A general study on the reactivity of different elements revealed to the earlier chemists that there is a definite class of elements which show almost no tendency to combine with other elements. These elements, as we know today, are the members of noble (or inert) gases. The atoms of these elements except that of helium have eight electrons in their outermost (or valence) shells. Since inert gases do not exhibit any tendency to combine with other atoms, the arrangement of eight electrons in the outermost shell was considered to be the most stable electronic configuration. It was also realized that the atoms of other elements show chemical reactivity in order to acquire the stable electronic configuration of the nearby inert gas atom. Primarily, this arrangement is achieved in two different ways as described below
- The mutual sharing of electrons between atoms.
- The complete transfer of electron(s) from one atom to another.
Consider the following situation in which atoms A, B and an inert gas is arranged in order of increasing atomic numbers.
Atom A Inert gas Atom B
Obviously, atom B has one more electron than the preceding inert gas whereas atom A is short of one electron as compared to the succeeding inert gas. Now both the atoms A and B can attain the stable electronic configuration of the inert gas if the extra electron in atom B is completely transferred to the atom A. In doing so, atom B acquires a unit positive charge whereas atom A acquires a unit negative charge, i.e. ions are formed. These ions are held together by electrostatic attraction. This type of bonding is termed as electrovalent or ionic bonding. One of the common examples of an ionic bond is sodium chloride. The electronic configuration of sodium and chlorine atoms is as follows:
11Na 1s2, 2s2, 2p6, 3s1
17Cl 1s2, 2s2, 2p6, 3s2, 3p5
Sodium has one extra electron as compared to the neon atom (atomic number 10) whereas chlorine has one electron less than the argon atom (atomic number 18). Both these atoms can acquire inert gas configurations provided one electron from sodium is completely transferred to the chlorine atom. In this process, sodium atom becomes sodium ion, whereas the chlorine atom becomes chloride ion. In Lewis structure, the formation of an ionic bond can be represented as follows:
Sodium atom is an electropositive atom (one which can easily give out its electron to form a cation, i.e. an atom having a low ionization energy), whereas chlorine atom is an electronegative atom (one which can easily accept electron to form an anion, i.e. an atom having high electron affinity). Hence, an ionic bond is formed between an electropositive atom and an electronegative atom. In general, atoms lying immediately after inert gases are electropositive atoms and those lying before are electronegative atoms. The other examples involving the transfer of more than one electron are MgCl2, CaO, K2S and AlCl3. The ionic bond formation in these molecules can be represented as follows:
Unlike covalent bonds, the ionic bond is non-directional in nature as the electrostatic interactions can be experienced all around the ions.