Further Lewis structures

In Level 3 Lewis structures of compounds will have expanded octets, due to atoms being in the third period. There are a number of methods to ensure the Lewis structure is correct:

1.Using formal charge.

2.Drawing the ground state of the surrounding atoms and central atom (valence shell only), then promoting electrons from the ground state to ensure it can bond with the outer atoms.

At Level 3 Formal charge is not used but it is a relatively straightforward method to arrive at the correct Lewis structure.

1. Formal Charge

In reality it is difficult to state where electrons actually are in relation to the atoms in a molecule. Formal charge is the charge that a molecule, and atoms have, if all the electrons are shared equally. When drawing a Lewis structure formal charge can be used to deduce the most probable structure, where the charge separation across the molecule is as low as possible. In the case of a polyatomic ion the only charge from the different formal charges around each atom should be the same as the ion's charge.

Formal charge is easy to calculate using the equation below:

2. Using ground state electron configurations and promotion of electrons

Atoms in the third period and above of the periodic table can hold more than 8 electrons in the valence shell; they have expanded octets. This is because they have d sub orbitals which can hold electrons; electrons can be promoted to the d orbitals if they are needed for bonding (very simplistic) in an expanded octet.

The same method can be used for polyatomic ions. Do not forget to add or subtract an electron / s depending on the ion's charge. Below are a few examples, showing SO₃^2- , PCl₆^- PO₄^3- and ClO₄^-