Structures of solids

Question 1

What happens to al inorganic substances if cooled sufficiently

Answer 1

1. Form a solid phase
2. The majority are crystalline solids
3. The atoms are molecules pack together in regular repeating units

Question 2

What are the four main solid types

Answer 2

1. Ionic solid e.g. NaCl
2. Metal e.g. Fe
3. Covalent network e.g. Si
4. Molecular solid e.g. H2O

Question 3

Briefly describe an ionic solid

Answer 3

1. Strong ionic (electrostatic interactions) between oppositely charged ions
2. High melting point
3. Conduct electricity in solution and the liquid phase but not normally as a solid

Question 4

Briefly describe a metal

Answer 4

1. Bonding via delocalised electrons
2. Wide range of melting points
3. Conduct electricity when a potential difference is applied

Question 5

Briefly describe a covalent network

Answer 5

1. Strong covalent bonds

2. Very high melting points

Question 6

Briefly describe a molecular solid

Answer 6

1. Strong intramolecular forces
2. Weak intermolecular interactions
3. Low melting point

Question 7

Give four examples of where solid materials are important for research and technology

Answer 7

1. Heterogeneous catalysts- e.g Fe, Pt, CeO2 (>80% Industrial chemicals)
2. Semiconductors- e.g. Si, GaAs
3. Ionic conductors in batteries e.g. LiCoO2
4. Solar cells e.g Si, CdTe, perovskite, CH3NH3PbI3

Question 8

What are the two ways of close packing

Answer 8

1. Cubic close packing (ccp) or (fcc)- face centred cubic

2. Hexagonal close packing (hcp)

Question 9

What is close packing

Answer 9

1. The most efficient way of packing atoms so the empty space is minimised

Question 10

Describe hexagonal close packing

Answer 10

1. First start with a single layer of atoms
2. Second layer placed above spaces in the bottom layer
3. Third layer directly above first layer
4. ABABAB

Question 11

What is the coordination number for hexagonal close packing and give 4 examples

Answer 11

1. 12

2. Mg, Zn, Ti, Co

Question 12

Describe cubic close packing/ face centred cubic

Answer 12

1. Same as hcp but third layer is not directly above previous layer
2. Layer sequence= ABCABC

Question 13

What is the coordination number for cubic close packing and give 4 examples

Answer 13

1. 12

2. Cu, Ag, Al, Ni

Question 14

What is the coordination number in a solid state structure

Answer 14

1. Number of nearest neighbours an atom has

Question 15

Why do hcp and ccp/fcc have a coordination number of 12

Answer 15

1. There are 6 nearest neighbours in the same row

2. And 3 in row above and below

Question 16

Why is ccp also known as fcc

Answer 16

1. There are atoms on each face of the cube

2. As well as the 8 vertices

Question 17

Describe the body centred cubic structure bcc

Answer 17

1. Not as common as hcp or ccp
2. Has lattice site at centre of a cube
3. Other lattice sites at the corner

Question 18

What is the coordination number for body centred cubic and give 3 examples

Answer 18

1. 8

2. Fe, Na, K

Question 19

What are the gaps between atoms called

Answer 19

Interstitial sites

Question 20

What are the two types of interstitial site

Answer 20

1. Octahedral sites

2. Tetrahedral sites

Question 21

What is a tetrahedral hole

Answer 21

1. Formed by a planar triangle of atoms capped by a single atom
2. So it is surrounded by 4 atoms

Question 22

What is an octahedral hole

Answer 22

1. Lies between two oppositely directed planar triangles of atoms
2. Surrounded by 6 atoms

Question 23

How many octahedral and tetrahedral sites do you get per atom of a close-packed structure

Answer 23

1. One octahedral site

2. Two tetrahedral sites

Question 24

What is a unit cell

Answer 24

1. Smallest repeating units that shows the fully symmetry of the solid structure
2. Repeated in 3D gives infinite solid lattice

Question 25

How many atoms are in one unit cell of the ccp

Answer 25

1. Looks like 14- one on each of 8 vertices and one on each of the 6 faces
2. But all of the atoms are shared with other unit cells, so don’t contribute fully to each cell
3. Total number is 4

Question 26

What is the total number of atoms contributing to the unit cell of a vertex atom

Answer 26

1. 8 atoms in the vertices
2. Each atom contributes 1/8 to each unit cell
3. Total number of atoms contributing to the unit cell= 8*1/8= 1

Question 27

What is the total number of atoms contributing to the unit cell of a face atom

Answer 27

1. 6 atoms at the faces
2. Each atoms contributes 1/2 to each unit cell
3. 6*1/2 =3

Question 28

How can you find the volume of atoms in a unit cell

Answer 28

1. Multiply the number of atoms e.g. 4 by 4/3pir^3

Question 29

What is a cell projection diagram

Answer 29

1. 2D representation of the unit cell- view from above
2. The x- and y- coordinates are shown on the projection as normal
3. But the z-coordinate is given as a number between 0-1 for each atom

Question 30

What do the z-coordinates represent

Answer 30

1. 0 is the bottom of the unit cell

2. 1 is the top of the unit cell

Question 31

How many different crystal systems are there

Answer 31

1. There are 7 different geometric shapes of unit cells so 7 different crystal systems

Question 32

What are the 7 different crystal systems

Answer 32

1. Cubic
2. Tetragonal
3. Orthorhombic
4. Monoclinic
5. Triclinic
6. Hexagonal
7. Trigonal/ rhombohedral

Question 33

What are the parameters that define a unit cell

Answer 33

1. Cell lengths (a,b,c)

2. Cell angles ( alpha, beta, gamma)

Question 34

What are the 4 lattice sites and where are the lattice points for each

Answer 34

1. P= primitive= lattice points on cell corners only
2. I= body-centered = lattice points on cell corners + centre of cell
3. F= face-centered= lattice points on cell corners + centre of each face
4. C= Base-centred= lattice points on cell corners + each face of one pair of opposite faces