Tug

swing the arm until the chalk arrow is gone.

every bond is a tug of war over its electron pair; swing a molecule's arms until the pulls cancel, and bet which of two molecules pulls harder, across ten rounds

In each bond the more electronegative atom pulls the shared electrons toward itself, so every bond has a direction, a dipole. A molecule's dipole moment is those pulls added as arrows. In a symmetric shape they cancel: CO2 is straight and BF3 is flat at 120°, so both are 0 D, and trans-1,2-dichloroethene is 0 while cis is 1.90 D. A bent molecule cannot cancel: SO2 is 1.63 D where straight CO2 is 0, and H2O (1.85) beats H2S (0.97) because O pulls far harder than S. Swap one Cl of CCl4 (0 D) for H and CHCl3 is 1.04 D. A lone pair pulls too: in NH3 it pulls the same way as the bonds (1.47 D), in NF3 against them (0.23 D). And size matters as well as pull, since moment is charge times distance: CH3Cl (1.89 D) edges CH3F (1.86 D) because C-Cl is so much longer.

Concepts: chemistry, symmetry, electricity.

An interactive you play with one thumb, on Wundr.