Formal Charge And Resonance - Chemistry Worksheet printable pdf download

Chemistry Resource

NAME: _____________________________

Block: _________

Formal Charge and Resonance

Formal Charge

Formal charge is a means of identifying the “best” Lewis dot structure when more than one

valid dot structure can be drawn for a molecule or molecular ion. The formal charge can be

assigned to every atom in a electron dot structure. The formal charge assigned for a particular

atom is calculated from the number of valence electrons the atom would have and the number of

electrons around the atom when it is bonded in the molecule. The calculate the formal charge of

an atom, start with the number of valence electrons, N

then subtract the number of unshared

v.e,

, and half of the bonding electrons, ½ N

electrons, N

us.e

b.e

– ½ N

Formal charge = N

– N

v.e.

us.e.

b.e.

The formal charge is a hypothetical charge assigned from the dot structure. The formal charges

in a structure tell us the “quality” of the dot structure. Some practice of assigning formal charge

is necessary before you master this technique.

If you can draw multiple valid Lewis dot structures for a particular molecule or ion dot

structures, then together they form a set of possible resonance structures. So, how does one

choose which structure is the most reasonable choice – that is to say, which is the most stable?

Another tool for your toolkit is called formal charge. Formal charge is a useful way to compare

the stability for a set of valid Lewis dot structures. After calculating the formal charge on each

atom for each of the structures in the set, the most stable structure(s) are determined by

application of the formal charge rules

Structures with the lowest magnitude of formal charges are more stable.

•

More electronegative atoms should have negative formal charges.

•

Adjacent atoms should have opposite formal charges (or zero formal charge).

•

Example: Draw Lewis dot structure for SO

Write out the atoms with the correct number of valence electrons:

. .

: O :

: S :

: O :

Arrange the atoms to make a skeletal structure, then complete the Lewis dot structure –

verify that the structure satisfies the octet rule:

. .

: O : : S : : O : ← octet rule not satisfied

0 formal charge

Adjust bonding electrons so that octet rules apply to all the atoms:

. . . .

. .

← octet rule satisfied

: O : S : : O :

' '

–1 +1 0

formal charge

Formal Charge And Resonance - Chemistry Worksheet

Related Articles

Related forms

Related Categories