 8 electrons, carbon can share the 4 electrons in its outermost shell. Having 4 electrons to share gives a carbon atom the ability to bond with other atoms in a large number of different ways—including in four different directions—and makes a huge variety of complex molecules possible (FIGURE 2-7).

Carbon atoms most commonly bond with oxygen, hydrogen, nitrogen, and other carbon atoms. Chains formed of carbon atoms bonded to other carbon atoms are very common—and each carbon bound to two other carbon atoms can still bind with one or two additional atoms. These carbon chains are present in most organic molecules.

Typically, an atom has the same number of electrons as protons. Sometimes an atom may have one or more extra electrons or may lack one or more electrons relative to the number of protons. An atom with extra electrons becomes negatively charged, and an atom lacking one or more electrons is positively charged. Such a charged atom is called an ion (FIGURE 2-8). Due to their electrical charge, ions behave very differently from the atoms that give rise to them. As we see later in the chapter, ions are more likely to interact with other, oppositely charged ions.