Which of these substances contains a nonpolar covalent bond?

• A. NaCl
• B. O2
• C. HBr
• D. NH3

Answer: (B)

When two atoms have very similar electronegativities, they share electrons almost equally, forming a nonpolar covalent bond. The oxygen molecule is made of two identical oxygen atoms, so the electrons are shared equally and there’s no net separation of charge. The molecule has no dipole moment, which is the hallmark of a nonpolar covalent bond.

In contrast, sodium chloride is an ionic compound—electrons are transferred from sodium to chlorine, creating charged ions rather than a covalent bond. Hydrogen bromide has a noticeable difference in electronegativity between hydrogen and bromine, giving a polar covalent bond with partial charges. Ammonia has covalent bonds that are also polar, and its shape leads to a net dipole moment.

So the substance with a nonpolar covalent bond is the oxygen molecule.