Which of the following pairs of elements can be joined by a covalent bond?

A. Na and C

B. Mg and C

C. Li and Cl

D. N and C

Your teacher is probably expecting D.

Explanation:

If you have learned that metal + nonmetal ⟶ ionic bond, the answer is D, because N and C are the only pair of nonmetals.

However, if you have learned that a bond is ionic if the electronegativity difference (ΔEN) between the two atoms is greater than 0.7, the answer is A, B, and D.

The Li-Cl bond is definitely ionic.

For Na and C, ΔEN = 1.62, so the Na-C bond is a highly polar covalent bond.

For Mg and C, ΔEN = 1.24, so the Mg-C bond is a polar covalent.