Which of the following molecules is considered a polar molecule? (A) N2, (B) H2, (C) HF, or (D) CH4.
For a molecule to be considered polar, it needs to have a net dipole moment. There is a net dipole moment when dipoles act together when they don’t cancel each
other out. A dipole is formed when atoms in a molecule differ in their electronegativities. A molecule may have one or more dipoles depending on the bonded atoms.
Let’s now look at the answer choices given. In a nitrogen molecule N2, the two nitrogen atoms have the same
electronegativities. Hence, no dipoles are formed. No dipoles means that there will not be a net dipole moment. And no dipole moment indicates the molecule is nonpolar. Thus, answer choice (A), N2, is incorrect. The same is the case with a hydrogen molecule H2, which has no dipoles and hence no
dipole moment. Thus, answer choice (B), H2, is not correct either.
In a molecule of hydrogen fluoride, HF, the fluorine atom is more electronegative
than the hydrogen atom. And the pair will form a permanent dipole and exhibit a dipole moment. And since fluorine is the most electronegative element, the resulting dipole moment
is also stronger. This makes HF a polar molecule. Thus, it seems as though answer choice (C) is the answer to this question.
But to confirm, let’s look at answer choice (D), CH4. Atoms of carbon and hydrogen in a methane molecule, CH4, differ slightly in their
electronegativities. It is not considered to be a polar molecule. The difference in electronegativities is very small. So four very weak dipoles are created. Due to the differing directions of these weak dipoles, a net zero dipole moment is
created. Thus, methane is nonpolar. Therefore, we can be sure that answer choice (D), CH4, is not the answer to this
Hence, the molecule listed which is considered to be a polar molecule is (C), HF.