Shapes of Molecules and Polyatomic Ions

Question 1

How do you calculate the number of electron pairs in a molecule?

Answer 1

First you calculate the number of outer electrons on the central atom and add one electron for each of the other atoms attached. If it is an ion with a +1 charge you subtract one from the total and vice versa. Finally divide the total number by 2 to get the total number of electron pairs. You can then tell the number of lone pairs by seeing if the number is equal to the number of pairs the central atom can make.

Question 2

What are the repulsion strengths of the pairs in order?

Answer 2

Non-bonding/Non-bonding > Non-bonding/Bonding > Bondng/Bonding.

Question 3

What shape is adopted by a molecule with 2 bonding pairs?

Answer 3

Straight/Linear.

Question 4

What shape is adopted by a molecule with 3 bonding pairs?

Answer 4

Trigonal (or triagonal planar).

Question 5

What shape is adopted by a molecule with 2 bonding pairs and 1-2 lone pairs?

Answer 5

Bent.

Question 6

What shape is adopted by a molecule with 4 bonding pairs?

Answer 6

Tetrahedron/Tetrahedral.

Question 7

What shape is adopted by a molecule with 3 bonding pairs and 1 lone pair?

Answer 7

Trigonal pyramidal.

Question 8

What shape is adopted by a molecule with 5 bonding pairs?

Answer 8

Trigonal Bipyramidal.

Question 9

What shape is adopted by a molecule with 6 bonding pairs?

Answer 9

Octahedral.