Question

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

Answer 1

Metals contain metallic bonds and more the strength of metallic bonds harder will be the metal. The actual bonding in the metallic bonds are the electrostatic forces between positive ions (cations) and free/delocalized electrons. So, if the metal can donate more valence electrons, the metallic bonds will be stronger and vice-versa.

Complete step by step answer:
Let us first discuss what makes a metal hard or soft.
Metals contain metallic bonds and more the strength of metallic bonds harder will be the metal. The actual bonding in the metallic bonds are the electrostatic forces between the positive ions (cations) and the free/delocalized electrons. So, if the metal can donate more valence electrons, the metallic bonds will be stronger and vice-versa.
Sodium metal has only one valence electron which it can donate for the metallic bonding. As there is only one electron for each positive cation, the strength of metallic bonds is not so stronger as compared to the other metals. Therefore, sodium metal is soft.
Hence option A is correct.

Note:
Sodium metal is an alkali metal which is as soft that we can cut it into parts with a knife. It has a bright silvery luster. Although this is a soft metal but it is highly reactive in nature. It even catches fire when kept in air i.e. reacted with oxygen. When it is heated it burns with its characteristic orange coloured flame. But sodium metal has several important applications such as it is a component of salt which is produced millions of tons every year.