CBSE Class 10 Science 2019 · Paper 31/3/3 · Question 11

Periodic Classification of Elements 3 marks Medium

(a)How is possible valency of an element determined from the electronic configuration of its atom ?
(b)Determine the valency of an element X whose atomic number is 15.

Answer

Valency depends on the valence electrons; X ( $2, 8, 5$ ) has valency $8 - 5 = 3$ .

(a) The valency of an element is determined by the number of electrons in the outermost shell (the valence electrons) of its atom. If the atom has

1

4

valence electrons, its valency equals that number; if it has

5

8

, its valency is

8

minus the number of valence electrons, that is, the number of electrons it must gain to complete its octet. (b) Atomic number

15

: the electronic configuration of X is

2, 8, 5

, so it has

5

valence electrons. Valency of X

= 8 - 5 = 3

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