This type of bond is exhibited by atoms which can either lose electrons to form positively charged ions (cations) or gain electrons to form negatively charged ions (anions). The atom which can lose electrons is said to be electropositive or basic and the atom capable of gaining one or more electrons is referred. as electronegative. The more electropositive atom has always low value of ionization potential and is thus capable of losing electrons with greater ease. The electrons lost by electropositive atoms are completely transferred to other atoms which show greater electro negativities. The bond formed by complete transfer of electrons from electropositive atom to more electronegative atom is called ionic or electrovalent bond The electropositive elements in energy terms should have higher energy states than those of electronegative elements. This ‘energy difference will be responsible for the flow of electrons from higher energy states to lower energy states. The two atoms are held together by electrostatic forces of attraction acting between such atoms.
M° + X° M+ : X (3.1)
The energy required to completely separate the ions from a diatomic molecule is given by the following expression:
Potential energy = Electrostatic energy + Van der Waals’ energy.
The electrostatic energy — q1 q2 e R
where q1 and q2 are charges on atoms M and X and R is the internuclear separation.
The general tendency of various atoms to form molecules is to attain inert gas configuration, being the most stable. The atoms of the inert gases have outermost p orbitals completely filled. Such a configuration will not easily lose or gain any electron because very high ionization potentials (or electro negativities) will be required to remove an electron or gain any additional electron. Let us consider potassium and chlorine atoms which would combine to form potassium chloride molecule. Potassium atom (Atomic number of K = 19) has the electronic configuration 1s2 2s2 2p6 3s2 3p6 4 • si. Chlorine (Atomic number of Cl = 17) has electronic configuration 1s2 2s2 2p6 3s2 3p5. None of these has an inert gas structure. But they possess an incomplete shell of electrons and orbitals. The nearest inert gas to both is argon having electronic configuration 1s2 2s2 2/ 3s23p6. Thu:s the lois of one electron from potassium and gain of this lost electron by chlorine would leave both the atoms with argon configuration. During this process, IC+ and would be produced and ‘the electrostatic attraction between these oppositely charged Ions should be responsible for a stable ionic bond.
Let us take some more examples to elaborate the ionic bonding situation.
- Formation of Sodium Chloride Molecule
- Formation of Magnesium Chloride Molecule
- Formation of Aluminium Fluoride
Transition metal ions which do not resort to inert gas configuration attain their stability through the formation of complex ions. The ions with odd number of s or p electrons are not known, but an odd number of d electrons is found in transition metal ions.
It should be noted that important forces between atoms or groups of atoms are electrostatic in nature.