explain the covalent character of Agl
Answers
Just as a covalent bond may have partial ionic character an ionic bond may also show a certain degree of covalent character. When two oppositely charged ions aproach each other closely, the cation would attract the electrons in the outer shell of the anion and simultaneously repel its nucles. This produces distortion or polarization of the anion, which is accompanied by some sharing of electrons between the ions, i.e., the bond acquires a certain covalent character. The formation of a covalent bond between two ions may be illustrated with reference to formation of Agl.
(b) Factors influencing ion – Deformation or increasing covalent character
(i) Large charge on the ions: -The greater the charge on the cation, the more strongly will it attract the electrons of the anion. For example, Al3+ can distort Cl– ion more than Na+ ion. So aluminium chloride is a covalent compound whereas NaCl, AlF3, AgF are ionic.
(ii) Small cation and large anion:- For a small cation, the electrostatic force with which its nucleus will attract the anion will be more. Moreover a large anion cannot hold the electrons in its outermost shell, especially when they are attracted by a neighbouring cation. Hence there will be increased covalent charcter with a small cation and a large anion, as in AgI.
(iii) Cation with a pseudo-inert gas type of electronic configuration:- A cation with a 18 electron in outermost shell such as Ag+ ([Kr] 4d10) polarizes anions more strongly than a cation with a 8 electron arrangement as in K+. The 'd' electrons in Ag+ do not screen the nuclear charge as effectively as the 's' and 'p' electron shell in K+. Thus AgI is more covalent than KI, although Ag+ and K+ ions are nearly of the same size. Cuprous and mercurous salts are covalent.
The above statements regarding the factors, which influence covalent character, are called Fajans' rules. It can thus be seen easily that there is nothing like a purely ionic compound or a purely covalent compound.
Polarisation power of a cation is usually called ionic potential or charge density.
Application of the concept of polarisation :
(a) To determine covalent and ionic character of molecule : -
From left (larger size) to right (smaller size) in a period f increases so covalent character increases.
-charge increases
- size decreases
- Covalent character increases with particular anion
From top to bottom in a group decreases so covalent, character decreases.