CLASSIFICATION OF ELEM ENTS AS METALS, NON-METALS AND SEMI-MTALS
In addition to the classification of elements into s-, p-, d and f-blocks, it is possible to divide them into metals, non-metals and metalloids. More than 78% of the elements are metals. Metals are present on the left side and the centre of the periodic table.
Metals are the elements which are malleable and ductile, possess luster, are good conductors of heat and electricity and have high densities. Metals usually have high melting and boiling points, and are generally solids at room temperature. Mercury is the only metal which is liquid at room temperature. Gallium (303 K) and caesium (302 K) also have very low melting points.
Non-metals are much less in number than metals. There are only about 20 non-metals. Non-metals are located at the top right hand side of the Periodic Table. Non-metals have low melting and boiling points. They are usually solids or gases at room temperature. Non-metals are neither malleable nor ductile. They are poor conductors of heat and electricity. In a period, the non-metallic character increases as we move from left to right. In a group; the non-metallic character decreases and metallic character increases on going down a group. There is no sharp line dividing metals from non-metals. A zig-zag line separates metals from non-metals as shown in Fig. 6.3. The borderline elements such as silicon, germanium, arsenic, antimony and tellurium exhibit characteristic properties of metals as well as non-metals. These elements are called semi-metals or metalloids.
Fig. 6.3. Position of metals, non-metals and metalloids in the periodic table.
Example 6.1. Write the electronic configuration of the elements given below and also predict their period, group and block. A (At. No. 16), E (At. No. 37) and G (At. No. 30).
Solution. The electronic configurations of the elements are:
16A : 1s2 2s2, 2p6, 3s2, 3p4
37E : ls2, 2s2, 2p6, 3s2, 3p6, 3d10, 4p2, 4p6, 5s1
30G : 1s2, 2s2, 2p6, 3s2, 3 p6, 3d10, 4s2.
As is evident from the electronic configurations:
(i) 1 6A receives last electrons in 3p sub-shell, therefore, its:
block= p -.
group= 10 +Number of valence electrons
= 10 + 6 = 16.
(ii) 37E receives last electrons in 5s-orbitals. Hence its:
group = Number of valence electrons
(iii) 30G receives last electrons in 3d-orbitals. Hence, its:
Since its valence shell is 4th. Therefore,
group = Number of n s-electrons
+ Number of (n- 1) d-electrons
= 2 + 10 = 12.
Example 6.2 How would you justify that there are only 18 elements in the fifth period of the periodic table?
Solution. The fifth period begins with the filling of 5s orbital and continues till the filling of sixth energy level (6s) starts. The subshells which follow 5s are, 4d, 5p, 6s …… Thus, the elements which involve filling of 5s, 4d and 5p sub-shells are accommodated in the .fifth period. These sub-shells have nine orbitals that can accept 18 electrons in all. Hence, there are 18 elements in the fifth period.
PERIODIC TRENDS IN
Most of the properties of the elements such as atomic volume, atomic size, ionization enthalpy, electron affinity and electronegativity are directly related to the electronic configuration of the atoms. These properties undergo periodic variation with the change in the atomic number within a period or a group. These properties indirectly control the physical properties such as melting point, boiling point, density, etc. Let us now proceed to study the variation of some of the atomic properties in the periodic table.