Solid State Structures Flashcards
which combination of cation/anion gives highest degree of polarisation/high covalent character?
small highly charged cations and large polarisable anions
large covalent character + Born-Landé equation
underestimates lattice energy (actual value obtained from a Born-Haber cycle = higher)
drawback of Madelung constant + interionic distance (r)
requires detailed knowledge of structure which may not be available
value of ‘k’ in Kapustinskii equation
107 900 pm kJ mol-1
predicting bond types - greater electronegativity
greater ionic character
predicting bond types - high electronegativity values
tends to be covalent
predicting bond types - low electronegativities
substance with be an alloy with metallic bonding
iodine
strong covalent bonds between atoms
weak intermolecular interactions
low mp/bp
sublimes with mild heating at atmospheric pressure
silicon
covalent network
each atom forms 4 covalent bonds
covalent network structures
infinite
atoms linked by covalent bonds
examples = boron, phosphorus + boron nitride
allotropy
elements that can form more than 1 structure
e.g. carbon (diamond, graphite, Buckminsterfullerene)
allotropes of tin
β tin (= white) - stable at rtp, metallic
α tin (= grey) - stable below 13°C, non-metallic
polymorphism
compounds that can also form more than 1 structure
e.g. quartz and β-cristobalite = polymorphs of SiO2
unit cell
smallest possible repeating unit for a structure
unit cell - vertex
1/8
unit cell - edge
1/4
unit cell - face
1/2
unit cell - body
1
cell projection diagrams - 0 and 1 meaning
0 = bottom of cell
1 = top of cell
interstitial sites
free space between spheres
how many octahedral sites are there in cubic-close packing?
4
how many tetrahedral sites are there in cubic-close packing?
9
what is an alloy?
contains 2 or more metals or metal + non-metal
why are alloys more commonly used than pure metals?
stronger - different sized atoms ∴ slide less easily
less susceptible to corrosion
substitutional alloys
atoms of 1 metal exchanged for those of another metal
usually occurs when atoms are similar in size (alloy formation doesn’t cause major distortions to lattice)
interstitial alloys
atoms of one metal are present in interstitial sites of another metal lattice
binary compounds
contains 2 elements
close-packed structure (one element in interstitial sites)
most ionic + some covalent described in this way
limiting radius ratio
smallest value of radius ratio - if radius is any smaller, cation-anion contact is lost
enthalpy change of formation
change in enthalpy when 1 mole of a compound is formed in a reaction between its elements in their standard states
enthalpy change of atomisation
change in enthalpy when 1 mole of an element is converted from solid to one mole of gas = 1/2 bond dissociation enthalpy
first ionisation energy
change in enthalpy when one mole of gas is converted to one mole of gaseous cations
electron gain enthalpy
change in enthalpy when 1 mole of gas is converted to 1 mole of gaseous anions = minus the e- affinity
lattice enthalpy
enthalpy change for conversion of 1 mole of ionic solid to gaseous ions
= +ve
value for charge on electron
1.6022 x10^-19 C
value for permittivity of vacuum
8.8542 x 10-12 C2 J-1 m-1
polarisable
large anions - e- density is further from nucleus + less controlled
small cations with higher charges - high charge density
kapustinskii equation
allows ionic radii for polyatomic ions to be estimated
Madelung constant requires detailed knowledge of structure - may not be available
madelung constant
scale factor
accounts for interactions in crystal
-> cations + anions = attractive
-> same charge = repulsive
