what makes sodium and potassium soft metal?

Hi,

 

Hardness of any metal is determined by the cohesive forces acting between the atoms. The cohesive force between atoms of sodium and potassium is very less. The cohesive energy is the force holding the atoms together in a metal. It depends upon the electron participating in bonding. The bonding in potassium and sodium is weak and electrons are not located over a single atom but over the number of atoms. The weak bonding results in softness of metal.

 

Hope it helps.

Cheers!