How to find a formal charge. Calculate the number of valence electrons the atom should have, based on its position in the periodic table. Count the number of electrons the atom actually has in the molecule, including lone pairs and bonds.
Subtract the number of electrons the atom has in the molecule from the number of valence electrons it should have. The result is the formal charge.
What is a Formal charge?
The formal charge is a chemical concept used to assign a charge to each atom in a molecule or ion. It helps to determine the most stable arrangement of atoms and electrons in a molecule, which is important in understanding the chemical properties and reactivity of the molecule.
The formal charge is calculated by taking the difference between the number of valence electrons of an atom and the number of electrons it “owns” in a molecule. The electrons an atom owns are calculated as the sum of the number of lone pair electrons and half the number of electrons in covalent bonds the atom shares with other atoms.
The formula for the formal charge is:
Formal charge = valence electrons – (lone pair electrons + 1/2 bonding electrons)
The sum of the formal charges of all the atoms in a molecule or ion must add up to the total charge of the molecule or ion. For example, if the molecule is neutral, the sum of all formal charges must be zero. If the molecule is a charged ion, the sum of all formal charges must equal the charge on the ion.
The most stable Lewis structure of a molecule or ion is the one that has the smallest formal charges on each atom. The formal charge helps to determine the placement of lone pairs and the distribution of electrons among atoms, which is critical to understanding the stability and reactivity of the molecule.
An easy method to find out the formal charge
To find the formal charge of an atom in a molecule, you can use a simple formula:
Formal charge = valence electrons – (lone pair electrons + 1/2 x bonding electrons)
Here’s an easy step-by-step method to find the formal charge of an atom:
Determine the number of valence electrons the atom has. You can find this by looking at the periodic table and identifying the group number of the element. Count the number of lone pair electrons on the atom. These are electrons that are not involved in bonding and are associated with the atom.
Count the number of electrons the atom shares in covalent bonds. This includes both electrons the atom shares with other particles and the electrons in multiple bonds. Plug these values into the formula and subtract them to find the formal charge.
If the formal charge is zero, the atom has the correct number of electrons in its valence shell. If the formal charge is positive, the atom is missing electrons, and if it is negative, the atom has more electrons than it would have in a neutral state.
Remember, it is the sum of formal charges on all atoms in a molecule that should add up to the overall charge of the molecule, whether it’s zero for a neutral molecule or some number for an ion
Some Other Examples of formal charge
Here are some examples of formal charges for atoms in different molecules:
In the molecule, NH3 (ammonia), the nitrogen atom (N) has 5 valence electrons. It is bonded to three hydrogen atoms, each with 1 valence electron, and has one lone pair of electrons. The formal charge on nitrogen can be calculated as follows:
Formal charge on nitrogen = 5 – (0 + 6/2) = 0
Therefore, the formal charge of nitrogen in ammonia is zero.
In the molecule, CO2 (carbon dioxide), the carbon atom (C) has 4 valence electrons. It is double-bonded to two oxygen atoms, each with 6 valence electrons. The formal charge on carbon can be calculated as follows:
Formal charge on carbon = 4 – (0 + 8/2) = 0
Therefore, the formal charge on carbon in carbon dioxide is zero.
In the molecule, HNO2 (nitrous acid), the nitrogen atom (N) has 5 valence electrons. It is bonded to one hydrogen atom, with 1 valence electron, and one oxygen atom, with 6 valence electrons. It also has one lone pair of electrons. The formal charge on nitrogen can be calculated as follows:
Formal charge on nitrogen = 5 – (2 + 4/2) = 0
Therefore, the formal charge on nitrogen in nitrous acid is zero.
In the molecule, NH4+ (ammonium ion), the nitrogen atom (N) has 5 valence electrons. It is bonded to four hydrogen atoms, each with 1 valence electron. The formal charge on nitrogen can be calculated as follows:
Formal charge on nitrogen = 5 – (0 + 8/2) = +1
Therefore, the formal charge on nitrogen in ammonium ions is +1.
In the molecule, SO4^2- (sulfate ion), the sulfur atom (S) has 6 valence electrons. It is double-bonded to two oxygen atoms, each with 6 valence electrons, and single-bonded to two other oxygen atoms, each with 6 valence electrons. The formal charge on sulfur can be calculated as follows:
Formal charge on sulfur = 6 – (0 + 8/2) = +2
Therefore, the formal charge of sulfur in sulfate ion is +2.
Lewis’s structure of the formal charge
In a Lewis structure, a formal charge is assigned to each atom to help determine the most likely structure. The most stable Lewis structure will have the smallest possible formal charges on each atom.
For example,
in the molecule H2O, the Lewis structure shows that oxygen has two lone pairs of electrons and is single-bonded to each of the two hydrogen atoms. The formal charge on the oxygen atom can be calculated as follows:
Valence electrons of oxygen = 6
Lone pair electrons on oxygen = 2
Bonding electrons on oxygen = 4 (2 from the single bonds with hydrogen)
Formal charge = 6 – (2 + 4/2) = 0
Therefore, the formal charge on the oxygen atom in the H2O molecule is zero, which suggests that this is the most stable Lewis structure for the molecule.
Overall, the formal charge helps to determine the most stable Lewis structure by indicating how the electrons are distributed among the atoms in the molecule.
