3.1.3.5 Shapes Of Simple Molecules And Ions

Question 1

What is the central atom?

Answer 1

The central atom is the atom that forms covalent bonds with all other atoms in the molecule. The shape of the molecule is determined by the number of electron pairs, and the nature of these pairs, in the outermost shell around the central atom.

Question 2

What is a lone pair?

Answer 2

A lone pair is a pair of electrons that are not shared with another atom in a bond.

Question 3

What is a bonding pair?

Answer 3

A bonding pair is a pair of electrons that is shared between two atoms in a bond.

Question 4

What is an electron charge cloud?

Answer 4

An electron charge cloud is a region of negative charge around the nucleus, linked to an atomic orbital. There is a high chance of an electron pair being present in this region, either a bonding pair or a lone pair.

Question 5

What is VSEPR?

Answer 5

Valence shell electron pair repulsion theory

Question 5

Describe the repulsion between pairs of electrons?

Answer 5

Electron pairs are far apart to minimise repulsion between them. Lone pair electrons repel each other more than bonding pair electrons, as they are more electron dense (larger electron charge clouds).

Question 6

Describe the principles of VSEPR?

Answer 6

-Valence electrons are those which are in the outer shell of an atom
-electron pairs repel each other as they have the same negative charge
-lone pair electrons repel each other more than bonding pair electrons, which reduces the bond angles in the shape by 2.5*
-the pairs of electrons will arrange as to minimise repulsion (as far apart as possible)

Question 7

State the 7 steps in determining the the shape of a molecule

Answer 7

1) identify the central atom
2) count the number of outer shell electrons in the central atom
3) add one electron for each covalent bond that the central atom makes with other atoms
4) add or subtract relevant charges if ion charges are present
5) divide the total of these electrons by 2 to find the number of pairs
6) find out how many pairs are lone pairs and how many are bonding pairs
7) use this to then predict the shape of the molecule using the core shapes (-2.5* from angles for every lone pair present)

Question 8

Describe a linear molecule?

Answer 8

2 bonding pairs
No lone pairs
Angles of 180*

Question 8

Describe a trigonal planar molecule?

Answer 8

3 bonding pairs
No lone pairs
Angles of 120*

Question 9

Describe a tetrahedral molecule?

Answer 9

4 bonding pairs
No lone pairs
Angles of 109.5*

Question 10

Describe a trigonal bipyramid molecule?

Answer 10

5 bonding pairs
No lone pairs
Angles of 90* and 120*

Question 10

Describe an octahedral molecule?

Answer 10

6 bonding pairs
No lone pairs
Angles of 90*

Question 11

Describe a trigonal pyramid molecule?

Answer 11

3 bonding pairs
1 lone pair
Angles of 107*
(Core shape = tetrahedral so -2.5*)

Question 12

Describe a bent linear molecule?

Answer 12

2 bonding pairs
2 lone pairs
Angles of 104.5*
(Core shape = tetrahedral so -5*)