Inorganic - Metallic bonding and semiconductors Flashcards

1
Q

describe what orbitals look like in metallic bonding

A

electrons in large delocalised orbitals, the size of the piece of metal

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2
Q

how is strength of metallic bonding measured?

A

looking at the energy required to vaporise a metal (solid –> gaseous atoms)

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3
Q

three patterns of packing in metallic bonding

A

hip, ccp, bcc
hexagonal close-packed
cubic close packed
body centred cubic

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4
Q

key feature of metallic structures

A

large number of “bonds” associated w each atom – 8-12 nearest neighbours

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5
Q

rationalise why some metallic rather than covalent bonding is more favourable for elements

A

energetically more favourable for some element to have 8 slightly weaker bonds in metallic form than one stronger bond

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6
Q

how is metallic liquid hydrogen formed? what is its structure?

A

formed when solid H2 is subject to extreme pressure, interatomic distances become equal and metallic liquid hydrogen is formed

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7
Q

why are elements on the LHS and bottom of the table more likely to form metallic bonding?

A

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

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8
Q

what do orbitals of 1-D metals form?

A

bands
spacing between individual crystal orbitals (COs) is so small that it is effectively a continuous range of energies

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9
Q

energy difference between lowest and highest energy orbitals in an infinite chain vs in a diatomic

A

exactly twice that of the diatomic

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10
Q

define band width

A

the difference in energy between the lowest bonding orbital and the highest anti bonding orbital

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11
Q

what factor does band width depend on?

A

depends on the degree of possible overlap between the AOs

greater degree for the smaller orbitals causes greater band width

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12
Q

explain conductivity in terms of bands

A

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

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13
Q

why can Be still conduct electricity even though it has 2 s e-s?

A

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

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14
Q

how does internuclear separation affect band width and gap?

A

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

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15
Q

describe the features of semimetals

A

fewer nearest neighbours than metals
usually poorer conductors (due to very small band gap or overlap)
more localised bonding compared to metals

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16
Q

as the temperature increases, the conductivity of both metals and semimetals…

A

decreases
degree of orbital overall disrupted by thermal vibrations

17
Q

features of semiconductors

A

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)

18
Q

features of non-metals

A

electrical insulators
filled valance band and vacant conduction band separated by more than 4eV

19
Q

main cause of transition from metals to nonmetals

A

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

20
Q

how do MOs of non-metals interact in solid form?

A

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

21
Q

describe the MOs in graphite and explain why it conducts electricity

A

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

22
Q

what are dopants and what do they do? (graphite)

A

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