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 ions.

• Position least electronegative atom in the centre.~central atom usually first in formula

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 –

SeCl2 : 6 +7 + 7 = 20 - 4 = 16

Se – ClCl –

• double check to make sure each atom in the compound has 8 electrons around it (Hydrogen is an exception – it only needs 2)

• Place remaining e- around atoms to fill valence Fill surrounding atoms BEFORE central atom

Draw the Lewis structure for CO2:

CO2 : 4 + 6 + 6 = 16

C – OO –

- 4 = 12

– –

• create double/triple bonds by moving an electron pair from a surrounding atom to share with the central atom.

• Too few electrons? - give central atom a complete octet by creating a double or triple bond.

S – OO ––

O– O

Draw the Lewis structure for SO42- :

SO42- : 6 + (4)6 + 2 = 32 - 8 = 24

2-

• place brackets around charged structures (ions)

BeCl2 : 2 + 7 + 7 = 16

– ClCl –

Be

- 4 = 12 Ionic Bond

-1

Cl

+2

Be

-1

Cl

• all atoms of an ionic bond are in brackets to distinguish the “giving” and “taking” nature of the bond.

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.

SeCl2 : 6 +7 + 7 = 20 - 4 = 16

Se – ClCl –

• VSEPR shape is determined by counting bonding pairs and lone pairs around the CENTRAL atom only.

• A line bond counts as 1 bonding pair• A lone pair is a pair of non-bonding electrons

• You will always have this (check the back of the booklet)• Count the bonding and lone pairs and read the shape.

AXE method:

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

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

- 6 = 18

– FF – B

F

–

*Unusual(Does not have octet)

3 – bonding pairs (X3)0 – lone pairs

Trigonal Planar

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

5 – bonding pairs (X5)0 – lone pairs

Trigonal Bipyramidal

6 – bonding pairs (X6)0 – lone pairs

Octahedral

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

*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

Treat double/triple bonds as single bonds:

CO2 : 4 + 6 + 6 = 16 - 4

Double Bonds

2 – bonding pairs (X2)0 – lone pairs

Linear

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