Inorganic and Materials Chem 2 Flashcards
what is the basic idea behind metallic bonding, how can we get an estimate of the strength of metallic bonding
- metallic bonding is what is seen in metals
- we can obtain an estimate of the strength by looking at the energy required to vaporise the metal
what are the three main metallic structures, what do we mean when we speak about bonds in metals/ why is this slightly problematic
- fcc, bcc, hcp
- when we use the word ‘bonds’ for metals, we don’t necessarily mean the standard 2-centre 2-electron bond that we tend to deal with
- it usually just means the number of nearest neighbours
how can we think of metallic bonding, use lithium as an example to show the difference between metallic and simple covalent bonding
- we can consider it in the same way as normal covalent bonding
- normally lithium is a metal but under certain conditions it can be a diatomic
- Li2 has a bond strength of 238kJ/mol, i.e. formation of 1 mole is 119kJ/mol
- for metallic lithium, enthalpy of atomisation is about 159kJ/mol
- this shows for some elements it is more energetically favourable to have 8 slightly weaker bonds than one stronger bond
using orbital sizes explain why the left side of the periodic table and the bottom of the p-block are metallic
- from left to right, Zeff increases and orbital size decreases, smaller orbitals have more effective overlap, eventually it is efficient enough for localised bonding
- on the right and at the bottom, orbital sizes are greater, this means that bonding is weaker so it is better to have less-directional, delocalised bonding - metals
what happens to the VDW bonding radius and metallic bonding radius and their ratio moving down a group (particularly halogens)
- they both increase but the VDW radius increases slower so their ratio gets closer to 1
- in a true metal, the ratio is 1
what happens to the separation of MO energies when moving from Li2 to Li3 Li4 etc. up to infinity, how does this contribute to a ‘band’ theory
- the energy difference between the most B MO and most AB MO increases as the number of atoms in the chain increases
- at infinity the energy difference is twice that of in Li2
- this is because in Li2 all atoms only form one B or AB interaction
- whereas in at an infinite chain length all atoms form 2 B or AB interactions
- the separation of the orbital energies also decreases so as you tend to infinity, we end up with a virtually continuous range of MO energies - bands
what do we mean by orbital bands, what are its limitations
- when we have a huge metallic lattice e.g. 10^23 atoms, the spacing between individual COs (crystal orbitals) is so small that there is effectively a continuous range of energies from the lowest bonding CO to the highest antibonding CO
- these continuous sections are called bands
NOTE: there is not really a continuous range of energy levels within a band but it is close
what does a density of states graph show
- you can calculate the number of energy levels within a given energy range
define band width
what does it depend on
band width is the difference in energy between the lowest bonding orbital and highest antibonding orbital
i.e. the energy range of the band
- it depends on the degree of overlap possible between the AOs, (greater overlap for smaller orbitals etc.)
when considering energy and orbital bands rather than discrete orbitals, where are the net bonding and net antibonding regions
- the net bonding region is the bottom half of the band
- the net antibonding region is the top half of the band
what can we say about crystal orbitals and the movement of electrons, what condition does this put on conductivity
- COs come in degenerate pairs
- one corresponds to electrons moving one way and the other corresponds to electrons moving the other way
- this means for a completely filled band, there is no net movement of electrons so no conductivity
- if a band is partially empty, when an electric field is applied, some electrons near the top of the filled part of the band can move into the vacant higher energy orbitals and there is a net movement of electrons
- hence, a metal conducts if it has a partially filled band
when are there deviations from our predicted filled bands = monoatomic gas rule
- consider Be, you expect all of the 2s orbital band to be filled leading to net AB and hence monoatomic gas
- This does not occur because at the equilibrium bond length there is an overlap of the 2s and 2p bands
- this means the highest energy 2s band electrons move to the lowest energy 2p band places
- this leads to a net lowering of energy
at what point does an element with a filled valence shell become conductive
- at the point where the ‘HOMO’ band e.g. 2s for Be just starts to overlap with the ‘LUMO’ band e.g. 2p for Be
define/classify metals
- have delocalised bonding with each atom typically having eight to twelve nearest neighbours. The presence of partially filled bands or overlapping bands allow conductivity
define/classify semimetals
- typically have structures with fewer nearest neighbours than metals and are usually poorer conductors of electricity than true metals.
- either due to presence of V small band gap or bands which overlap to a very small degree
what occurs to the electrical conductivity of Metals, Semimetals and Semiconductors with a change in temperature
- metals and semimetals have lower conductivity at higher temps
- semiconductors have higher conductivity at higher temps
define/classify semiconductors
- conduct electricity poorly but the conductivity increases with temperature and when irradiated with light of a suitable wavelength
- band gap of 0.1-4eV
define/classify non-metals
- electrical insulators
- filled conduction band has large energy gap to vacant conduction band
- energy gap usually > 4eV
what does the trend of metals to non-metals follow and why
- the trend follows the trend in the sizes of valence orbitals
- on crossing a period, Zeff increases, valence orbital size decreases, better overlap
- this gives more localised bonding, fewer nearest neighbours
- on descending a group, n increases so valence orbital gets bigger so tends towards metals
consider H2 molecules, explain when it has simple B and AB MOs and when we can consider bands, is solid H2 conductive?
- H2 molecules have B and AB MOs, when gaseous, the atoms are very far apart and do not easily interact
- when the pressure is increased and they are forced together, the MOs from different molecules start to interact to form new orbitals at different energies
- eventually we obtain bands
- the lower energy sigma band and higher energy sigma* band are totally separated with a large energy gap so it is a conductor (unless under extreme pressure)
explain what the form of the MOs at the top and bottom of the sigma and sigma* band in solid hydrogen
- our solid structure still contains molecules but all aligned regularly like in a solid
- bottom of sigma band = many H2 molecules each with bonding MOs all aligned such that there are no nodes between molecules
- top of sigma band = many H2 molecules with bonding MOs but with the maximum number of nodes between molecules
- bottom of sigma* band = many H2 molecules with antibonding MOs but with no nodes between molecules
- top of sigma* band = many H2 molecules with antibonding Mos and the max number of nodes between molecules
how can we consider more complicated structures diamond using a band theory like for solid hydrogen
- in reality it all gets v complicated, v quickly for 3D structures
- but we can take a simplified view and do the same thing
- i.e. with all sigma in phase at bottom of sigma band etc.
explain the structure of graphite
- in graphite there are hexagonal layers of sp2 carbons, ABAB structure
- each have one p-orbital contributing to a pi network over the whole molecule
what orbitals does each atom have in graphite, which bands does this form
- each atom has
three sigma B AOs
one pi B MO
one pi* AB MO
three sigma* AB MOs
this forms bands of
filled sigma band
filled pi band
empty pi* band
empty sigma* band
considering the band system of graphite (sketch out) explain why graphite conducts
- the delocalised pi system itself doesn’t make graphite conductive
- But when sketching out the bands we can see that the filled pi band and the empty pi* band just touch
- this means electrons can easily be promoted to higher energy levels so graphite will conduct in the pi plane
what type of material is graphite
- a semimetal
what is the purpose of doping
- doping is where materials can have improved qualities by treating them with other materials, dopants
explain how/why graphite is doped and what qualities this improves
- graphite is usually doped to improve it’s electrical conductivity
- this is done by either doping with atoms which will donate electrons to give a partially filled pi* band e.g. potassium
- or with atoms which will accept electrons to give a partially filled pi band, e.g. bromine
what other effects does doping graphite cause
- the added atoms are usually between layers increasing interlayer spacing
- it often changes the structure to an AAAA structure
- it increases both conductivity in the plane and perpendicular to the plane
what are the 4 main ways that we can describe bonding
1) LCAO, linear combination of atomic orbitals
2) HAOs
3) VB - Valence Bond model, keep track of all valence electrons using Lewis Structures
4) VSEPR - Valence shell electron pair repulsion - a kind of extension to HAOs but without much focus on known orbitals
Briefly explain the basis of VB
- VB (valence bond) model is a simple way to keep track of valence electrons
- always considers them as localised pairs
- either bonding pairs or non-bonding (lone) pairs
- what we use in curly arrow mechanisms
- resonance structures sometimes required
