How to Calculate the Formal Charge of CoCl2

Valence Electron Number

• 1

  Look up each atom in the periodic table of elements to determine the number of valence electrons.

• 2

  Remember that two electrons go in the first s shell, two electrons in the second s shell, six electrons in the first p shell, etc. As a reminder: 1s(^2)2s(^2)2p(^6)3s(^2)3p(^6)

• 3

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

• 4

  For CoCl2 (Phosgene gas): C = 4; O = 6; Cl = 7. The molecule is not ionized and has a neutral charge. Therefore, the total amount of valence electrons is 4 + 6 + (7x2) = 24.

Lewis Structure

• 5

  See the diagram for the Lewis structure of CoCl2 (phosgene gas). 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.

• 6

  Draw each atom and its valence electrons, then form bonds as needed.

• 7

  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.

Formal Charge of Each Atom

• 8

  Count the lone pairs of each atom in the Lewis structure. Assign each atom one electron from each bond in which it participates. Add these numbers together. In CoCl2:
  C = 0 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 1 electron from a bond with C = 7 electrons.

• 9

  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

• 10

  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.