Tendency of atoms of an element to lose electrons and form positive ion is known as electropositivity.
A more electropositive element has more metallic character.
Whether an element behaves as a metal or a non-metal is directly related to its ionization energy. The elements having low values of ionization energies are metals whereas e1ements having high values of ionization energies are non-metals. The border line elements behave as metalloids.
Variation of Metallic Character Across a Period
Metallic character decreases across a period from left to right. On the other hand non-metallic character increases with increase in atomic number across a period. For example, let us consider elements of second and third periods.
In the second period, lithium and beryllium are metals, boron is a metalloid while carbon, nitrogen, oxygen, fluorine and neon are non-metals.
In the third period, sodium, magnesium and aluminium are metals, silicon is a metalloid while phosphorus, sulphur chlorine and argon are non-metals.
Variation of Metallic Character along a Group
On going along a group from top to bottom, the metallic character of elements increases.
In each group, the first element is least metallic while the last element is most metallic. For example, let us consider the elements of groups 14 and 15.
In group 14, the first element carbon is a non-metal, silicon and germanium are metalloids while tin and lead are metals.
In group 15, the first two elements, nitrogen and phosphorus, are non-metals while arsenic and antimony are metalloids and bismuth is a metal.
It may be mentioned here that metals generally form cations by losing electrons from the outermost shell while non-metals generally form anions by accepting one or more electrons. For example, alkali metals form M+ ions by losing
Example 6.12 Arrange the following elements in the increasing order of metallic character:
8, At, Mg, K
Solution. Metallic character increases on moving down the group and decreases on going across a period from left to right. Hence, the· order of increasing metallic character is B <Al < Mg < K. one electron while alkaline earth metals form M2+ ions by losing two electrons from the outermost shell. The nonmetallic elements of group 17 form anions (X-) by accepting one electron.
The periodic trends of various physical properties are given in Fig. 6.12.
Fig. 6.12. The general periodic trends of various physical properties in the periodic table.
Example 6.13 Arrange the following elements in the increasing order of non-metallic character:
B, C, Si , N, F
Solution . Non-metallic character increases across a period from left to right and decreases on moving down the group from top to bottom. Hence, the order of increasing non-metallic character is
Si < B < C < N < F.
Example 6.14 Among the elements B, AI, C and Si
( i) Which has the highest first ionization energy?
(ii) Which has the most negative electron gain energy?
(iii) Which has the largest atomic radius?
(iv) Which has the most metallic character?
Solution . (i) Carbon (C) has the highest first ionization energy.
(ii) Carbon (C) has the most negative electron gain energy.
(iii) Aluminium (AI) has the largest atomic radius.
(iv) Aluminium (AI) has the most metallic character.