The Ionic Model

Question 1

How are ionic solids held together?

Answer 1

They are held together by electrostatic interactions

Question 2

What are the assumptions of the ionic model?

Answer 2

Ions are assumed to be hard spheres with fixed sizes

Question 3

How is the distance between the centre of 2 ions measured?
What are the draw backs?
How is this overcome?

Answer 3

Using X ray crystallography
It is hard to tell individual ionic radio from this
Point of minimum electron density is used to define where cation starts and anion ends

Question 4

What can change ionic radii?

Answer 4

Ionic radii varies with the compound it is in

A difference in coordination number leads to significant changes

Question 5

Describe the trend in ionic radii going down the group

Answer 5

Ionic radii increases down the group with increase in n

Question 6

What is an isoelectronic ion?

Answer 6

This is an ion with the same electronic configuration

Question 7

Describe the difference in ionic radii for isoelectronic ions

Answer 7

The ion with the greater change has the smaller ionic radii

Question 8

Describe the most stable structure in terms of cation and anions and their contact

Answer 8

Maximum anion- cation contact to maximise attraction

Minimum anion- anion and cation- cation contact to minimise repulsions

Question 9

Describe the nearest neighbours of an ion in ionic model and what is the distance between the centres

Answer 9

Ions are assumed to be in cobalt with nearest neighbours of opposite charge
Distance between centres= E r+ + r-

Question 10

What happens if the cation shrinks?

Answer 10

As the cation shrinks, the anions get closer together which leads to more repulsions
If the cations shrink even more, the anions overlap leading to even more repulsions

Question 11

Which structure will the compound adopt?

Answer 11

Compounds will adopt the structure with the maximum coordinate number subject to the cations and anions being in contact with each other

Question 12

What is the radius ratio?

Answer 12

This is the ratio of the radius of the smaller Ion to that of the larger anion
The cations are normally smaller

r+/ r-

Question 13

How do you work out the radius ratio?

Answer 13

1) draw a right angle triangle with length r-, r++r- and angles 90 and 45
2) apply the sine rule
r+ + r-/ sin90= r-/ sin 45
- sin90 = 1
3) rearrange to get r+/ r-= (1/sin45)-1
= 0.414
Or divide the two ionic radii

Question 14

What is the limiting radius ratio?

Answer 14

For a particular coordinate type, you can work out geometrically the smallest possible value of the radius ratio for the anions and cations to be in contact
Any smaller and the cation anion contact is lost

Question 15

Describe the assumptions of the ionic model and how you can use it

Answer 15

Assume ions are hard sphere
Ions should be in contact with ions of opposite charge
Most stable structure maximises the number of cation- anion contact
Determine the radius ratio r+/ r-

Question 16

How can you predict structures from radius ratio?

Answer 16

If you know the radius ratio you can see which coordination number this corresponds to
<0.155= 2, 0.155-0.255 =3, 0.225- 0.414= 4, 0.414- 0.732= 6, 0.732-1.000= 8, >1.000= 12 (cuboctahedral)

Question 17

Describe the bonds in polarising species

Answer 17

Polarising species form bonds with covalent character

Question 18

Describe the drawbacks of using radius ratio to determine structures

Answer 18

Only works if the species is purely ionic
Significant covalent interactions can affect the structures adopted
Also problem determining the ionic radii as these differ for same atom in different compounds
- ions are not really hard spheres with fixed radii