How to Calculate the Formal Charge of CoCl2

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By Carla Boulianne
eHow Contributing Writer

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Calculating formal charge is easy with a periodic table

When determining the formal charge of a molecule such as CoCl2 (phosgene gas), you need to know the number of valence electrons for each atom and the Lewis structure of the molecule.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

Periodic Table of Elements

Determine the Valence Electron Number of Each Atom in CoCl2

Step 1

Look up each atom in a Periodic Table of Elements to determine the number of valence electrons.
Step 2

Remember that 2 electrons go in the first s shell, 2 electrons in the second s shell, 6 electrons in the first p shell, etc.

As a reminder:
1s(^2)2s(^2)3p(^6)3s(^2)3p(^6)
Step 3

Adjust for charge. If the molecule is an ion, add or subtract one or more electrons overall to account for the final charge.
Step 4

For COCl2 (Phosgene gas):

C = 4; O =6; Cl = 7

The molecule has is not ionized and has a neutral charge. Therefore, the total amount of valence electrons is 4 + 6 + (7x2) = 24

Draw the Lewis Structure for CoCl2

Step 1

The Lewis Structure represents the most stable and probable structure for a molecule. Atoms are drawn with paired valence electrons; bonds are formed between lone electrons to satisfy the octet rule.
Step 2

Lewis Structure for CoCl2

See the diagram for the Lewis Structure of CoCl2 (phosgene gas). First draw each atom and its valence electrons, then form bonds as needed.
Step 3

The chloride atoms share single bonds with the carbon molecule, while the oxygen atom forms a double bond with carbon. Each atom in the final structure satisfies the octet rule and has eight valence electrons allowing for molecular stability.

Calculate the Formal Charge of Each Atom in CoCl2

Step 1

Count the lone pairs of each atom in the Lewis structure. Assign each atom one electron from each bond it participates in. Add these numbers together.

In CoCl2:

C = zero lone pairs plus 4 electrons from bonds = 4 electrons
o = 4 electrons from lone pairs plus 2 electrons from bonds = 6 electrons
Cl = 6 electrons from lone pairs plus one electron from a bond with C = 7 electrons
Step 2

Formal Charge of Atoms in CoCl2

Subtract the sum from the number of valence electrons in the unbonded atom. The result is the formal charge for that atom.

In CoCl2:

C = 4 valence electrons (v.e.) in unbonded atom minus 4 assigned electrons in Lewis Structure (L.s.) = 0 formal charge
o = 6 v.e. - 6 L.s. = 0 formal charge
Cl = 7 v.e. - 7 L.s. = 0 formal charge
Step 3

Write these charges next to the atoms in the Lewis structure. If the overall molecule has a charge, enclose the Lewis Structure in brackets with the charge written outside the brackets in the upper right corner.