This problem is asking for the reason behind the nonexistence of elementary substances with ionic bonds. First of all, it is necessary to bear to mind that the definition of the type of bond in a compound depends on the difference electronegativity that is present between two of the atoms composing it.
For instance, in binary salts, such as KCl, NaCl, KBr and so on, we are able to know these are all ionic because they are composed by a metal and a nonmetal, which have the smallest and largest electronegativities, so that the difference electronegativity is large enough to produce an ionic bond.
In contrast, elementary susbtances, such as liquid bromine (Br2) and gases such as H2, N2, Cl2, F2 and O2, since they are all nonmetals and the same type, the electronegativity difference will be 0 and therefore, they will not be ionic but covalent.
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