In general, boiling points rise with increasing molecular weight, both because the
additional mass requires higher temperature for rapid movement of the molecules and
because the larger number of electrons in the heavier molecules provides larger London
forces. The difference in temperature between the actual boiling point of water and the
extrapolation of the line connecting the boiling points of the heavier analogous compounds
is almost 200" C. Ammonia and hydrogen fluoride have similar but smaller differences
from the extrapolated values for their families. Water has a much larger effect,
because each molecule can have as many as four hydrogen bonds (two through the lone
pairs and two through the hydrogen atoms). Hydrogen fluoride can average no more
than two, because HF has only one H available.