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How to calculate formal charge

How to calculate formal charge

 

Not all atoms within a neutral molecule need be neutral. An atom can have the following charges: positive, negative, or neutral, depending on the electron distribution. This is often useful for understanding or predicting reactivity. Identifying formal charges helps you keep track of the electrons.

The formal charge is the charge on the atom in the molecule. The term “formal” means that this charge is not necessarily on the presented atom because in some cases, it is also prevalent on other atoms present in the molecule. It is actually spread out through the other atoms and is not only on the one atom. Identifying a formal charge involves:

  1. Determining the appropriate number of valence electrons for an atom – This can be accomplished by inspecting the periodic table. The group number indicates the appropriate number of valence electrons for each atom.
  2. Determining whether the atom exhibits the appropriate number of electrons – In the Lewis structure, determine whether some of the atoms show an unexpected number of electrons.

The formal charge on an atom can be calculated using the following mathematical equation.

 

 

Lewis structures also show how atoms in the molecule are bonded. They can be drawn as lines (bonds) or dots (electrons). One line corresponds to two electrons. The nonbonding electrons, on the other hand, are the unshared electrons and these are shown as dots. One dot is equal to one nonbonding electron. The valence electrons are the electrons in the outermost shell of the atom.

 

 

Examples:

 

CH4, methane

A number of bonding electrons: 2 for H, 8 for C

A number of non-bonding electrons: 0 for both H and C

[Formal charge]H = 1 – (1/2) × 2 – 0 = ⇒ This applies to each hydrogen. These hydrogens are all zero.

 [Formal charge]c = 4 – (1/2) × 8 – 0 = 0

⇒ This molecule is neutral.

 

 

CH3+, methyl cation

 A number of bonding electrons: 2 for H, 6 for C

A number of non-bonding electrons: 0 for both H and C

[Formal charge]H = 1 – (1/2) × 2 – 0 = ⇒ This applies to each hydrogen. These hydrogens are all zero.

[Formal charge]c = 4 – (1/2) × 6 – 0 = 4 – 3 – 0 = +1 ⇒ This is a cation.

 

 

CH3‾, methyl cation

 A number of bonding electrons: 2 for H, 6 for C

A number of non-bonding electrons: 0 for H, 2 for C

[Formal charge]H = 1 – (1/2) × 2 – 0 = ⇒ This applies to each hydrogen. These hydrogens are all zero.

[Formal charge]c = 4 – (1/2) × 6 – 2 = 4 – 3 – 2 = -1 ⇒ This is a anion.