Engineering Chemistry S Datta

494 ENGINEERING CHEMISTRY
aggregate of a large number of atoms (n), each offering one orbital for the combination with
others, so there will be a total of n new energy levels similar to ‘n’ number of molecular orbitals
(M.O.s). These large number of energy levels form an energy band by spacing closely one upon
another. The valence electrons from all the atoms will enter every level and a metal thus
consists of energy bands formed by merging of individual atomic orbitals. Let us illustrate the
following cases:
Let us consider Li as an example. The electronic configuration of a Li atom is 1s 2 , 2s 1 .
So, two molecular orbitals are formed by two 2s atomic orbitals from two Li atoms. Similarly
three or four Li atoms would give rise to formation of three or four energy levels by combination
of their 2s-atomic orbitals. Now extending this idea for ‘n’-atoms, all 2s-atomic orbitals will
combine to give a 2s-energy band with n-energy levels. Similar combination of the p-atomic
orbitals will form a 2p energy band containing 3n-energy levels. Each Li atom has only one
valence electron and the total n-electrons from n-atoms will occupy doubly the lower n/2 energy
levels in the 2s-band. When the electrons gain thermal energy or are placed in an electric field.
The electrons are raised to higher unfilled energy levels. This phenomenon explains the high
thermal and electrical conductivities of metals.
–
2p band
E
–
+
+
Combination of
4-atomic orbitals
2s band
Fig. 22.18
Combination of
n Li atoms
An Mg atom has the electronic configuration 1s 2 .2s 2 .2p 6 .3s 2 . The 3s band in Mg is exactly
filled by the 2n electrons of the ‘n’-Mg atoms. One finds that the 3p band (formed by unoccupied
3p atomic orbitals of the Mg atoms) overlaps the 3s band in Mg and there is no energy gap
between the highest occupied and lowest vacant electronic energy levels and Mg is an excellent
conductor.
3p band
3s band
Fig. 22.19 Merger of the 3s and 3p bands in Mg.