Ionic Lattices And Close Packing

Question 1

What is the opposite of a periodically ordered array of atoms know as?

Answer 1

An amorphous solid lacking long-range order

Question 2

Define what a unit cell is

Answer 2

A unit cell is the smallest repeating unit of a periodic structure

Question 3

What do we need to describe the pattern of a crystalline structure?

Answer 3

The unit cell parameters and the contents of the unit cell

Question 4

What is the shape of a 3 dimensional unit cell?

Answer 4

Parallelepiped - cuboid

Question 5

How many crystal lattice structures are known as Bravias lattices?

Answer 5

14

Question 6

What are the 3 types of cubic packing?

Answer 6

Primitive cubic

Body-centred cubic

Face-centred cubic

Question 7

Draw out a body-centred cubic projection presentation

Answer 7

Question 8

When counting atoms in a unit cell, what is the relative value of each position to the unit cell?

Answer 8

Inside = 1

Face = 1/2

Edge = 1/4

Corner = 1/8

Question 9

What effect does metals being electropositive have on the structure they form?

Answer 9

Their valence electrons are less tightly held - hence bonding is non-directional

As a result atom tend to pack as densely as possible

Question 10

How does cubic close packing differ from hexagonal close packing?

Answer 10

CCP is where you have a stacking sequence of ABC - with C directly above A

HCP is where you have a stacking sequence of AB - with B in the gaps created by the A spheres below

Question 11

What unit cell does cubic close packing show?

Answer 11

Face-centred unit cell

Question 12

How do you find the packing efficiency of a unit cell?

Answer 12

Packing efficiency = (number of spheres per unit cell x volume of sphere) / volume of unit cell

Question 13

What is the coordination number and coordination of a simple cubic structure?

Answer 13

CN = 6

Coord = octahedral

Question 14

What is the coordination number and coordination of a body-centred cubic structure?

Answer 14

CN = 8

Coord = cubic

Question 15

What is the coordination number and coordination of a face-centred cubic structure?

Answer 15

CN = 12

Coord = cube-octahedral

Question 16

What holds ions together?

Answer 16

Electrostatic forces

Question 17

Why do ions form crystalline structures?

Answer 17

Because they tend to pack in such a way that maximises attractive forces between opposite ions, and repelling forces from same ions are minimised

Resulting in a crystalline solid packed in a regular periodic fashion

Question 18

Which are generally bigger anions or cations?

Answer 18

Anions are usually bigger, they form the sphere packing

Cations occupy the interstitial holes between them as they’re smaller

Question 19

What are the two types of interstitial holes in close-packing of equal spheres?

Answer 19

Octahedral holes and tetrahedral holes

Question 20

What is the coordination of octahedral holes?

Answer 20

Coordinated by 6 anions

Question 21

What is the coordination of tetrahedral holes?

Answer 21

Coordinated by 4 anions

Question 22

Give the coordination numbers of Rocksalt

Answer 22

NaCl
Both cations and anions have CN = 6

All octahedral holes in cubic close packing of anions is occupied by cations

Question 23

Give the coordination numbers of Fluorite

Answer 23

CaF2
Cations = 8
Anions = 4

Cation form the cubic close packing while anions fill all tetrahedral holes

Question 24

Give the coordination numbers of Antifluorite

Answer 24

LiO2
Cations = 4
Anions = 8

Anions form the cubic close packing while cations fill all tetrahedral holes

Question 25

Give the coordination numbers of Sphalerite

Answer 25

Cubic cp ZnS Both cations and anions have CN = 4 Half of tetrahedral holes in cubic close packing are occupied

Question 26

Give the coordination numbers of Wurtzite

Answer 26

Hexagonal cp ZnS Both cations and anions have CN = 4 Half of tetrahedral holes are filled

Question 27

Give the coordination numbers of CsCl type packing

Answer 27

Simple body-centred cubic CsCl Both cations and anions have CN = 8 (Primitive cubic not bcc as it is 2 simple cubic cubes interpenetrating each other)

Question 28

What are the coordination numbers of CdCl2

Answer 28

Distorted ccp CdCl2 Cation = 6 Anion = 3 Half of octahedral holes filled

Question 29

Give the coordination numbers of Rutile

Answer 29

Distorted hcp TiO2 Cation = 6 Anion = 3 Half of octahedral holes filled

Question 30

Give the coordination numbers of NiAs

Answer 30

hcp NiAs Both cations and anions have CN = 6 All octahedral holes filled

Question 31

How does the size of an ion change with its charge respectively?

Answer 31

Cations generally are much smaller while anions become much larger than their neutral atoms r(Na) = 191 pm vs r(Na+) = 102 pm r(Cl)=99pm vs r(Cl-)=181pm ( 2x larger due to extra electron)

Question 32

What does the ionic radius depend on?

Answer 32

The charge of the ion The coordination number of the ion (especially for cations) - the higher the CN the larger the ionic radius

Question 33

What structures does the number of interstitial holes increase in order of?

Answer 33

Tetrahedral —> Octahedral —> Cubic

Question 34

What are the radius ratios of the 3 interstitial hole types?

Answer 34

Tetrahedral - 0.225 Octahedral - 0.414 Cubic - 0.732

Question 35

How do you find the radius ratio of a binary ionic solid?

Answer 35

¥ = radius small ion / radius large ion

Question 36

What does the coordination number of the cation reflect?

Answer 36

Due to cations usually being smaller they fill the interstitial holes Hence the cation coordination number indicates the interstitial hole type. 4 = tetrahedral 6 = octahedral 8 = cubic

Question 37

Why is the structure of Fluorite a special case?

Answer 37

Because F- ions are larger than Ca2+ ions, so F- anions should make up the sphere packing The structure is usually referred to as a cubic close packing of cations in which anions fill the holes because the structure is face-centred cubic