Order of primary secondary tertery

Basicity of amines due to steric hindrence

Dear Student,
When comparing the basicity of aliphatic amines, we would anticipate tertiary amines to have the highest basicity, followed by secondary amine, and finally primary amine.

But this is not so. The order of basicity is
NH3< primary amine ~ tertiary amine < secondary amine
The following explanations explain why this "unexpected" sequence of basicity exists:
Steric hindrance
An alkyl group has a larger size than a hydrogen atom. As a result, an alkyl group would prevent a hydrogen atom from attacking the molecule, lowering its basicity. As a result, the higher the number of alkyl groups connected, the lower the basicity.

So, the decreasing order of basicity will be:

Primary amine > secondary amine > tertiary amine

Thank you