Structures Flashcards
learn for my exam
1
Q
we can classify solids in three ways
A
degree of order, chemical bonding, structures and symmetry
2
Q
what are the three types of degrees of order?
A
Long-range order refers to a consistent and repeating arrangement of particles eg, crystals
Long-range order with extended defects-Crystals with disorders in particle structure eg, ionic conductors
Short-range order local, limited arrangement of atoms, ions, or molecules consistent and organized over a short distance eg, Ampohomophus structures
3
Q
covealent bonding causes what
A
extreme hardness eg diamond
4
Q
what does covalent bonding do?
A
cause ionic conductiveness
5
Q
metal bonding does what?
A
high conductivity at low temps
6
Q
Crystalline:
A
A solid characterized by periodic or repeating arrays over large
atomic distances
7
Q
Amorphous (Non-crystalline):
A
A solid that may have short-range order, but
no long-range order
8
Q
what is the unit cell?
A
basic building block (simplest structural unit) of a crystal structure;
defines crystal structure geometry & atom position- parallel-sided region from which entire structure can be formed
9
Q
what is the motif?
A
Motif: characteristic structural features
10
Q
how many unit cell geometries are possible?
A
7
11
Q
in hexagonal what are the internal angles?
A
alpha and beta = 90
gamma= 120
12
Q
in cubic or isometic what are the interaxial angles?
A
all 90
13
Q
what about tetragonal?( think of a rectange)
A
all angles 90 but only a=b not also c
14
Q
what about trigonal?
A
alpa+beta=90 but not gamma= 120
15
Q
in fractional geomirtys what is an exact face
A
100 think central cees face is 100.
16
Q
a corner in fractional geometries?
A
(0, 0, 0) is at the corner.
17
Q
what is the occupancy factor
A
number of atoms on one particular side
18
Q
what are rectangular cells?
A
Unit cells that aren’t cubic
19
Q
What is the occupancy factor of the rectangular cells?
A
Atoms completely in unit cells = 1.0
Atoms on the face of the rectangle = 0.5
Atoms on the edge of the unit cell = 0.25
Atoms on the corner = 0.125
20
Q
what is the most accurate model for atom showing?
A
hard sphere is realistic, doesn’t reduce the atoms we can note how porous
21
Q
in the close packed structure what is visible
A
triangular vertices- vertex up is B vertex down is C- when adding the b layer note that c is still exposed. Ie the C vertices is still exposed.
22
Q
Hexagonal Close Packed Structure (hcp) is what kind of structure?
A
ABAB… Stacking Sequence.
Coordination numbers for each metal: [12]- indeed on the A face we even see- a 6 hexagon Atomic packing factor APF = 0.74
23
Q
give an example of HCP
A
Mg,zn cd,Ti
24
Q
what are the cubic close-packed structures?
A
Cubic Close Packed Structure (ccp or fcc); ABCABC…Stacking Sequence- Unit cell contains 4 atoms
25
give an example of the cubic packed structures?
Eg, Cu, Ag, Au
26
difference between hcp and ccp?
CCP: The packing sequence is ABCABC.- stacking sequence means that the third layer of atoms (C) is placed so it doesn’t lie directly over either of the previous two layers
HCP: The packing sequence is ABAB
27
what is a special feature of FCC?
FCC structures can be plastically be deformed;which means it means they can be permanently reshaped without breaking
28
The body centred cubic structure?
Sort of rare in chemistry due to its low efficiency of 68%, only 2 atoms, Coordination number [8]+[6] depending on how big the atom is
29
what are some examples of the BBC models
Fe, Cr. Mo,
W, Ta, Ba
30
Primitive Packing (α-Po structure)
Unit cell contains 1 atom this means that all the cornes make up only 1
Atomic packing factor APF = 0.52
Coordination number: [6] * rare
31
Zn, Cd:
distorted hcp; elongation in c-direction: c/a = 1.86
C.N. = [6] +[6]: 6 neighbours from adjacent planes; 6 ca. 10% further
away in co-planar planes
32
Hg:
distorted hcp; Compression in c-direction; C.N. = [6] + [6]
33
Ga:
Undulated sheets ; C.N. = [1] + [6]; Ga pairs; 1 neighbour from next
sheet + 6 within the sheet
34
Many metals are polymorph (polytype structures), what does this mean?
Depending on the temperature, metals have different structures
35
outline the polimorphic nature of iron.
α-Fe (bcc) heating
beta -Fe (bcc) heating
gamma-Fe (fcc)
36
what is the goldsmiths rule?
temp up - cn down
pressure up - cn up
37
what are alloys?
An alloy is a mixture of two or more elements, where at least one is a metal, and the result has metallic properties.
38
A1
FCC
39
A2
BCC
40
A3
HCP
41
NI and CU ( The alloy example ) Similar
FCC, crystallize in fcc lattices and their unit cells have very
similar lattice parameters , But if we heat Cu and Ni to melting, mix the melts, and cool slowly, the fcc packing is retained, but with a random placement of the two elements
42
Yet, if we, Elemental Cu and Au
each use fcc packing but have very different lattice parameters (Au >> Cu). Reaction (melting and cooling) yields a specifically ordered arrangement = an intermetallic compound, which may not conform to oxidation state rules
43
Solvent atoms vs solute atoms?
Solvent atoms = majority element;
Solute atoms = minority element
44
Substentutional solutions
Repaces atoms of one chemical species on the crystal lattice of with another species
Usually happens with atoms of compatible properties- will litrally nock out parts of the crystal structure
45
Random alloy
Atoms of two or more elements have a random distribution over lattice sites
46
interstitial phases- hagg structures
Metallic phases incorporate non-metal atoms (H, C, N), The Radii of the metal atom need to be large enough so that non-metal atoms can be accommodated in the interstitial spaces without significantly changing the overall structure
Let's say hole filling….
To see if these phases can form, we must consider the radii
47
what is the haggs border rules? Aswell as the critical radius?
r(non metal) /r(metal)- must be less than or equal to 0.59- metal's radius should be larger than 1.35 Å
48
to form a solid solution what is the rule?
Differences between atomic radii: max 15% Large electronegativity differences lead to strains within the lattice,
49
Ordered alloy
An ordered alloy is a solid solution where different types of atoms occupy specific, regular positions in the crystal lattice — forming an ordered pattern. Fully ordered structure forma s super lattice. CUAU
50
what are the properties of haggs phases?
Very hard materials (Mohs hardness greater than 8.5); the interstitial atoms hamper that layers can be displaced concerning each other
Fire resistant; Metallic appearance, conductivity (carbide>nitride)
Very high melting points (3000 – 4000 oC)
Mohs hardness is a qualitative scale that ranks minerals (or materials) based on their ability to resist scratching.
51
why isn't NACL and interstitial solid/
NaCl doesn’t have small atoms squeezing into interstitial gaps — instead, Na⁺ and Cl⁻ ions alternate in a fixed, repeating pattern based on electrostatic attraction, not size-based filling of gaps
52
if we have a Cubic close packing and we fill all octahedral holes-
NACL structure
53
why cant we fill all of the holes in the hexagonal close packing?
octahedral holes; directly above each other atoms come too close- but we can fill half to form- That means some C atoms will lie directly above each other,
But carbon atoms repel each other at such close distances,
Result? Unstable structure due to C–C crowding
M2C (M = V, Nb, Ta, W)
54
what are ionic carbides?
Ionic carbides are compounds formed between carbon and electropositive metals (typically alkali metals, alkaline earth metals, or early transition metals) where carbon acts as an anion( look at the peroidic table for this )what
55
what is needed for carbides?
Carbides of metals with rC < 1.30 Å (e.g. Cr, Mn, Fe, Co, Ni)
56
what are the features of interstitual carbides carbides?
Carbides of Cr, Mn, Fe, Co, Ni are more reactive than interstitial
carbides
Hydrolyse in diluted acid (liberation of H2; CH4; C2H6)
Reactivity between ionic and interstitial carbides, do not react with water; T > 1000oC: HNO3 or HF
attack carbides- c-c zig zag
57
what are the interstitial nitrides?
in metal nitrides with the general formulae MN, M2N, and M4N; N atoms occupy octahedral holes in ccp and hcp metal lattices
58
Square faces:
are cross-sections of octahedral holes
59
Triangular faces:
are cross-sections of tetrahedral holes
60
Interstitial Metal Phases- like previously what was mentioned?
triangular and square faces
61
Frank-Kasper-Phases
These phases never use square-faced polyhedra — they rely on geometries that optimize volume filling without gaps or awkward angles
62
Laves Phases
Laves phases are a group of intermetallic compounds with a specific stoichiometry and structure:
AB2
-A = larger metal atom( Cu/Zn/Ni)
-B = smaller metal atom (Mg)3 important structure types of intermetallic phases with composition AB2: MgCu2, MgZn2 and MgNi2.Extremely dense packing
63
laves phases form 2 tyoes of structures-
for cordination polihedrom -
1. Distorted Icosahedron
(CN 6 A +6 B) - Coordination polyhedra of smaller B atoms (Cu/Zn/Ni)
2. 2nd polyhedra CN =16: the bigger A-(Mg) atoms are surrounded by 12 B- und 4
A-atoms.
64
what is the 8-N rule for?
covalent bonds- Atoms form bonds to achieve a stable electron configuration, similar to a noble gas configuration.
65
what is the principle of maximum conductivity?
Elements in periods 3 and beyond, such as sulfur, phosphorus, silicon, etc,, (below) Atoms tend to prefer single bonds over double or triple bonds.
oxygen has 2 bonds as it is at the top of the period. - This is because the larger atomic size and the availability of d-orbitals in these elements allow for more flexible bonding but often favor single bonds to minimize strain or repulsion.Indeed, if we go to further down periods or higher periods, more metallic features are available.
66
pressure homogus rule-
If an element A (like Si) adopts a certain structure only under high pressure, then a heavier element B in the same group (like Ge or Sn) might already naturally crystallize in that structure at normal pressure
67
when does the α-Polonium Structure form
It also occurs in high-pressure modifications of P and Sb.
68
what is the length shortening occur?
There is a counterintuitive model here : When pressure is applied the short bond distances between the double bonds don't become shorted, they become longer. ( Single bond has the longest bond length)- “When the coordination number increases according to
the coordination number, the inter-atomic distances also increase
69
what happens with phosphourous?
Purple phosphorous forms by slow crystallisation at temperatures around 550oC
White phosphorus (P₄) can be converted into red phosphorus through heating under controlled conditions.
Black phosphorus Is the thermodynamically most stable form of phosphorous at normal conditions, key word; zig zag layer
70
what are fullerenes?
Modifications of carbon that consist of cage molecules c60 and c70
71
diamond like structures are like what? give examples?
Cubic Structure - Diamond, silicon, germanium and α-tin (stable below 13oC) and hexagonal cp
72
diamonds are what in nature?
Insulator, colourless.
C.N. = 4 (in graphite C.N. 3)
Bond distance 154 pm, bond angles 109.47o
there is also hexagonal diamond
73
how can you explain metal conductivity? -
Metal conductivity:
Arsenic (As), Antimony (Sb), and Bismuth (Bi) show metallic conductivity at high pressures. This can be explained by band theory, which suggests that at high pressure, the atomic structure becomes more compact, allowing electrons to move freely, leading to metallic behavior.
74
how do you form a wurzite structure?
In the hexagonal binary diamond substitute - Substituting alternately C atoms with S and Zn results in wurzite structure
75
how do you form a sphalerite, zinc blende structure?
Substituting alternately C
atoms with S and Zn
results in ZnS structure
76
Binary Diamond-type Compounds- what can they do for the industry?
nearly as hard as diamond: abrasives
SiC also used for heating devices for high temperature furnaces, as it is a
semiconductor with a sufficiently high conductivity at high temperatures, highly
corrosion resistant with low thermal expansion
CdS(yellow), CdSe (red): colour pigments
ZnS: luminophore in cathode ray displays
77
Octahedral Interstices in Hexagonal Close Packing
What happens if we fill them completely?
exactly 1 vertesces in this ababab packing- Central atoms come very close electrostatically, not ideal
If these interstices are filled with atoms, they will form octahedral coordination between atoms, meaning each atom in the interstice is surrounded symmetrically by six atoms (forming an octahedron).
When these interstitial atoms come too close to each other, the electrostatic repulsion between them can become significant. The atoms in the octahedral interstices are close enough to experience repulsive forces, which makes the arrangement less stable compared to configurations where atoms are further apart.
78
what size are tetrahedral holes?
Tetrahedral holes are
approx. ½ the size of
octahedral holes
79
if we fill the tetrahedral holes in hexagonal close pacing
If we occupy all the tetrahedral holes,
Exactly 2 interstices per sphere Face sharing tetrahedra central atoms come too close not stable
80
in the cubic close packed-
Exactly
1 OH and 2 TH interstice per sphere
No Face sharing polyhedra
81
Cubic Close Packing and fill all the octahedral holes what structure do we get?
NACL- this gives A γ B α C β A stacking sequence- Preferentially in ionic structures
82
filling all of the hexagonal octahedral-
Hexagonal Close Packing and fill all the octahedral holes →NiAs- Structure
Electrostatically, it is not so favoured since the Ni-Ni distances are rather close: Face-sharing octahedra
Formation of metal-metal bonds Structure just occurs if d electrons are available
83
MX2 compounds are what?
close-packed
Half of the octahedral holes are occupied- CdCl2 AγB□CβA□BαC□
Cdl2
MgCl2, FeCl2, Cs2O
AγB □- Every intermediate plane is half occupied- this gives layered matherials
84
what are Frank-Kasper-Phases
Coordination polyhedral do not contain
square faces
85
what is rutile
no square faces mx2 close packed filling half of the octahedral holes - (3,6)-connected net
86
MX3 compounds
are like what?
layered materials, 1/3 of the octahedral holes are occupied. AlCl3 Aγ2/3B□Cβ2/3A□Bα2/3C□
