58 STRUCTURE AND BONDINGBut it is found that certain very electronegative atoms or groups,
notably fluorine, can cause expansion of the valency shell, and
further orbitals of higher energy—the d orbitals—can be hybridised
with the 5 and p orbitals. Consider phosphorus, with five valency
electrons; these can be placed either in four tetrahedral sp^3 hybrid
orbitals (with one orbital doubly occupied) or singly in five orbitals
formed by hybridisation of one 3s, and three 3p and one 3d (sp^3 d)
to give a trigonal bipyramidal shape (Table 2,X). sp^3 mixing is
found in the phosphine molecule PH 3 , while sp^6 d is found in the
phosphorus pentafluoride molecule PF 5. Similarly with sulphur, sp^3
mixing with two lone pairs is found in the H 2 S molecule while
sp^3 d^2 mixing gives six octahedral orbitals as found in the SF 6
molecule. It will now become apparent that all the common molecular
shapes given in Table 2.8 can be accounted for by assuming appropriate
hybridisation of the orbitals of the central atom—sp, linear: sp^2 ,
trigonal planar: sp^3 , tetrahedral: sp^3 d, trigonal bipyramidal and
sp^3 d^2. octahedral.THE BONDING IN METALS
We have seen that in a metal the atoms are close-packed, i.e. each
metal atom is surrounded by a large number of similar atoms
(often 12, or 8). The heat required to break up 1 mole of a metal into
its constituent atoms is the heat ofatomisation or heat of sublimation.
Values of this enthalpy vary between about 80 and 800 kJ, for metals
in their standard states; these values indicate that the bonds between
metal atoms can vary from weak to very strong. There is a rough
proportionality between the m.p. of a metal and its heat of atom-
isation, so that the m.p. gives an approximate measure of bond
strength.
Here we can discuss the nature of metallic bonding only in a
qualitative way. The bulk metal may be pictured as consisting of
positively charged atoms embedded in a "sea' of free valency
electrons. There are, therefore, no localised bonds, as in a giant
covalent crystal like diamond. The freedom of the electrons is
shown by their ability to move in an electrical field, so bestowing
electrical conductance on the metal. The strength of the metal bond-
ing (as measured by the heat of atomisation) is determined essen-
tially by (a) the size of the atoms, increase in size decreasing the
heat ofatomisation and (b) the number of valency electrons, increase
in the number of valency electrons increasing the heat of atomisa-
tion. In the close-packed metal structure of, for example, sodium.