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.

• A. Tungsten is much harder than lithium or sodium because it has a higher atomic number. 
• B. Tungsten is much harder than lithium or sodium because it has a higher melting point. 
• C. Tungsten is much harder than lithium or sodium because it has more unpaired electrons in the d orbital. 
• D. Tungsten is much harder than lithium or sodium because it has no unpaired electrons in the d orbital.

Answer: (C)