Why does hardness increase with shorter distances between the metal and the oxygen?

a.

The metallic bonding is optimal when the distances between the metal and the oxygen are the smallest.

b.

The electrostatic forces are the strongest when the metal and oxygen are closest together, making the compound harder.

c.

The chemical repulsion is the greatest when the metal and oxygen are closest to each other, making the compound harder.

d.

The magnetic attraction is the strongest when the metal and oxygen are closest together, making the compound harder.

Answer: Option (A)

The metallic bonding is optimal when the distances between the metal and the oxygen are the smallest.

Explanation:

Hardness increases with respect to shorter distances between the metal and the oxygen compounds, the reason for the increase in hardness is due to metallic bonding (electrostatic force between electrons and positive ions) which is optimal between them.

As the distance decreases, hardness increases and for increasing distances hardness decreases. So when the molecular weight is large, it has smaller valence or less charge on the metal which makes the bond weaker and less hard.