Question

why is iron(Fe) harder than sodium(Na) even when sodium is more reactive?

Answer 1

Iron as a material is hard because of the arrangement of the individual atoms. The iron atoms are dense and compact and the metallic bonds are extremely strong. I dont know if this is the answer you are looking for.

Answer 2

If you were referring to hard/soft acids then it depends on the oxidation state and ionization energy and not necessarily reactivity. Fe is softer than Na as an acid because of sodiums smaller radius and high ionization energy.