6.1 - Shapes of molecules and ions

Question 1

What is the electron pair repulsion theory?

Answer 1

That the shape of a molecule is determined by the electron pairs surrounding the central atom
- Electron pairs from other pairs
- They move as far apart to minimise repulsion

Question 2

What shape would a molecule have if the central atom has two pairs of bonding electrons around it? (no lone pairs) and the bond angle? CO2

Answer 2

Linear shape
180° bond angle

Question 3

What shape would a molecule have if the central atom has three pairs of bonding electrons around it? (no lone pairs) and the bond angle? BF3

Answer 3

Trigonal planar
120° bond angle

Question 4

What shape would a molecule have if the central atom has four pairs of bonding electrons around it? (no lone pairs) and the bond angle? CH4

Answer 4

Tetrahedral
109.5°

Question 5

What shape would a molecule have if the central atom has five pairs of bonding electrons around it? (no lone pairs) and the bond angle? PCl5

Answer 5

Trigonal bipyramidal
90° and 120°

Question 6

What shape would a molecule have if the central atom has six pairs of bonding electrons around it? (no lone pairs) and the bond angle? SF6

Answer 6

Octahedral
90°

Question 7

How does lone pairs affect bond angles?

Answer 7

Lone pairs repel more strongly than bonding pairs and this extra repulsion decreases other bong angles by 2.5°

Question 8

How to calculate bond angle of h2o?

Answer 8

h2o has two bonding pairs of electrons and two lone pairs
- four pairs of electrons = tetrahedral structure with 109.5° bond angle
- one lone pair decreases angle by 2.5°, as there are 2 you need to decrease by 2 x 2.5 (5)
- 109.5 - 5 = 104.5°