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