Shapes of Molecules

Question 1

What does VSEPR stand for
What is it used for

Answer 1

Valence Shell Electron Pair Repulsion
Used to determine the 3-D shape of a covalent molecule

Question 2

What is the general notation used in VSEPR

Answer 2

ABn

Question 3

Use VSEPR theory to work out the shape of BF₃

Answer 3

Electrons from Boron: 3
Number of B-F bonds: 3
Total electrons at Boron: 6
THREE electron pairs - therefore AB₃
Repulsion in minimised by a trigonal planar arrangement with bond angles of 120°

Question 4

Use VSERP theory to work out the shape of CH₄

Answer 4

Electron from Carbon: 4
Number of C-H bonds: 4
Total electrons at carbon: 8
FOUR electron pairs - therefore AB₄
Repulsion is minimised by a tetrahedral arrangement with bond angle all 109.5°

Question 5

Using VSEPR theory, work out the shape of PF₅

Answer 5

Electrons from Phosphorus: 5
Number of P-F bonds: 5
Total electrons at Phosphorus: 10
FIVE electron pairs - therefore AB₅
Repulsion is minimised by a trigonal bipyramidal arrangement with bond angles of 90° (ax) and 120° (eq)
Be aware you can also have a square-base pyramid configuration too

Question 6

Using VSEPR theory, work out the shape of SF₆

Answer 6

Electrons from Sulphur: 6
Number of S-F bonds: 6
Total electron at Sulphur: 12
SIX electron pairs - therefore AB₆
Repulsion is minimised by an octahedral arrangement

Question 7

Using VSEPR theory, work out the shape of NH₄⁺

Answer 7

Electrons from Nitrogen: 5
Number of N-H bonds: 4
SUBTRACT e- for +VE charge: -1(as charge is on central atom)
Total electrons at N: 8
FOUR electron pairs - therefore AB₄
TETRAHEDRAL arrangement

Question 8

Using VSEPR theory, work out the shape of PF₆¯

Answer 8

Electrons from Phosphorus: 5
Number of P-F bonds: 6
ADD e- for -VE charge: +1 (as charge is on central atom)
Total electrons at Phosphorus: 12
SIX electron pairs - therefore AB₆
Octahedral arrangement

Question 9

If some of the central atom electrons do not participate in bonding they are…

Answer 9

Lone pairs

Question 10

How do lone pairs affect VSERP theory, and what annotation is used to represent them

Answer 10

Lone pairs affect the shape of a molecule, and hence must be included in the electron count
A lone pair will be given the symbol L

Question 11

Using VSERP theory, describe the shape of SnCl₂

Answer 11

Electrons from Sn: 4
Number of Sn-Cl bonds: 2
Total electron at Sn: 6
THREE electron pairs, but only TWO bonds - therefore AB₂L
Shape BASED on trigonal planar - with one vertex occupied by the lone pair. Appears to be V-shaped

Question 12

Electron repulsion pair theory states that

Answer 12

Repulsion from lone pairs are greater than those from bonded pairs
Therefore the bond angles between two bonded pairs will be smaller, with the presence of a lone pair

Question 13

Using VSEPR thoery, work out the shape of NH₃

Answer 13

Electrons from Nitrogen: 5
Number of N-H bonds: 3
Total electrons at Nitrogen: 8
FOUR electron pairs, but only THREE bonds - therefore AB₃L
Shape BASED on tetrahedral - with one vertex occupied by the lone pair. So appear pyramidal
Because of the lone pair, the angle between the N-H bonds will be smaller

Question 14

Using VSEPR theory, what is the shape of H₂O

Answer 14

Electrons from Oxygen: 6
Number of O-H bonds: 2
Total electrons at Oxygen: 8
FOUR electron pairs, but only TWO bonds - therefore AB₂L₂
Shape based on tetrahderal - but with two verticies occupied by the lone pairs. Appears to be V-shaped
Smaller bond angle between O-H because of lone pair repulsion