Polarity and forces

A bond between two different elements is polar: the more electronegative atom takes a bigger share of the electrons. C–H is barely polar (2.55 vs 2.20). C–O and C–N are noticeably polar. C=O is strongly polar, and that polarity is the reason carbonyl chemistry exists — the carbon is left electron-poor and open to attack.

Between molecules, three important interactions are listed below. Their relative importance depends on molecular size, shape and polarity, so this is not a universal strength ranking:

1. London dispersion — everything has it; grows with size and surface contact.
2. Dipole–dipole — needs a permanent molecular dipole; polar bonds can cancel by symmetry, as in CO₂.
3. Hydrogen bonding — the strongest common introductory examples have H attached to N, O or F and an appropriate acceptor.

Ethanol and dimethyl ether have the same formula, C₂H₆O. Ethanol boils at 78 °C; the ether at −24 °C. Same atoms, same mass — but only ethanol can hydrogen bond to its neighbours.