Bonds in metals
A) Crystal structure of metals
- 60 % of metals have close packed
cubic- or hexagonal crystal structures:
ccp (ABCABC…), hcp (ABAB…)
- 40 % of metals have body centered
cubic structure: bcc
ccp, hcp: 74 % of available space is
occupied
bcc:
67 % “
“
“
“
“
ccp, hcp: coordination number is 12
bcc:
coordination number is 8
ccp:
Cu, Ag, Au, Pt
hcp:
Zn, Cd, Hg
The band theory
- Formation of bands
- metal – semiconductor –
insulator
0
0 – 5 eV
> 5 eV band
gap
- Doped semiconductors
Overlapping of
filled “3s”band
with empty “3p”
band of Mg
crystal. As a
result ...
Overlapping of half-filled
“3s” band with empty “3p”
band of Na crystal.
Consequences of metallic structure
1.Metals can conduct electricity.
2.Metals are conductors of heat.
They absorb heat as electrons become
thermally excited to low lying vacant
orbitals in a conduction band.
3.Metals have lustrous appearance.
Mobile electrons can absorb a wide range
of wavelength of radiant energy as they
jump to higher energy levels. They can
immediately emit photons of visible light
and fell back to lower levels within the
conduction band.
Consequences of metallic structure
(continuation)
4. Metals are malleable and ductile.
As bond are broken, new ones are
readily
formed.
Other crystalline solids with
metal properties
1.Alloys – solid solutions, intermetallic
compounds (fixed stoichiometry)
e.g. – TiAl – titanium aluminide
2.Interstitial solid solutions
- small non-metal atoms (H,B,C,N)
occupy interstitial positions
(interstices) in the crystal lattices
of d-metals
- octahedral sites
- tetrahedral sites
Noncrystalline metals
1) Amorphous metals, metallic glasses,
nanocrystalline metals
X-ray diffraction (0.1 nm, Roentgen);
Bragg eqn.
2d sin = (d is distance between
layers, at
angle the X-rays interfere
positively)
To obtain:
- extremely rapid
cooling
- electrolysis
2) Covalent bond between metal atoms,
Na
2
X-ray diffraction (Roentgen)
2d sin = (d is distance between
layers, at
angle the X-rays
interfere positively)
Amorphous alloys
A new promising alloy (1992):
Liquidmetal is more than twice as strong as
titanium and steel, doesn’t rust and can be cast
like plastic and honed to an edge as sharp as
glass.
The alloy is made of elements that fit very
poorly together: titanium, copper, nickel,
zirconium and beryllium. These elements’
atoms are of different sizes so they don’t
readily form crystals, even when cooled slowly.
Liquidmetal doesn’t shrink when it solidifies.
Can be cast with a precision down to 1 micron.
Metal clusters
Formation of metal clusters
Icosahedron, 13
atoms,
dwudziestościan
Mass
spectrum