Question

Which of the following metals is soft?
A. $Na$
B. $Pb$
C. $Al$
D. $Cu$

Answer 1

The d-block metals are usually hard and have much higher density than other metals as they have very strong intermolecular forces or cohesive forces between them as there are unpaired electrons present to overlap and form strong bonds. The –block elements are usually soft and are less dense compared to the d block elements in the same period.

Complete step by step answer:
Sodium is the softest metal among all the options given. It can be easily cut through a knife. In some metals the force which is needed to make the layers’ slide over each other is small. This explains why some metals are soft. Sodium has larger atomic size because of less force of attraction between the valence electron and the nucleus and lesser are the cohesive forces, hence it is soft. For example, Sodium being an alkali metal is very soft and can cut through a knife while iron which is a transition metal which is very hard and requires great energy to break apart. Iron is hard, tough and strong compared to sodium because of the strong metallic atom-atom bonding.
So, the correct answer is “Option A”.

Note: The metallic bonding decreases with increase in atomic size and thus, the tendency to show metallic bonding among alkali metals decreases from $Li\,to\,Cs$ and thus, close packing of atoms in crystal lattice decreases from $Li\,to\,Cs$ resulting in an increase in softness. Potassium is softer than Sodium, Rubidium being softer than Potassium, and this goes on. The increasing atomic radius means weaker forces between the atoms and so a lower melting and boiling point.