Question

According to hard and soft acid base principle, a hard acid:
(a)- has low charge density
(b)- shows preference for soft bases
(c)- shows preference for donor atoms of low electronegativity
(d)- is not polarizable

Answer 1

Hint: Hardness or softness is defined on the basis of electron clouds associated with a particular species. A tightly held electron – cloud having low polarizability makes the species “hard” while a readily polarizable electron cloud makes the species “soft”.

Complete step by step solution:
Let us see the differences between Hard and Soft acids:

Hard Acids	Soft Acids
Low electronegativity (in the range of 0.7-1.6)	Intermediate to high electronegativity ( in the range of 1.9-2.5)
Relatively small size	Large size
Relatively high charge (> 3+)	Low charge (1+, 2+)
Absence of any outer electrons	Presence of several excitable valence shell electrons.
Not easily polarizable	Easily polarizable
Prefer to coordinate with hard bases	Prefer to coordinate with soft bases
Examples: \[N{{a}^{+}},M{{g}^{+}},F{{e}^{3+}},A{{l}^{3+}}\]	Examples: \[C{{u}^{+}},H{{g}^{2+}},A{{u}^{+}},A{{g}^{+}},P{{b}^{+}}\]

Now, let us look into soft bases and hard bases:

Hard Bases	Soft Bases
Very high electronegativity (in the range 3.4-4)	Intermediate to high electronegativity (2.1-3.0)
Relatively small size	Large size
Presence of filled orbits; empty orbitals may exist at high energy levels.	Partially filled orbitals, empty orbitals are low- lying.
Low polarizability.	High polarizability.
prefer to coordinate with hard acids.	Prefer to coordinate with soft acids
Examples: \[{{O}^{2-}},{{F}^{-}},C{{O}_{3}}^{2-},P{{O}_{4}}^{3-}\]	Examples: \[{{S}^{2-}},{{I}^{-}},B{{r}^{-}}\]

So, the correct option is (d).

Note: Apart from the fundamental "hard" and "soft" categories, two additional categories are useful.
Borderline acids, which are in between hard and soft acids.
Borderline bases, which are in between hard and soft bases.