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[edit] structure of metals

The structures[1] adopted at normal temperatures are colour coded and shown below, the only exception is mercury, Hg, which is a liquid and the structure refers to the low temperature form. The melting points of the metals (in K) is shown above the element symbol.

bcc
body centred cubic		 hcp
hexagonal close packed		 fcp
face centred cubic (cubic close packed)		 unusual structure not known / uncertain non metal



H He
453.69
Li
bcc		 1560
Be
hcp		 B C N O F Ne
370.87
Na
bcc		 923
Mg
hcp		 933.47
Al
fcc		 Si P S Cl Ar
336.53
K
bcc		 1115
Ca
fcc		 1814
Sc
hcp		 1941
Ti
hcp		 2183
V
bcc		 2180
Cr
bcc		 1519
Mn		 1811
Fe
bcc		 1768
Co
hcp		 1728
Ni
fcc		 1357.8
Cu
fcc		 692.68
Zn
 301.91
Ga		 Ge As Se Br Kr
312.46
Rb
bcc		 1050
Sr
fcc		 1799
Y
hcp		 2128
Zr
hcp		 2750
Nb
bcc		 2896
Mo
bcc		 2430
Tc
hcp		 2607
Ru
hcp		 2237
Rh
fcc		 1828
Pd
fcc		 1235
Ag
fcc		 594
Cd		 430
In		 505
Sn		 904
Sb		 Te I Xe
302
Cs
bcc		 1000
Ba
bcc		 2506
Hf
hcp		 3290
Ta
bcc		 3422
W
bcc		 3186
Re
hcp		 3033
Os
hcp		 2446
Ir
fcc		 1768
Pt
fcc		 1337.33
Au
fcc		 234.32
Hg		 577
Tl
hcp		 600.61
Pb
fcc		 544.7
Bi		 Po At Rn
Fr Ra
bcc		 Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uup Uuh Uus Uuo
↓
La
 Ce
fcc		 Pr
 Nd
 Pm
hcp		 Sm Eu
bcc		 Gd
hcp		 Tb
hcp		 Dy
hcp		 Ho
hcp		 Er
hcp		 Tm hcp Yb
fcc		 Lu hcp
Ac
fcc		 Th
fcc		 Pa U Np Pu Am
hcp		 Cm
hcp		 Bk Cf Es Fm Md No Lr


[edit] Close packed metal structures

A simple model for the structure of metals is to assume that the metal atoms are spherical and are packed together in the most efficient way which is termed close packing or closest packing. In this every atom has 12 equidistant nearest neighbours, and therefore a coordination number of 12. One way of describing the differing possible arrangements is to consider the structure as being composed of layers that differ only in respect of how one is placed relative to the others. Whilst there are many ways can be envisaged for a regular build up of layers the two most common in metals are:

  • A,B,A,B, ......... termed hexagonal close packing (also termed P63/mmc, Pearson symbol hP2, strukturbericht A3. Alternate layers are directly above/below each other.
  • A,B,C,A,B,C,.......termed face centred cubic (also termed cubic close packing, Fm3m , Pearson symbol cF4, strukturbericht A1) Every third layer is directly above/below each other.

These two structures are adopted by many metallic elements.

[edit] Hexagonal close packed

In the ideal hcp structure the unit cell axial ratio is 1.633, However there are deviations from this in some metals where the unit cell is distorted in one direction but the structure still retains the hcp space group. In others e.g. zinc the deviations from the ideal change the symmetry of the structure.

[edit] Face centered cubic (cubic close packed)

Close acked planes --implications for glide/slide.

[edit] Body centred cubic

This is NOT a close packed structure. In this each metal atom is at the centre of a cube with 8 nearest neighbours, however the 6 atoms at the centres of the adjacent cubes are only approximately 15% further away so the coordination number can therefore be considered to be 14 when these are included. Note that if the if the body centered cubic unit cell is compressed along one 4 fold axis the structure becomes face centred cubic (cubic close packed).

[edit] Trends in melting point

Melting points are chosen as a simple, albeit crude, measure of the stability or strength of the metallic lattice. Some simple trends can be noted. Firstly the transition metals have generally higher melting points than the others. In alkali metals (group 1) and alkaline earth metals (group 2) the meltingpoint decreases as atomic number increases, but in the transition metals the melting points if anything increase.

[edit] Unusual structures

Metal structure family coordination number notes
Mn cubic distorted bcc - unit cell contains Mn atoms in 4 different environments [1]
Zn hexagonal distorted from ideal hcp. 6 nearest neighbours in same plane- 6 in adjacent planes approx. 10% further away[1]
Ga orthorhombic each Ga atom has one nearest neighbour at 244pm, 2 at 270pm, 2 at 273, 2 at 279pm. [1] The structure is related to Iodine.
Cd hexagonal distorted from ideal hcp. 6 nearest neighbours in the same plane- 6 in adjacent planes approx. 10% further away[1]
In tetragonal slightly distorted fcc structure[1]
Sn tetragonal 4 at 302pm; 2 at 318pm; 4 at 377; 8 at 441pm [1]
Sb rhombohedral puckered sheet; each Sb atom has 3 neighbours in the same sheet at 290.8pm; 3 in adjacent sheet at 335.5 pm. [1] grey metallic form.
Hg rhombohedral 6 nearest neighbours this structure can be considered to be a distorted hcp lattice with the nearest neghbours in the same plane being approx 16% further away [1]
Bi rhombohedral puckered sheet; each Bi atom has 3 neighbours in the same sheet at 307.2 pm; 3 in adjacent sheet at 352.9 pm. [1]
Po cubic
La hexagonal 12 nearest neighbours "double hcp" with a layer structure ABAC...[2]
Pr hexagonal 12 nearest neighbours "double hcp" with a layer structure ABAC... [2]
Nd hexagonal 12 nearest neighbours "double hcp" with a layer structure ABAC...[2]
Sm hexagonal 12 nearest neighbours complex hcp with 9 layer repeat, ABCBCACAB....[2]
Pa tetragonal body centred tetragonal unit cell, which can be considered to be a distorted bcc
U orthorhombic
Np orthorhombic [3]
Pu monoclinic

[edit] Properties of metals

H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uup Uuh Uus Uuo
↓
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr


[edit] metallic bond

Simple view that sea of electrons. Other theories used to explain are: Free electron theory http://www.teknik.uu.se/ftf/education/ftf1/forelasningar/overview/Freelectronmodel.pdf "


The free electron theory of metals, first propounded by Drude and Lorentz in the early years of the 20th century, considers that some electrons act as if they were able to move freely within the solid. Free electrons are held in the solid by a potential well or box, but are not affected by the local potential associated with individual atoms.

The Bloch wave theory represents a development of the free electron theory."

Band theory (Bloch, Brillouin)

The nearly free electron model

Atomic orbital akin to MO theory- tightly bound model







[edit] References

  1. ^ a b c d e f g h i j Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
  2. ^ a b c d A.F Wells (1962) Structural Inorganic Chemistry 3d Edition Oxford University Press
  3. ^ hume rothery

[edit] Disulfur dinitride

[edit] Disulfur dinitride Intro

[edit] Preparation

Passing gaseous S4N4 over silver metal wool at 250-300 at low pressure (1mm Hg) yields the unstable dimer S2N2.
The silver first becomes sulfided due to thermal decomposition of the S4N4, and the resulting Ag2S catalyzes the conversion of the S4N4 into the four-membered ring S2N2, [1]

S4N4 + 8 Ag → 4 Ag2S + 2 N2
S4N4 → 2S2N2

An alternative uses the less explosive S4N3Cl.

[edit] Structure and bonding

The S2N2 molecule is virtually square and planar. The S-N bond lengths are 165.1pm and 165.7pm and the bond angles are very close to 90. The bonding has been investigated using a spin-coupled valence bond method [2] and is described as having four framework sigma bonds, with the N atoms bearing a high negative charge and the S atoms a corresponding positive charge. Two π electrons from the sulfur atoms are coupled across the ring making the molecule overall a singlet diradical.

[edit] Reactions

S2N2 decomposes explosively above 30, and is shock sensitive. It forms adducts with Lewis acids via a nitrogen atom ,e.g. S2N2.BCl3, S2N2.2AlCl3, S2N2.SbCl5, S2N2.2SbCl5[3]

==See also== ==References== Wiberg holleman Inorganic Chemistry 2001 ElsevierISBN 0123526515