6.1 Shapes of molecules and ions

Question 1

What is the electron-pair repulsion theory?

Answer 1

Groups of electrons surrounding the centre atom of a molecule will determine its shape.

Question 2

What causes different shapes in a molecule?

Answer 2

Different number of electron groups will result in different shapes
Different combinations of bonded groups and lone pairs also results in different shapes

Question 3

Why do electrons repel one another?

Answer 3

To arrange themselves as far apart as possible in order to minimise repulsion and thus hold bonded atoms in a definite shape

Question 4

Describe the structure, shape and bond angle of methane CH4

Answer 4

symmetrical with 4 C-H covalent bonds
-4 bonded pairs of electrons surrounding the central carbon atom
-tetrahedral shape with 4 equal H-C-H bond angles of 109.5 degrees

Question 5

How are 3D shapes represented on a flat sheet of paper?

Answer 5

SOLID LINE: a bond in the plane of the paper
SOLID WEDGE: comes out of the plane of the paper (towards you)
DOTTED WEDGE: goes into the plane of the paper (away from you)

Question 6

What is a lone pair?

Answer 6

a pair of valence electrons that are not shared with another atom in a covalent bond

Question 7

How does a lone pair affect repulsions?

Answer 7

slightly closer to central atoms, occupies more space than bonded pairs,
=> lone pairs repel more strongly so they repel bonded pairs slightly closer together and decrease the bond angle

Question 8

What is the scale of repulsion?

Answer 8

bonded-pair/bonded-pair < bonded-pair/lone-pair < lone-pair/lone-pair

————————————>
increasing repulsion

Question 9

For every lone pair, how much does the bond angle reduce by?

Answer 9

approx 2.5 degrees per lone pair

Question 10

Common shapes that must be memorised
(no lone pairs)
pairs/ name of shape/ bond angle

Answer 10

2 bond regions: linear/ 180
3 bond regions: trigonal planar/ 120
4 bond regions: tetrahedral/ 109.5
5 bond regions: trigonal bipyramidal aka T shape / 90 & 120
6 bond regions: octahedral / 90

Question 11

Common shapes ( with lone pairs)
pairs/ name of shape/ bond angle

Answer 11

3 bond pairs and 1 lone pair: pyramidal/ 107
2 bonded pairs & 2 lone-pairs: non linear/ 104.5

Question 12

What if molecules contain multiple bonds?

Answer 12

Each multiple bond is treated as a bonding region
e.g. carbon dioxide has 4 covalent bonds yet it is considered to have only have 2 bonding regions

Question 13

Why is an octahedral shape produced from six bond pairs?

Answer 13

e.g. SF6
6 fluorine atoms are positioned at the corners of an octahedron. Joining all the corners creates an octahedron shape