Introduction:
When drawing Lewis structures, it is often possible to have multiple valid arrangements of atoms and electrons. These different arrangements are called resonance structures. However, not all resonance structures are equal in terms of stability and energy. Formal charges are used to select the lowest energy structure from a number of possible Lewis structures.

Explanation:
1. Definition of Formal Charge:
Formal charge is a concept used to determine the distribution of electrons in a molecule or ion. It helps to evaluate the stability and energy of different resonance structures. Formal charge is calculated by subtracting the number of lone pair electrons and half the number of bonding electrons from the valence electrons of an atom.

2. Purpose of Formal Charges:
The main purpose of formal charges is to identify the most stable resonance structure for a molecule or ion. The lowest energy structure is the one with the least formal charges or the structure with formal charges closest to zero.

3. Evaluating Resonance Structures:
When there are multiple resonance structures, the one with the lowest energy is preferred. Formal charges help in evaluating the stability of these structures and selecting the most favorable one. Here's how formal charges aid in this selection process:

3.1. Rule of Minimizing Formal Charges:
The structure with the fewest formal charges is typically the lowest energy structure. This is because minimizing formal charges helps distribute the electrons more evenly and reduces electron repulsion.

3.2. Rule of Closest to Zero Formal Charges:
If there are multiple structures with the same number of formal charges, the one with formal charges closest to zero is preferred. This is because atoms strive to achieve a neutral or near-neutral formal charge, which enhances stability.

4. Examples:
Let's consider the example of the nitrate ion (NO3-). There are three possible resonance structures for this ion:

- Structure 1: Nitrogen with a single bond to one oxygen and a double bond to another oxygen, and a single bond to the third oxygen.
- Structure 2: Nitrogen with a double bond to one oxygen and a single bond to another oxygen, and a single bond to the third oxygen.
- Structure 3: Nitrogen with a single bond to one oxygen and a double bond to both other oxygen atoms.

4.1. Calculating Formal Charges:
By calculating the formal charges, we can compare the stability of these structures.

In Structure 1:
Formal charge of Nitrogen = 5 - 0.5(6) - 6 = 0
Formal charge of Oxygen (single bond) = 6 - 0.5(2) - 2 = 0
Formal charge of Oxygen (double bond) = 6 - 2(2) - 6 = -1

In Structure 2:
Formal charge of Nitrogen = 5 - 0.5(4) - 6 = +1
Formal charge of Oxygen (double bond) = 6 - 2(2) - 6 = -1
Formal charge of Oxygen (single bond) = 6 - 0.5(2) - 2 = 0

In Structure 3:
Formal charge of Nitrogen = 5 - 0.5(2) - 6 = -1