As seen in the previous section on the octet rule, atoms tend to lose or gain electrons in order to attain a full valence shell and the stability a full valence shell imparts. Because electrons are negatively charged, an atom becomes positively or negatively charged as it loses or gains an electron, respectively. Any atom or group of atoms with a net charge (whether positive or negative) is called an ion. A positively charged ion is a cation while a negatively charged ion is an anion. In this section, we briefly look at some of the processes through which electrons are gained and lost in the formation of ions.
The process of gaining or losing an electron requires energy. There are two common ways to measure this energy change: ionization energy and electron affinity.
The ionization energy is the energy it takes to fully remove an electron from the atom. Ionization energy is a property that varies predictably across the periodic table. Group I and II elements with few electrons in their outer shell have very low ionization energies, while ionization energies increase dramatically moving right along the periodic table. The octet rule gives a straightforward (albeit simplified) explanation of this trend: elements with few valence electrons (those on the left of the periodic table) readily give them up in order to attain a full octet within their inner shells.
When several electrons are removed from an atom, the energy that it takes to remove the first electron is called the first ionization energy, the energy it takes to remove the second electron is the second ionization energy, and so on. In general, the second ionization energy is greater than first ionization energy. This is because the first electron removed feels the effect of shielding by the second electron and is therefore less strongly attracted to the nucleus.
An atom's electron affinity is the energy change in an atom when that atom gains an electron. The sign of the electron affinity can be confusing. When an atom gains an electron and becomes more stable, its potential energy decreases, meaning that upon gaining an electron the atom gives off energy and the electron affinity is negative. When an atom becomes less stable upon gaining an electron, its potential energy increases, which implies that the atom gains energy as it acquires the electron. In such a case, the atom's electron affinity is positive. An atom with a negative electron affinity is far more likely to gain electrons.
Like ionization energy, electron affinity exhibits periodic trends, with electron affinities becoming increasingly negative from left to right. Remember, as the electron affinity of an atom becomes more negative, it becomes more likely for an atom to gain an electron.
An ionic bond is comprised of the electrostatic attraction of positively and negatively charges ions which holds them together. A common example of a compound held together by ionic bonds is table salt (NaCl), which consists of Na+ cations and Cl- anions held together in a solid crystal. The attractive force between positive and negative ions stabilizes the crystal.
It is important to remember that ionic bonds, unlike covalent bonds, are adirectional, meaning that ionic bonds occur between the ion and all other ions surrounding it. Hence ionic compounds do not occur as discrete units but as large aggregates. Furthermore, when ionic compounds are placed in water or other polar solvents they dissociate into their component ions. When you encounter ionic compounds in the context of organic reactions, they will almost always occur as free ions in solution. We will see that in the context of organic chemistry, covalent bonding is far more important than ionic bonding.