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CH1.2 foundations of chemistry > solids - structure > Flashcards

solids - structure Flashcards

(56 cards)

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1
Q

what are the 4 main types of solid structure?

A

molecular solid
covalent network structure
metal lattice structure
ionic solid/lattice structure

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2
Q

outline the structure of a molecular solid structure

A

strong covalent bonds hold together atoms in each molecule, molecules are held togehter by weak intermolecular interactions

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3
Q

how does the structure of molecular solids impact their physical properties?

A

weak IMFs between molecules means solids have low m.p/b.p and often tend to sublime when heated
insulating materials
quite soluble in water and especially in organic solvents

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4
Q

outline the structure of a covalent network structure

A

each atom forms 4 covalent bonds to other atoms - goes on infinitely

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5
Q

how does the structure of a covalent network impact their physical properties?

A

to melt/boil all covalent bonds must be broken, these are very strong therefore m.p/b.p are very high
usually insulating materials
form hard brittle solids that are generally insoluble

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6
Q

outline the structure of a metal lattice structure

A

metal cations are held together by a sea of delocalised electrons, bonds are not covalent

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7
Q

how does the structure of a metal lattice impact their physical properties?

A

lots of energy needed to break apart metallic bonding structure and sea of electrons so m.p/b.p is high
good conductors as delocalised electrons can carry a charge
hard solids, generally insoluble, but can be malleable and ductile

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8
Q

outline the structure of an ionic lattice structure

A

ions are held together by electrostatic interactions

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9
Q

how does the structure of an ionic lattice impact their physical properties?

A

strong ionic bonds need to be broken to melt/boil so m.p/b.p is high
good conductors when molten as ions are free flowing and can carry charge
form hard + brittle solids that sometimes dissolve in water but are usually insoluble in organic solvents

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10
Q

allotropy definition + example

A

when elements can form more than one structure
e.g. carbon can form diamond (covalent network structure), graphite, buckminster fullerene

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11
Q

polymorphism definition

A

when compounds can form more than 1 structure, as opposed to elements

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12
Q

crystalline structure definition

A

structures with regular arrangements with long range order

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13
Q

amorphous solids definition

A

solids which don’t have long range order, only short range, so cannot predict the structure from a sample from another area of the strucutre

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14
Q

what is close-packing?

A

the idea of packing atoms as closely as possible, minimising empty space between - think of packing hard spheres

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15
Q

what is cubic close packing?

A

a close packing method:
layer 1 - spheres are packed as closely as possible
layer 2 - spheres are packed on top in gaps again minimising space between
layer 3 - spheres are packed in gaps in both layers, carried from the first layer
forms an ABCABC arrangement

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16
Q

what is hexagonal close packing?

A

a close packing method:
layer 1 - spheres are packed as closely as possible
layer 2 - spheres are packed on top in gaps again minimising space between (same as cubic)
layer 3 - spheres are packed in a gap only in the second layer, directly above the first layer
forms an ABABAB arrangement

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17
Q

unit cell definition

A

the smallest repeating unit for a structure, these can be stacked together to build the overall structure

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18
Q

what are the 3 types of cubic unit cells?

A

face centred cubic
body centred cubic
primitive/simple cubic

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19
Q

describe face centred cubic unit cell structure

A

identical to cubic close packed
coordination number = 12
there is an atom on every vertex and the centre of every face (therefore face centred)

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20
Q

describe body centred cubic unit cell structure

A

coordination number = 8
there is an atom on every vertex and in the centre of the cell

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21
Q

describe primitive cubic unit cell structure

A

coordination number = 6
atoms are only located on every vertex

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22
Q

coordination number definition

A

refers to the number of nearest neighbouring atoms any on atom anywhere in the structure would have

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23
Q

are face centred cubic, body centred cubic, primitive cubic, and hexagonal cubic close packed structures?

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A

fcc - yes
bcc - no
pc - no
hcp - yes

24
Q

how are the unit cells for hexagonal close packing different to that of cubic close packing?

Study These Flashcards
A

cubic unit cells are perfect cubes
unit cells for hexagonal close packing have a rhombus face with angles 60 and 120, 3 unit cells join to make a hexagon prism shape

25
what kind of unit cell is a hexagonal close packing unit cell?
body centred - atoms only on the vertex and one in the centre
26
what kinds of solid structures have the highest packing efficiency?
close packed structures
27
what is the equation for packing efficiency
packing efficiency = (volume of atoms in unit cell/total volume of unit cell) x100 as atoms are spherical, volume of atoms in unit cell = no. atoms x 4/3 π r³
28
how much do atoms at vertices, edges, faces and central body represent?
vertices = 1/8 edge = 1/4 face = 1/2 body = 1
29
density definition
mass per unit volume of a substance ρ = m/v
30
how can unit cells be used to determine density?
this can be done if the dimensions/content of the cell is known mass unit cell = (no. atoms in unit cell x Mr)/avogadros number
31
intersitial sites definition
these are the gaps between the atoms - the left over % of packing efficiency
32
what are the 2 types of intersitial sites?
octahedral - in contact with 6 atoms, 3 above + below tetrahedral - in contact with 4 atoms, 3 above + 1 below these are defined by their geometries
33
describe where the intersitial sites would be located for ccp structure
fcc structure means there are atoms on the vertices and faces octahedral sites lie on the edges, in between all atoms, they appear on the same spots as the atoms on a cell projection tetrahedral sites are in the centres, but 1/4 + 3/4 of the way through on every side, between all the octahedral sites/atoms ratio for atoms : octahedral : tetrahedral = 1:1:2
34
what is the difference between intersitial sites in cubic close packed structure and hexagonal close packed structure?
in cubic close packed - the octahedral sites and tetrahedral sites share edges, not stacked in hexagonal close packed - the octahedral sites share faces, stacked on top of their own kind
35
substitutional alloy definition + example
metal materials in which atoms of a metal are exchanged for atoms of another e.g. brass
36
why do most substitutional alloys consist of elements of similar size?
normally substitutional alloys occur when sizes of atoms are similar so as to not cause major distortions to the structure
37
intersitial alloy definition + example
metal materials where atoms of an element are present in the intersitial sites of a metal lattice e.g. steel
38
binary compound definition
compounds containing 2 elements
39
what kind of structure is usually adopted by binary compounds?
usually adopt structures similar to close packed structures, with 1 element places in atom positions and the other element in intersitial sites - most commonly ionic compounds are arranged in this way
40
describe the expected binary compound structure of ionic compound NaCl
close packed large Cl- anions with smaller Na+ cations in intersitial sites - remember ratio atoms : octahedral sites = 1:1 so Na would be in octahedral sites
41
describe the expected binary compound structure of ionic compound CaF2
close packed large F- anions in tetrahedral intersitial sites and smaller Ca2+ cations are in the ccp structure atom : tetrahedral sites = 1:2 this specific arrangement of anions/cations = antifluorite structure
42
describe the expected binary compound structure of the ionic compound NiAs
similae to ccp NaCl structure, with As in hcp arrangement and Ni in octahedral intersitial sites, this is a more common structure for compounds with more covalent character
43
why is there no hcp equivalent to the antifluorite structure?
this is because in hcp structure, tetrahedral sites share a face, meaning they are quite close together the similarly charged ions of the same element that would be placed there would be too close and experience repulsion, so this doesn't work in practise
44
do all the intersitial sites need to be filled in binary compound structure?
no, it is common for only 1/2 the tetrahedral sites to be filled for example
45
how many intersitial sites foes a primitive cubic structure have + what kind?
they have 1 cubic intersitial site, which is the centre of the unit cell
46
can non-binary compounds adopt structures based on close packing?
yes - they can sometimes be described with multiple unit cell structures which build up to form the same overall structure
47
how are unit cells unrealistic?
they are perfect models - in reality solid structures aren't perfect and often have defects in the arrangements of atoms/ions
48
point defect definition
defects that occur around an atom/ion - can be intrinsic or extrinsic
49
what is the difference between intrinsic defects and extrinsic defects?
intrinsic defects do not change the composition of the solid whereas extrinsic defects involve the insertion of foreign atoms
50
what are the 2 types of point defect?
schottky defects frenkel defects
51
what are schottky defects
aka vacancy defects - these occur when some lattice sites are unoccupied, there needs to be vacancies from all ions in formula unit to maintain charge neutrality (so 1 missing anion per missing cation /vice versa)
52
what are frenkel defects?
these occur when an atom/ion moves into an intersitial site creating a vacancy at it leaves its own spot - in ionic solids this normally involved movement of a cation as they tend to be smaller
53
how do structures with frenkel defects compare energetically to perfect unit cell structures?
a structure with a frenkel defect would be higher in energy as energy is needed to move one ion into the intersitial site however energetically a perfect solid is impossible, as although it is favoured by enthalpy, it is disfavoured by entropy structures with frenkel defects have greater entropies as any ion could be moved around into an incorrect position
54
how do defects affect stability of structures?
defects are entropically favoured, so they increase stability/decrease gibbs energy
55
when do extrinsic defects occur?
these occur when impurities are present - this is why gemstones exist
56
give one benefit of extrinsic defects
extrinsic defects are helpful for improving conduction if more conductive elements substitute with ions/atoms the substance - this is useful for electronics

CH1.2 foundations of chemistry (32 decks)

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