1.1 LEWIS STRUCTURE

1.1
LEWIS STRUCTURE

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LEWIS STRUCTURE

• Representation of covalent bonding in which shared electron
pairs are shown either in lines or as pairs of dots between two
atoms, and lone pairs are shown as pairs of dots on individual
atoms.

• Only valence electrons are shown.

•• • •• •
• •
HOH H O H

•• ••

• Illustrate the octet rule.
• Single bond = 2 atoms share a pair electron.
• Double bond = 2 atoms share two pairs of electrons
• Triple bond = 2 atoms share three pairs of electrons.

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WRITING LEWIS STRUCTURE Eg: NF3

1 Count the total no of VE of the molecule. Octet achieved
for all atoms.
2 Draw the skeletal structure of the compound

Complete the duplet (H) and octet for all
3

terminal atom.

4 Excess VE are placed on the central atom

5 If central metal atom do not achieved octet,
make double/triple bond with terminal atom.
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FORMAL CHARGE

• Electrical charge difference between the valence electrons in
an atom and the number of electrons assigned to that atom in
a Lewis structure.

• Molecules : sum of the formal charge must zero.

• Cations : sum of the formal charge must equal to positive
charge

• Anions : sum of the formal charge must equal to negative

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FORMAL CHARGE

Atom Ve e FC
N assigned

550

Atom e Atom e
F Ve assigned FC F Ve assigned FC
770 770

Atom e
F Ve assigned FC
770

Since NF3 is a molecule, so the sum of the formal charge must zero.

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LEWIS STRUCTURE & FORMAL CHARGE

• Select the most plausible Lewis structure by following this:

LESS PLAUSIBLE STRUCTURE PLAUSIBLE STRUCTURE

Molecule with FC Molecule without FC

Large FC Small FC
(+2, +3, -2, -3 and so on)
Negative FC are placed on the more
electronegative atoms.

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RESONANCE

• The use of two or more Lewis structure to represent a
particular molecule.

• The double head arrow indicates that the structures shown
are resonance structures.

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DETERMINE THE POLARITY

1) Draw the Lewis structure.
2) If there are no lone pairs on the central atom, and if all the

bonds to the central atom are the same, the molecule is
nonpolar.
3) If the central atom has at least one lone pair and if the
groups bonded to the central atom are not all identical, the
molecule is probably polar.

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DETERMINE THE POLARITY

Molecule Lewis structure Polarity
CH4 Non polar
AlF3 Non polar
H2O
NH3 polar
NO2F polar
polar

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