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CHEM111 > Ionic Lattices And Close Packing > Flashcards

Ionic Lattices And Close Packing Flashcards

1
Q

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

A

An amorphous solid lacking long-range order

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2
Q

Define what a unit cell is

A

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

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3
Q

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

A

The unit cell parameters and the contents of the unit cell

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4
Q

What is the shape of a 3 dimensional unit cell?

A

Parallelepiped - cuboid

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5
Q

How many crystal lattice structures are known as Bravias lattices?

A

14

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6
Q

What are the 3 types of cubic packing?

A

Primitive cubic

Body-centred cubic

Face-centred cubic

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7
Q

Draw out a body-centred cubic projection presentation

A
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8
Q

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

A

Inside = 1

Face = 1/2

Edge = 1/4

Corner = 1/8

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9
Q

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

A

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

As a result atom tend to pack as densely as possible

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10
Q

How does cubic close packing differ from hexagonal close packing?

A

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

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11
Q

What unit cell does cubic close packing show?

A

Face-centred unit cell

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12
Q

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

A

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

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13
Q

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

A

CN = 6

Coord = octahedral

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14
Q

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

A

CN = 8

Coord = cubic

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15
Q

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

A

CN = 12

Coord = cube-octahedral

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16
Q

What holds ions together?

A

Electrostatic forces

17
Q

Why do ions form crystalline structures?

A

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

18
Q

Which are generally bigger anions or cations?

A

Anions are usually bigger, they form the sphere packing

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

19
Q

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

A

Octahedral holes and tetrahedral holes

20
Q

What is the coordination of octahedral holes?

A

Coordinated by 6 anions

21
Q

What is the coordination of tetrahedral holes?

A

Coordinated by 4 anions

22
Q

Give the coordination numbers of Rocksalt

A

NaCl
Both cations and anions have CN = 6

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

23
Q

Give the coordination numbers of Fluorite

A

CaF2
Cations = 8
Anions = 4

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

24
Q

Give the coordination numbers of Antifluorite

A

LiO2
Cations = 4
Anions = 8

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

25
Q

Give the coordination numbers of Sphalerite

A

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

26
Q

Give the coordination numbers of Wurtzite

A

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

27
Q

Give the coordination numbers of CsCl type packing

A

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)

28
Q

What are the coordination numbers of CdCl2

A

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

29
Q

Give the coordination numbers of Rutile

A

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

30
Q

Give the coordination numbers of NiAs

A

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

31
Q

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

A

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)

32
Q

What does the ionic radius depend on?

A

The charge of the ion

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

33
Q

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

A

Tetrahedral —> Octahedral —> Cubic

34
Q

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

A

Tetrahedral - 0.225

Octahedral - 0.414

Cubic - 0.732

35
Q

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

A

¥ = radius small ion / radius large ion

36
Q

What does the coordination number of the cation reflect?

A

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

37
Q

Why is the structure of Fluorite a special case?

A

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

CHEM111 (12 decks)

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