Dihydrogen phosphate (H2PO4?) is a polyatomic ion with 1- charge. It acts as a weak acid, forms common salts like sodium dihydrogen phosphate and potassium dihydrogen phosphate, and serves as a vital pH buffer in biological and food systems. The picture below is its lewis structure.

Step 1: Calculate total valence electrons

P: Group VA, 5 valence electrons

O: 6 × 4 = 24

H: 1 × 2 = 2

Ion carries a -1 charge; add 1 extra electron

Total valence electrons: 5 + 24 + 2 + 1 = 32 e?

Step 2: Skeleton and central atom

P has lower electronegativity and serves as the central atom;

Bonding skeleton: Central P bonded to 4 O atoms; two O atoms are each bonded to an H (hydroxyl O), and the other two are terminal O atoms; resonance exists.

Preferred bonding: One P=O double bond, three P–O single bonds (two P–OH hydroxyl bonds, one P–O? anionic terminal bond).

Step 3: Tally all σ bonds

σ bond list:

P bonded to 4 O atoms: 4 P–O σ bonds; 2 O–H σ bonds; total of 6 σ bonds

σ bonding electrons: 6 × 2 = 12 e?

Remaining electrons: 32 - 12 = 20 e?

Step 4: Introduce P=O double bond to optimize formal charges

Add a π bond to one terminal O to form a P=O double bond;

Total bonding electrons = 12 + 2 = 14 e?

Remaining electrons: 32 - 14 = 18 e?

Step 5: Assign lone pairs to all oxygen atoms

Double-bonded terminal O (P=O): 4 bonding electrons, assign 2 lone pairs (4 e?)

Two hydroxyl O atoms (P–O–H): 4 bonding electrons each (P–O + O–H), assign 2 lone pairs to each, total 2 × 4 = 8 e?

Anionic terminal O (P–O?): 2 bonding electrons, assign 3 lone pairs (6 e?)

Total lone pair electrons: 4 + 8 + 6 = 18 e?; all electrons accounted for.