Formal charge is the charge assigned to an atom in a molecules.

Or

Formal charge is the charge in which positive charge and negative charge which must be assigned certain atoms within the molecules and completely described with the help of Lewis structure of molecules.

for example,

CH4 (mathane)

lets draw the Lewis structure of CH4

According to Lewis structure of CH4,

• According to periodic table, carbon have 4 valence electron.

• All this bonding of carbon is bonding electron, so there is no any one non bonding electrons.

• carbon have 8 bonding electrons.

Formal charge = valence electron – non bonding electrons – bonding electrons /2.

Formal charge = 4 – 0 – 8/2

= 0

so, the formal charge on carbon atom is zero.

Formal charge equation

This equation is written on the basis of formal charge. Such as,

FC = V – N – B/2

where,

FC – Formal charge

V – Valence electron of an atom

N – Non bonding electrons

B – Bonding electrons

Let’s know with examples

SO2 (sulfur dioxide)

According to Lewis structure of SO2,

If we want to find the value of formal charge of that molecules with the help of formal charge equation.

We can find formal charge of carbon atom and oxygen with the help of Lewis structure.

Formal charge equation is,

FC = V – N – B/2

where,

FC – formal charge,

V – valence electron

N and B – Non bonding electrons and Bonding electrons

Now, we can find formal charge of that molecules (SO2).

first, we will find formal charge of sulfur (S).

• sulfur have 6 valence electron

• it has 4 non bonding electrons

• it has 4 bonding electrons /2

so, formal charge = 6 – 2 – 8/2

= 0

Now find, formal charge of that atoms,

(O)

formal charge 6 – 4 – 4/2 = 0

(O)

formal charge 6 – 4 – 4/2 = 0

Formal charge rules

The best way for calculating formal charge, when we talk the number of valence electrons of a neutral atom, substract the number of bond connected to that atom in the Lewis structure.

Another way, if you want to calculating formal charge of a resonance structure, the difference between the atoms number of valence electrons and the number it owns is the formal charge.

The formal charge of a atom in a molecules is the charge that would reside on the atom if all of the bonding electrons where shared equally.

Another best way (another rule) to for calculating formal charge, 1. In molecular structure, all the formal charge is zero to one in which some formal charge is not zero. 2. If the Lewis structure have nonzero formal charge, the arrangement with the smallest non zero formal charge is preferable.

Resonance structure rules

Resonance structure are not actual structures. the best examples is, – (benzene).

All the resonance structure must be bonafied Lewis structure.

In writing the resonance structure we only allowed to more electrons.

All canonical forms must have same number of Un-paired electrons.

All atoms involved in resonance lie in same plane.

The hybrid structure is always more stable then its canonical forms.

All the resonating structure do not contribute equally to the actually molecules.

Question And Answer

1. The formal charge of hexacyanaoferrate is?

The basic or general formula of hexacyanaoferrate is [Fe (CN)6]3- and You know that, it is an anion. and the formal charge of hexacyanaoferrate is -3.

2. Calculate the formal charge of NH3?

first, i m going to draw Lewis structure of nh3, then we find formal charge for this atoms.

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