CBSE Class 10 Science 2017 · Paper 31/2/2 · Question 7

Metals and Non-metals 3 marks Medium

The atomic number of an element is 19. (a) Write the electronic configuration of this element and determine

(i)the valency of this element, and
(ii)whether this element is a metal or a non-metal. (b) Write the formula of the oxide of this element. (c) Is this element more reactive or less reactive than Na (atomic number 11) ? Justify your answer giving example.

Answer

The element is potassium ( $2, 8, 8, 1$ ): valency 1, a metal, oxide $K_{2} O$ , and more reactive than sodium.

(a) Electronic configuration: 2, 8, 8, 1 (the element is potassium, K). (i) Its outermost shell has one electron, which it loses, so its valency is 1. (ii) It is a metal, since it forms a positive ion by losing its outer electron. (b) The formula of its oxide is

K_{2} O

(potassium, valency 1, with oxygen, valency 2). (c) It is more reactive than Na (2, 8, 1). Its valence electron is in the fourth shell, farther from the nucleus than sodium's, which is in the third, so it is held less strongly and lost more easily. Example: potassium reacts with cold water even more vigorously than sodium,

2 K + 2 H_{2} O \to 2 KOH + H_{2}

, and it stands above sodium in the reactivity series.

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CBSE Class 10 Science 2017 · Paper 31/2/2 · Question 7

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