# Which of these hydrides is the MOST polar: ammonia; methane; PH_3; AsH_3?

Nitrogen is much more electronegative than the lower Group atoms. The lone pair on nitrogen is smaller and more charge dense. The result is that ammonia is the most polar molecule. This is also reflected by the high boiling point of the molecule, $\text{b.p. } - 33.3$ ""^@C.