Basic Strength Order (Group 15)

$$\text{NH}_3 > \text{PH}_3 > \text{AsH}_3$$

Concept: Lewis basic strength decreases down the group because nitrogen’s small atomic radius concentrates its lone pair into a small, dense region of space, whereas larger central atoms dilute the lone pair into more diffuse valence orbitals.

Notation:

• $$\text{NH}_3$$ = Strongest Lewis base in Group 15;
• $$\text{AsH}_3$$ = Weak Lewis base due to low electron density.