The Shapes of Molecules and Ions

Question 1

What are the rules that govern the shapes of simple molecules?

Answer 1

1. Each pair of electrons around an atom will repel all other electron pairs.
2. The pairs of electrons will therefore take up positions as far apart as possible to minimise repulsion.

Question 2

How does the number of electron pairs around an atom affects the shape of the molecule?

Answer 2

-Two pairs of electrons = linear, 180 degrees
-Three pairs of electrons = trigonal planar, 120 degrees
-Four pairs of electrons = tetrahedral, 109.5 degrees
-Five pairs of electrons = trigonal bipyramidal, 120 degrees
-Six pairs of electrons = octahedral, 90 degrees

Question 3

What happens to the shape of a molecule when lone pairs of electrons start to become involved?

Answer 3

-3 pairs of electrons, 1 lone pair = triangular pyramid, 107 degrees
-2 pairs of electrons, 2 lone pairs = V-shaped, 104.5 degrees
-4 pairs of electrons, 2 lone pairs = square planar, 90 degrees

Question 4

Draw out the following shapes:
-4 bonding pairs
-5 bonding pairs
-1 lone pair, 3 bonding pairs
-1 lone pair, 4 bonding pairs
-2 lone pairs, 3 bonding pairs

Answer 4

-Tetrahedral 109 degrees
-Trigonal Bipyramidal 90, 120 degrees
-Pyramidal 107 degrees
-Seesaw 104.5 degrees
-T-shaped 87.5 degrees

Question 5

What is the formula for calculating the number of lone pairs?

Answer 5

Group number (+/- electron charge) - the number of bonds // 2