PO3- molecules are excellent sequestrants, emulsifiers, and water softeners, they are widely used in commercial, cosmetic, and food processing applications. The picture below is its lewis structure.

Step 1: Calculation of total valence electrons

P: Group VA, 5 valence electrons

O: 6 × 3 = 18

Ion carries a -1 charge; add 1 extra electron

Total valence electrons: 5 + 18 + 1 = 24

Step 2: Selection of central atom

P has lower electronegativity, so it serves as the central atom; skeleton: central P bonded to 3 O atoms.

Step 3: Construct the σ-bond skeleton

Draw three P–O σ single bonds

Bonding electrons: 3 × 2 = 6e?

Remaining electrons: 24 - 6 = 18e?

Step 4: Distribute lone pairs to the peripheral oxygen atoms first

If all bonds were P–O single bonds: each O would require 3 lone pairs (6e?) to complete its octet.

Total electrons in O lone pairs: 3 × 6 = 18e?

Electrons are fully utilized; central P has no lone pairs.

At this stage, all atoms satisfy the octet rule, but formal charges are high; a more stable resonance structure exists: one P=O double bond and two P–O? single bonds. Optimize for the stable resonance structure (standard Lewis model)

Bonding: one P=O double bond, two P–O single bonds

Bonding electrons: 4 + 2 + 2 = 8e?

Lone pair distribution

Double-bonded O: 2 lone pairs (4e?);

Two single-bonded O atoms: 3 lone pairs each (6e? each);

Total lone pair electrons: 4 + 6 + 6 = 16e?

Total electrons: 8 + 16 = 24; total count matches.

Central P has no lone pairs; P is in Period 3, with a total of 10 bonding electrons, allowing for an expanded octet.