[6.1] shapes of molecules and ions

Question 1

what is the shape, name, and bond angle for molecules with 2 electrons pairs/bond regions?

Answer 1

• shape: X —- A — X
  > if there are double bonds show them as = etc.
• name: linear
• bond angle: 180°

Question 2

what is the shape, name, and bond angle for molecules with 3 electron pairs/bond regions

Answer 2

• shape: solid lines at 3, 7, and 11
• name: trigonal planar
• bond angle: 120°

Question 3

what is the shape, name, and bond angle for molecules with 4 electron pairs/bond regions?

Answer 3

• shape: solid line at 12 and 4, solid wedge at 7, dotted wedge at 8
• name: tetrahedral
• bond angle: 109.5°

Question 4

what is the shape, name, and bond angle for molecules with 5 electron pairs/bond regions?

Answer 4

• shape: solid line at 12, 3, and 6, solid wedge at 8:30, dotted wedge at 9:30
• name: triangular pyramid
• bond angles: 90° and 120°

Question 5

what is the shape, name, and bond angle for molecules with 6 electron pairs/bond regions?

Answer 5

• shape: solid line at 12 and 6, solid wedge at 3:30 and 8:30, dotted wedge at 2:30 and 9:30
• name: octahedral
• bond angle: 90°

Question 6

how are molecules represented in 3D shape?

Answer 6

• a solid line represents a bond in the plane of the paper
• a solid wedge represents a bond coming out of the plane of the paper
• a dotted wedge represents a bond going into the plane of the paper

Question 7

why are electron pairs spread out as far as possible?

Answer 7

• to minimise repulsive forces
• the shape adopted by a simple molecule or ion is that which keeps repulsive forces to a minimum

Question 8

how do lone electron pairs affect bond angle?

Answer 8

• bonding electron pairs repel
• lone (non-bonding) electron pairs are held closer to the nucleus and repel more
• each extra lone pair reduces the expected bond angle by ~ 2.5°

Question 9

what is the shape, name, and bond angle for molecules with 3 bonded pairs and 1 lone pair?

Answer 9

• shape: lone pair at 12, dotted wedge at 4, solid wedge at 5, solid line at 8
• name: pyramidal
• bond angle: 107°

Question 10

what is the shape, name, and bond angle for molecules with 2 bonded pairs and 2 lone pairs?

Answer 10

• shape: lone pair at 11 and 2, solid wedge at 4:30, solid line at 8
• name: non-linear
• bond angle: 104.5°

Question 11

what is the shape, name, and bond angle for molecules with 4 bonded pairs and 2 lone pairs?

Answer 11

• shape: lone pairs at 12 and 6, solid lines at 10:30 and 1:30, solid wedges at 7:30 and 4:30
• name: square planar
• bond angle: 90°

Question 12

how do you draw the dot and cross diagrams for ions?

Answer 12

• draw the number of outer shell electrons (eg. N = 5)
• for every +/- charge, add/remove electrons (eg. NH₄⁺ = remove one electron)
• pair up electrons the usual way
• inside the centre circle, add a charge (eg. N⁺)

Question 13

how can chemists predict the shapes of molecules based on the number of electron pairs surrounding the central atom?

Answer 13

• pairs of electrons surrounding a central atom repel
• the shape is determined by the number of bond pairs and the number of lone pairs of electrons