How to Calculate the Formal Charge of CoCl2

Written by carla boulianne
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How to Calculate the Formal Charge of CoCl2
(molecules of the chemical formula image by Oleg Verbitsky from Fotolia.com)

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.

Skill level:
Moderately Easy

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Things you need

  • Periodic table of elements

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Instructions

    Valence Electron Number

  1. 1

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

  2. 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. 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. 4

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

    Lewis Structure

  1. 1

    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.

  2. 2

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

    How to Calculate the Formal Charge of CoCl2
  3. 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.

    Formal Charge of Each Atom

  1. 1

    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.

  2. 2

    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

    How to Calculate the Formal Charge of CoCl2
  3. 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.

Tips and warnings

  • Calculating the formal charge for molecules containing transition metals can be tricky. The number of valence electrons for transition metals will be those outside the noble gas-like core.

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