topic 5🅱️

Question 1

ionic solids

Answer 1

cations and anions held together by electrostatic interactions between opposing charged ions

Question 2

what does the ionic model assume

Answer 2

ions are hard spheres that have no covalent character with fixed ionic radii

Question 3

how do we determine the ionic radii

Answer 3

by determining the distance between the nuclei of the 2 ions very accurately using x ray crystallography.

Question 4

what is distance ‘d’

Answer 4

the sum of the cat ionic and anionic radii

radius of cation + radium of anion.

Question 5

what is difficult about ‘d’

Answer 5

it’s hard to be sure where the boundary between the 2 ions occurs.

Question 6

what are the value of ionic radio dependent on

Answer 6

the compound used to determine them

Question 7

what does the radius of an ion depend on

Answer 7

• coordination geometry
• ion charge
• electron configuration

Question 8

coordination number and ionic radii

Answer 8

increase in coordination number = increase in ionic radii

Question 9

charge of the ion and ionic radii

Answer 9

increase the positive charge = decrease in radius

diff oxidation states = diff radii

Question 10

electron configuration and ionic radii

Answer 10

electron configuration and ionic radii

Question 11

ionic radii trends

Answer 11

• change in e- number = change in radii
• increase electrons = larger radii (anion forms,, Zeff decreases, e- e- repulsion increases)
• decrease electrons = smaller radii (cation forms,, Zeff increase,, e- e- repulsion decreases)

Question 12

ionic radii trends down a group

Answer 12

increase in ionic radii

larger principle quantum number, ‘n’.

Question 13

ionic radii trends: ionic radii across a period

Answer 13

if they have the same charge,, the radius will decrease across a period

electrons in the same shell but an increase in protons. poor shielding from electrons in the same shell.

feels more Zeff so radius decreases

Question 14

what are isoelectronic ions

Answer 14

same number of electrons but different number of protons.

ions have different charges that allows them all to have the same number of e-.

Question 15

ionic trends: isoelectronic ions

Answer 15

greater z = smaller radius: larger Zeff

Question 16

predicting ionic structures

Answer 16

• most stable structure is formed

Question 17

what is the most stable structure that is most likely to form

Answer 17

• the structure that maximises all cation-anion contacts
• the structure with the highest coordination number will still allowing cations and anions to remain in contact.
• 4 below,, 1 above.
  burger: bottom bun is always massive,, top bun disappeares.

Question 18

what is the radius ratio

Answer 18

the ratio of the smallest ion radius to the largest ion radius.

Question 19

which ion usually has the smaller ratio

Answer 19

the cation,, lacking in e-,, greater Zeff,, less e- e- repulsions

Question 20

radius ratio equation

Answer 20

radius of cation // ratio of anion

Question 21

what is the limiting radius ratio

Answer 21

smallest possible value of the radius ratio that still allows the cations and anions to be in contact

Question 22

limiting radius ratio

Answer 22

• defined for a particular geometry around the cation

table needs to be memorised <3

Question 23

what is the radius ratio rule

Answer 23

predicting a compounds structure from the radius ratio of its ions

Question 24

limiting radius ratio rule steps

Answer 24

always gonna be 2 right handed triangles between the 2 anion and 1 cation radii.

use sin rule

r+/r- = (1/sinX) -1

where X is the angle that doesnt equal 90

Question 25

radius ratio values

Answer 25

less than 0.1555
0.155 - 0.225
0.225 - 0.414
0.414 - 0.732
0.732 - 1.000
larger than 1.000

Question 26

coordination number: radius ratio values

Answer 26

2
3
4
6
8
12

Question 27

geometry of cation: radius ratio

Answer 27

2 = linear <0.155
3 = trigonal 0.155 - 0.225
4 = tetrahedral 0.225 - 0.414
6 = octahedral 0.414 - 0.732
8 = cubic 0.732 - 1.000
12 = cuboctahedral >1.000

Question 28

does the radius ratio rule always work

Answer 28

nope

Question 29

why doesnt the radius ratio rule not apply in all cases

Answer 29

relies on the ionic model,, the ionic model is based on a lot of assumptions

Question 30

what are some assumptions of the ionic model + their correction

Answer 30

• ions in crystals arent hard spheres
• we dont know the exact radii of individual ions (radii lengths are dependent on the compound theyre in)
• not all compounds are 100% ionic,, there is a degree of covalency dpending on the electronegativity difference between the 2 atom types.

Question 31

compounds that deviate from ionic model + radius ratio

Answer 31

LiI
LiCl

both said to be octahedral coordination + adopt rock salt structure (NaCl)

but LiCl is octahedral
and LiI is tetrahedral coordination

Question 32

what causes discrepancies between real and predicted structure types

Answer 32

ionic + covalent character

electronegativity differences.

Question 33

why does LiI go against the radius ratio predicted structure

Answer 33

difference in electronegativity,, allows a degree of polarizability.

in LiCl there is also some covalent character.

larger ions are more polarisable than smaller ions

smaller,, highly charged cations are strongly polarizing

the small Li+ cation can polarise the diffuse e- cloud on the larger I- ion leading to a higher degree of covalent character.

the bonding in LiI isnt well described by the ionic model as there is a strong degree of covalent character.