Which of the following pairs of atoms are most likely to form covalent bonds?

Question

which of the following pairs of atoms are most likely to form covalent bonds?

A. oxygen and nitrogen

B. oxygen and magnesium

C. oxygen and krypton

Answer 1

A. Two nonmetals; two elements of similar, high electronegativity (3.5 vs. 3.0).

Answer 2

A. oxygen and nitrogen.  This is because both are gases, (which is where covalent bonds happen), and neither have 8 valance electrons.  

Oxygen (gas) and magnesium (a metal) would form an ionic bond.  Oxygen and krypton are both gases, but since krypton is a noble gas, it has 8 valence electrons and is already stable, so wouldn't need to bond with oxygen.