Why is sodium light and iron hard in nature?

Sodium is an alkali metal and is very soft and can cut through knife while iron is a transition metal which is very hard and requires great energy to break apart.

Iron is hard, tough and strong compared with sodium because of the strong metallic atom-atom bonding.

The strong attractive force between the atoms is only weakened at high temperatures; hence iron has the high melting points and boiling points.

This strong bonding between atoms holding them together in iron gives it high density. For example: iron has a density of 

7.9 g/cm3 and sodium has a density of 0.97 g/cm3 (less than water hence floats on water)