Chemical Bonding and Molecular Geometry

Ionic Bonds

Ionic bonds form between atoms with large differences in electrostatic forces of oppositely charged ions. Electrons are transferred in ionic bonds, and oppositely charged ions result.

An ionic bond is an electrostatic force that holds cations (positively charged ions) and anions (negatively charged ions) together. This type of bond typically forms between atoms with very high electronegativity, such as chlorine, and atoms with low electronegativity, such as sodium. When atoms with widely different electronegativities are in contact with each other, each element forms an ion. Ionic compounds form when cations and anions become ionically bonded. They are typically solids at room temperature and form extended crystalline lattice structures. In such a solid, each ion is surrounded by ions with the opposite charge. Ionic solids have numerous bonds, resulting in high melting and boiling points. Table salt is an example of an ionic solid.

An ionic compound forms most often between the most common ions of the elements involved. Alkali metals, such as sodium and lithium, have ions with 1+ charge. Alkaline earth metals, which are the second group in the periodic table, form ions with 2+ charge. Halogens, group 17 elements such as fluorine, form ions with 1– charge. Ionic compounds have a balanced total charge: The overall positive and negative charges of the ions is zero. Magnesium chloride, for example, consists of magnesium ions with 2+ charge and chlorine ions with 1– charge. In magnesium chloride, two chlorine anions are present for each magnesium cation, resulting in the chemical formula MgCl2 and a particle with zero overall charge between the ions.

There are different theories describing how ionic bonds form. One theory, proposed by John Polanyi, Hungarian Canadian chemist and winner of the 1986 Nobel Prize in Chemistry, states that electron transfer occurs at ranges much higher than bond formation. According to Polanyi, ions form first, and then are strongly attracted toward each other because of the difference in electrostatic charge.

When two ions approach each other, they experience both attraction and repulsion forces. The protons of the first ion repulse the protons of the second ion. Similarly, the electrons of the two ions also repulse each other. At the same time, protons of each ion attract the electrons of the other. The sum of these repulsion and attraction forces changes according to the distance between the two ions. The point where the forces are in equilibrium represents a distance at which the total energy of the atoms is the lowest. Once at this distance, the atoms require an input of energy to get closer to each other. They also require an input of energy to get farther away from each other.
During the formation of an ionic bond, the potential energy is first a positive quantity that begins to decrease and becomes negative as the distance between the ions decreases. The bond length is the distance at which the potential energy is minimized.
One important point in ionic bonding is that formation of the ions always requires a net input of energy. Formation of a cation, a positively charged ion, involves removing an electron from an element. Removing an electron always requires energy, even in alkali metals, such as lithium and sodium, which have low ionization energies. Formation of an anion, a negatively charged ion, involves adding an electron to an element. This step releases energy, but generally, not as much as required for cation formation. In ionic bonding, ion formation requires an input of energy. So, it is not the driving force of the process. Instead, the energy released during the actual formation of the ionic bond becomes the main driving force in ionic bonding. The process produces the solid lattice