Impact : very large.... ΔpKa 60 units (i.e. full range) e.g. consider CH₄ (pKa = 50) vs HI (pKa = -9)

The first thing to look for when considering the acidity of an H atom is the nature of atom to which the H are attached since this is the single most significant factor.

ACIDITY : Look for factors that stabilise the conjugate base...we do this by simply removing the H+:

1. The atoms are in the same row of the periodic table: e.g. CH₄, NH₃, H₂O and HF

The acidities (pKa values) are CH₄ (50), NH₃ (35), H₂O (14) and HF (3). The conjugate bases are CH3-, NH2-, HO- and F-

2. The atoms are in the same group (column) of the periodic table e.g. HF, HCl, HBr and HI