Define formal charge.

Formal charge is a concept in chemistry that helps to predict the distribution of electrons in molecules. It is calculated for an atom in a molecule as follows:

1. Start by determining the number of valence electrons in the free atom. This is typically equal to the group number of the atom in the periodic table.

2. Next, calculate the number of valence electrons in the bonded atom. This is done by counting the number of non-bonding (lone pair) electrons and adding half the number of bonding electrons.

3. Subtract the number of valence electrons in the bonded atom from the number of valence electrons in the free atom. The result is the formal charge.

The formal charge is used to determine the most likely Lewis structure for a molecule. The structure with the smallest formal charges on the atoms is usually the most stable and therefore the most likely.