Acid Strength and Molecular Structure
The structural factors that determine how readily a molecule releases a proton.
Unit
Dimensionless
In depth
Acidity increases with the electronegativity of the atom bearing the hydrogen across a period, and with the size of that atom down a group, because a larger anion delocalises charge better. Electron-withdrawing groups and resonance stabilisation of the conjugate base both increase strength markedly.
Examples in the real world
HI is stronger than HF; trichloroethanoic acid (pKa 0.7) far exceeds ethanoic acid (pKa 4.76).