Electronegativity Definition

Second ionization enthalpy of carbon atom is less than that of boron atom.

Increasing order of ionic radii: $$\mathrm{Al^{3+}} < \mathrm{Mg^{2+}} < \mathrm{Na^{+}}$$

Under identical conditions, in solid state, the density of potassium metal is more than density of sodium metal.

The H$$-$$H bond is weaker than F$$-$$F bond.