3.26 Sspd_chapter 1_part 11_solid state of matter_diifferent  (Page 3/5)

Being the first elements of each period, alkali metals have the largest atomic and ionic radii in their respective periods. As we move within a period, the atomic radius and ionic radius tend to decrease due to increase in the effective nuclear charge. On moving down the group, there is increase in the number of shells and, therefore, atomic and ionic radii increase.

Table 1.16. Electronic configurations and ionization energies of Alkali Metals.

Within the crystalline lattice, conducting electrons can freely move around hence alkaline metals are conductors but as they come to the boundary they are faced with surface barrier potential φ _BS and the energy required to overcome this surface barrier is known as work function W _F = q φ _BS. This sea of conductin electrons belong to the whole lattice and hold together the positively charged ionic cores by what we know as metallic bond.. What is the physical basis of this metallic bond?

In the five alkaline metals, in atomic state the outer most electron has zero kinetic energy and negative potential energy decided by the principal quantum number. The principal quantum numbers are 2, 3, 4, 5 and 6 for Li, Na, K, Rb and Cs. As they come together in sold state. The outermost electron acquire Kinetic Energy which is (3/5)E _F . In Cu Fermi Energy is 9.04eV and average Kinetic Energy is 4.44eV. But sum of Potential Energy and Kinetic Energy is at its minimum . Hence metallic solid configuration is stable. But as we move to Group II and Group III, Kinetic Energy increase due to large number of outer orbital electrons or valence electrons hence metallic bond becomes weaker and eventually it is non-existent. Hence metallic bond exists from Group I to Group II ( transition metals). It also exists in Group III namely Al, Ga, In and Ti. But not beyond this.

Because of the sea of conducting electrons in conduction band, metals have high electrical conductivities as well as thermal conductivities.

Bonding energies and melting points are shown in Table 1.17.

Table 1.17. Bonding Energies and Melting Temperatures of selected substances.

[ “Rudiments of Material Science” by Pillai and Pillai, Talble 1.A, New Age International Publishers 2005 ]

Because of orderly arrangement of the positive ionic centers in a spatial lattice network, metals are highly crystalline.