Drawing Lewis Structures and VSEPRDrawing Lewis Structures and VSEPR

• Draw basic Lewis dot structures of atoms and compounds.

• Using VSEPR, predict bond shape from electron arrangement in Lewis dot diagram.

Additional KEY Terms

OCTET RULE:• atoms will gain, lose or share electrons to fill their

valence shell (noble gas)

use only valence electrons in Lewis dot.

STEPS / RULES TO LEWIS DOT:

• Determine total valence electrons for all atoms. ~Watch for negative/positive/polyatomic ions.

• Position least electronegative atom in the centre.~central atom usually first in formula (unless H/ F).

CH2ClF, SeCl2, O3 (CO2, NH3, PO43-)

• Write other atoms around central with line bond.~line represents 2 e- (must be subtracted from total)

Se – ClCl –

• Place electrons around each atom to fill valence.

• Too few electrons - give each atom a complete octet by adding double or triple bonds.

SeCl2 : 6 +7 + 7 = 20 - 4

Se – ClCl –

Draw the Lewis structure for CO2:

CO2 : 4 + 6 + 6 = 16

C – OO –

- 4

– –

S – OO ––

O– O

Draw the Lewis structure for SO42- :

SO42- : 6 + (4)6 + 2 = 32 - 8

2-

BeCl2 : 2 + 7 + 7 = 16

– ClCl –

Be

- 4 Ionic Bond

-1

Cl

+2

Be

-1

Cl

VSEPRValence Shell Electron Pair Repulsion Theory

• Predicts 3D structures of molecules.

• Shape calculated by comparing bonding and lone

pair electrons around the central atom.

• Minimizes the electrostatic repulsion between electrons in valence sublevels.

AXE method:

A: Central atomXn: # of bonding pairsEn: # lone pairs

Draw the Lewis structure for BF3:BF3 : 3 + (3)7 = 24

- 6

– FF – B

F

–

*Unusual(Does not have octet)

3 – bonding pairs (X3)0 – lone pairs

Trigonal Planar– 120o bond angle

Central Atom with only bonding pairs:

Draw the Lewis structure for CH4:

Draw the Lewis structure for PCl5:

Draw the Lewis structure for SF6:

4 – bonding pairs (X4)0 – lone pairs

Tetrahedral – 109.5o

5 – bonding pairs (X5)0 – lone pairs

Trigonal Bipyramidal – 90o and 120o

6 – bonding pairs (X6)0 – lone pairs

Octahedral – 90o

Central Atom with LONE pairs:Lone pairs have a greater repulsion force and distort the predicted bond angles.

H2S : 1 + 1 + 6 = 8

– HH –

S

- 4

2 – bonding pairs (X2)2 – lone pairs (E2)

Bent – 104.5o bond angle

*Predict Linear

Draw the Lewis structure for PF3:

PF3 : 5 + (3)7 = 26

- 6

– FF – P

F

–

*Predict Trigonal Planar

3 – bonding pairs (X3)1 – lone pairs

Trigonal Pyramidal – 107o

Treat double/triple bonds as single bonds:

CO2 : 4 + 6 + 6 = 16 - 4

Double Bonds

2 – bonding pairs (X2)0 – lone pairs

Linear – 180o bond angle

C – OO –

– –

CAN YOU / HAVE YOU?

• Draw basic Lewis dot structures of atoms and compounds.

• Using VSEPR, predict bond shape from electron arrangement in Lewis dot diagram.

Additional KEY Terms