Question

The hardest substance amongst the following is?
a.) $B{{e}_{2}}C$
b.) Titanium
c.) ${{B}_{4}}C$
d.) Graphite

Answer 1

Hint: Diamond is one of the hardest materials known to date. Diamond has both high thermal conductivity and electrically insulating properties. Other than the Diamond, Carbides of some elements are hardest in nature.
Complete answer:
In Hardest substance the elements should have a strong covalent bond.
If the electronegativity difference of the atoms present in a substance is too low. Then the substance is called the hardest substance.
Coming to the given options. Option A, $B{{e}_{2}}C$. It is called beryllium carbide. In this compound the electronegativity difference is not low. Because beryllium belongs to s-block and carbon belongs to p-block. So, this is a wrong option.
Coming to option B, Titanium. It is the hardest substance but compared to diamond it is less hard.
Coming to option D, Graphite. Graphite is not harder because the hybridization of carbon in graphite is $s{{p}^{2}}$. So, Option D is wrong.
Coming to option C, ${{B}_{4}}C$. Both boron and carbon belong to the same group and electronegativity difference is also too low.
${{B}_{4}}C$is the one of the hardest materials known, ranking third behind diamond and cubic boron nitride.
So, the correct option is C.
Note: Don’t be confused with the hard property of diamond and ${{B}_{4}}C$. Both substances are hard in nature.
But Diamond is purely made up of carbon and ${{B}_{4}}C$is made up of boron and carbon. So, the diamond is harder in nature when compared to ${{B}_{4}}C$.
Covalent bond strength in diamond is high compared to ${{B}_{4}}C$.