Geology Reference
In-Depth Information
Box 7.6 The energy-level structure of lithium metal
Box 7.3 showed that the 1 s orbital overlap between hydro-
gen atoms generates two molecular orbitals: a bonding
orbital (1s σ ), having a lower energy than the atomic orbit-
als, and an anti-bonding orbital (denoted 1s σ *), with a
higher energy. the same happens when two lithium atoms
form a bond.
Because these 2s σ molecular orbitals extend across
the two atoms, the electrons in them are common to
both. When three Li atoms are close enough for overlap
to occur, they share three electrons and form three
molecular orbitals with three distinct energy levels. Four
atoms form four orbitals; five atoms form five; and n
atoms n orbitals. In a crystal of appreciable size, the
value of n is effectively infinite, resulting in a virtually
continuous energy band (bottom of Figure  7.6.1). as it
originates from interaction between s-orbitals, it is called
the s-band. the n valence electrons occupy the lower
(bonding) half. these lower levels represent fixed elec-
tron states, each associated with a particular location in
the crystal.
an electron migrates by transferring to a new molecular
orbital. the only vacant ones are those with energies in
the upper (anti-bonding) part of the band, and promotion of
electrons to these accessible upper levels explains the
electrical conductivity of metals (and the optical proper-
ties derived from it - Chapter 8). electrons may also have
access to higher bands; in Li the hypothetical overlap of
empty p-orbitals makes available a 'p-band', partly
interleaved with the s-band (Figure  7.6). these vacant
levels constitute the conduction band .
Assemblage
Energy levels
Molecular
atomic
2s
Anti-bonding
2s σ *
2s
2s σ
Bonding
2s
2s
2s
Figure 7.6.1 Illustrating the origin of conduction bands in
Li metal.
(a)
(b)
(c)
Empty
conduction
band
Prohibitive
energy jump
to conduction
band
Excitation of electron
to adjacent
conduction band
Conduction band
Full levels
Full levels
Figure 7.6 Energy bands in (a) a conductor,
(b) an insulator, and (c) a semiconductor.
Li metal
conductor
Diamond
insulator
Germanium
semi-conductor
Search WWH ::




Custom Search