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

Taking into account the definition of covalent bond, the correct answer is option D. N and C.

A covalent bond is a force that joins two atoms of non-metallic elements to form a molecule.

That is, the covalent bond occurs when two atoms of non-metallic elements combine with each other, since they need to gain electrons to reach the noble gas configuration and the only way to do it is by sharing electrons.

Then  the covalent bond occurs between non-metallic atoms that share electrons, which are attracted to the nuclei of the two atoms.  Atoms share pairs of electrons from their most superficial shell (called the valence shell) to achieve the stability of the molecule that has been formed with the bond, thus complying with the octet rule.

So, in this case, the correct answer is option D. N and C.  Both elements are nonmetals, so they will share their valence shell electrons to form a molecule.

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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.