Tungsten, used in the aerospace and automotive industries, is much harder than lithium or sodium. Provide an explanation as to why this is the case.

Tungsten is much harder than lithium or sodium because it has a higher atomic number. 

Tungsten is much harder than lithium or sodium because it has a higher melting point. 

Tungsten is much harder than lithium or sodium because it has more unpaired electrons in the d orbital. 

Tungsten is much harder than lithium or sodium because it has no unpaired electrons in the d orbital.