3.3 Ionic Bond

Ionic bond in binary compound is formed by attraction of oppositely charged ions. Electronegativities of atoms forming ionic bond should differ significantly (see previous paragraph). Representative elements (s- and p-) often have ns²np⁶ (n2) electron configuration in ionic compounds. For d- and f-elements the octet rule doesn’t work well, instead, particular stable electron configurations (for example, nd⁵ or nd¹⁰) are formed.

For most of nonmetallic elements, addition of an electron to the atom is energetically favorable; therefore these elements readily form anions. Thus, formation of an anion with a charge of 2- or larger is always endothermic. When a valent shell is completed, any additional electron must enter the next higher shell. This also requires a very large amount of energy. As successive ionization energies increase in magnitude, formation of highly charged cations is energetically favorable.

Attractions between opposite charged ions, which occur when the ionic compound is formed, produce a large decrease in the potential energy (greater, then total energy required to form the ions). This is the principle reason of the stability of ionic compounds. A crystalline lattice of ionic compound is composed of ions of opposite charges, regularly arranged to achieve a minimum of potential energy. The arrangement of ions in the lattice depends on the charges of the ions and their radii, but is practically unaffected by electronic structures of the ions. The energy delivered when a crystalline lattice is formed from gaseous ions is called the lattice energy. Ionic bond is nonsaturable and nondirectional, in contrast to covalent bond.

Polarization of an anion by the cation is a concept used to explain ionic-covalent character of the bonds in metal - nonmetal compounds. Cations of metallic elements are almost always smaller than anions of nonmetallic elements. The positive charge of the cation is therefore concentrated in a rather small volume, while the negative charge of the anion is spread over a much larger volume. When a cation is located near an anion it pulls part of the electron density into the region between the two nuclei. The electron cloud of the anion becomes distorted (non-spherical) and the anion is said to be polarized by the cation. Since polarization increases the electron density between nuclei, the greater the degree of polarization, the greater the degree of covalent character of the bonds.

There are two major factors that contribute to cation’s ability to polarize a given anion. All other things being equal, a cation with a 2+ charge will distort the electron cloud of an anion more, that one with 1+ charge. The second factor is the size of the cation. In a small cation the positive charge is highly concentrated and has a strong effect on the anion. The same positive charge on a larger cation will be more spread out, so it won’t distort the anion’s electron cloud as well. A cation effect on the anion is therefore directly proportional to its positive charge and inversely proportional to its radii. This ratio is called the ionic potential ():

 = q/r

The ionic potentials of Be²⁺ and Li⁺ are high because of their radii are small (e.g., LiI is ionic but has some covalent bonding present).

Going down the periodic table group, the charge on the cations stays the same (for example, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺). The sizes of the cations increase, so their ionic potential decrease. Therefore, going down the group, compounds with the same anion should become more and more ionic (for example, MgCl₂ should be more ionic, than BeCl₂).

The atomic radii decrease going from left to right across the period, while the charges of the cations increase (Na⁺ — Mg²⁺ — Al³⁺). This causes a rapid increase of the ionic potential and corresponding increase of the covalent character of the bonds. Simple ions with large positive charges like C⁴⁺, N⁵⁺, S⁶⁺, Cl⁷⁺, Ti⁴⁺, V⁵⁺, Cr⁶⁺, Mn⁷⁺, couldn’t exist. If the oxidation state of an atom is high, polar covalent bond is formed.