Ions and Ionic Bonding
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.
Ionization Energy and Electron Affinity
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.
Ionization Energy
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.
Figure 3.1: Comparing the ionization energies of lithium, carbon, and
fluorine. Lithium is the only one with a tendency to ionize to form a
cation, since
the ionization energies of carbon and fluorine are so much higher.
Electron Affinity
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.
Figure 3.2: Comparing electron affinities of lithium (Group I), carbon (Group
II), and fluorine (Group VII). Of these, only fluorine has a tendency to ionize
to form anions because it has a very negative electron affinity.
Ionic Bonding
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.
