Transcript Ionic coordination numbers

coordination numbers
A coordination number CN =6 is widely come across in ionic
compounds. Silicon in SiO2 on the other hand has a CN = 4
because Si4+ ion is too small to accommodate 6 oxygen ions.
r/R for silicon is approximately 0.3.
To calculate r/R ratio for a four-fold coordination (i.e. when
CN = 4)
The larger atoms are located at the four corners of a regular
tetrahedron. The small atoms sits in the body centre of the
tetrahedron and touches each of the four atoms at the
corners. The distance between any two corner atoms = 2R
See below
The four corner atoms may be
visualized as occupying the
corners of a cube
Hence, it may be written that the body diagonal
d = 2R + 2r, also d = a √3
But a √2 = 2R or a = 2R/√2
2(R+r) = a √3 = 2R/√2
Or R+r/R = √3/2
Hence, 1 + r/R = 1.224
And r/R = 0.224
Coordination of 12:
all the atoms are the same
size,
each atom have 12 immediate
neighbors.
Solid circles: four neighbors in
the same plane as the central
atom. Dashed circles: four
neighbors above, and four
neighbors below. Each
neighbor also will be
coordinated with 12 neighbors.
Metallic Bonding
- Unlike ionic and covalent solids where the valence
electrons are localized
- The nature of bonding is different from both types.
- like the bi electrons in benzene valence electrons
are delocalized, it can easily move under the effect of
electric field.
- it is best described as positive ions surrounded by a
sea of delocalized electrons.
- CN can be >12 it can exceed 100
Properties and atomic bonding
• Density: determined at at. Wt., at. Radius and the
coordination number (Sig)
• Melting & boiling: depth of the energy well = bond
energy
• Hardness: the height of the total force curve
• Elasticity: the slope of the sum curve, where the net
force is zero. It s also related to the bond energy.
• Thermal expansion: inversely related to the melting
temperature. See Figure
Conductivity of Metals
• Electrical conductivity depends on the nature of atomic
bons.
• Ionic and covalent materials are poor conductors in the
solid state.
• In metals the delocalized electrons can free move along
potential gradient.
• Thermal conductivity is high in metallic bonds, due to
the delocalized electrons are efficient carriers of thermal
as well as electrical energy.