Alkyl groups increase basicity because: 1) They push electron density onto the nitrogen atom, which makes the nitrogen lone pair more attractive to hydrogen ions. 2) They stabilise the positive charge on the nitrogen after the ion is formed.Aromatic groups decrease basic properties because the delocalisation of the benzene ring withdraws electron density from the nitrogen atom, which makes the nitrogen lone pair less available for protons. Therefore, primary aliphatic amines are more basic than ammonia, while primary aromatic amines are less basic than ammonia.