Inorganic - Metallic bonding and semiconductors Flashcards
describe what orbitals look like in metallic bonding
electrons in large delocalised orbitals, the size of the piece of metal
how is strength of metallic bonding measured?
looking at the energy required to vaporise a metal (solid –> gaseous atoms)
three patterns of packing in metallic bonding
hip, ccp, bcc
hexagonal close-packed
cubic close packed
body centred cubic
key feature of metallic structures
large number of “bonds” associated w each atom – 8-12 nearest neighbours
rationalise why some metallic rather than covalent bonding is more favourable for elements
energetically more favourable for some element to have 8 slightly weaker bonds in metallic form than one stronger bond
how is metallic liquid hydrogen formed? what is its structure?
formed when solid H2 is subject to extreme pressure, interatomic distances become equal and metallic liquid hydrogen is formed
why are elements on the LHS and bottom of the table more likely to form metallic bonding?
Zeff increases across the table and orbital size degreases, eventually small enough for EFFICIENT OVERLAP –> localised bonding
larger orbitals –> weaker bonding, better to have larger number of less directional delocalised bonding interactions
what do orbitals of 1-D metals form?
bands
spacing between individual crystal orbitals (COs) is so small that it is effectively a continuous range of energies
energy difference between lowest and highest energy orbitals in an infinite chain vs in a diatomic
exactly twice that of the diatomic
define band width
the difference in energy between the lowest bonding orbital and the highest anti bonding orbital
what factor does band width depend on?
depends on the degree of possible overlap between the AOs
greater degree for the smaller orbitals causes greater band width
explain conductivity in terms of bands
if a band is completely full –> insulator
partially empty: e- field applied, e- near the top of the filled part can move into vacant higher energy orbitals
net mvmd of e- in one direction, ie. conducts
why can Be still conduct electricity even though it has 2 s e-s?
in theory, 2s band would be filled and hence not conduct
BUT 2p bands must be considered - the 2s and 2p bands overlap (at equilibrium bond length)
hence some e- occupy the strongest bonding part of the 2p band instead, leaving some of both bands vacant
how does internuclear separation affect band width and gap?
greater overlap = increased band width
hence as internuclear separation gets further away from equilibrium bond length, less band overlap and eventually a band gap forms
describe the features of semimetals
fewer nearest neighbours than metals
usually poorer conductors (due to very small band gap or overlap)
more localised bonding compared to metals
as the temperature increases, the conductivity of both metals and semimetals…
decreases
degree of orbital overall disrupted by thermal vibrations
features of semiconductors
conduct electricity very poorly
band gap 0.1-4 eV separate filled valence band and vacant conduction band
conductivity increases w/ temp or irradiated with light of suitable wavelength (allows e- to jump the band gap, leading to partially filled bands)
features of non-metals
electrical insulators
filled valance band and vacant conduction band separated by more than 4eV
main cause of transition from metals to nonmetals
the trend in the size of valence orbitals
bigger Z(eff) –> smaller orbital, more localised bonding and fewer nearest neighbours
going down, bigger orbitals –> more delocalised bonding and nearest neighbours
how do MOs of non-metals interact in solid form?
in gas, large separation, separate MOs. in solid, MOs interact and form bands
stronger interaction = larger band width
but large energy difference between sigma and sigma star MOs –> bands do NOT touch, cannot conduct –> insulator
describe the MOs in graphite and explain why it conducts electricity
say each C is sp2 hybridised –> three sigma bonding, 1 pi bonding, one pi* a-b, three sigma* a-b MOs which form four bands in the solid
bonding bands filled
the filled pi and empty pi* bands JUST touch so e- can be promoted to nearby energy levels
conducts in the pi plane
what are dopants and what do they do? (graphite)
doping = treat solid w/ potassium vapour
makes graphite a much better electrical conductor as K atoms fit between C layers and donate an extra e- to the C layer, into the vacant pi* band
also works w Br, which accepts e- instead but still leads to partially empty band
