To draw the Lewis structure for phosphoric acid (H₃PO₄), we start by determining the total number of valence electrons available for bonding. For H₃PO₄, we have:
- Hydrogen (H): 3 atoms × 1 electron = 3 electrons
- Phosphorus (P): 1 atom × 5 electrons = 5 electrons
- Oxygen (O): 4 atoms × 6 electrons = 24 electrons
Adding these together gives us a total of:
3 + 5 + 24 = 32 valence electrons.
Next, we will use these electrons to construct the structure:
- Place phosphorus (P) at the center, as it is the central atom. Then, connect three hydrogens (H) to phosphorus using single bonds.
- Connect the oxygen atoms (O) around phosphorus. Two of the oxygen atoms will be double-bonded (O=P), while the other oxygen will be bonded through a single bond (O-P) and share a hydrogen.
The initial drawing looks as follows:
This gives us an initial structure, which can be represented as:
H H O : H-O-P=O :O H
To ensure the octet rule is satisfied, we verify each atom:
- Hydrogen (H): Each has 2 electrons, satisfying its duet rule.
- Phosphorus (P): Has 8 electrons (3 from H and 5 for O), satisfying the octet rule.
- Oxygen (O): Each double bonded oxygen has 8, and the single bonded with a hydrogen also has 8 electrons from the bond and lone pairs, satisfying the octet rule.
Now let’s calculate formal charges for all atoms:
- For Hydrogen (H): 0 (1 valence – 0 non-bonding – 0.5 from bond) = 0
- For Phosphorus (P): 5 (5 valence – 0 non-bonding – 4 from bonds) = +1
- For Oxygen (double bond): 6 (6 valence – 0 non-bonding – 4 from bonds) = 0
- For Oxygen (single bond with hydrogen): 6 (6 valence – 2 non-bonding – 2 from bonds) = 0
After reviewing the structure, we find:
- H: 0
- P: +1
- O: 0 and 0
This results in a total formal charge of +1 for the phosphoric acid molecule, indicating that while we have satisfied the octet rule, we also maintained transparency with formal charges. Hence, the Lewis structure for H₃PO₄ is displayed correctly, demonstrating how each atom conforms to the octet rule adequately.