CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 8

Periodic Classification of Elements 3 marks Medium

The elements Li, Na and K each having one valence electron are in the

2^{nd}

3^{rd}

and

4^{th}

period respectively of Modern Periodic Table. Answer the following questions giving reasons :

(i)In which group should they be placed ?
(ii)Which one of these elements is least reactive ?
(iii)Which one of these elements has the largest atomic radius ?

Answer

Li, Na and K belong to group 1; lithium is the least reactive and potassium has the largest atomic radius.

(i) Group 1 of the Modern Periodic Table. Elements with the same number of valence electrons are placed in the same group, and each of these has one valence electron (Li: 2, 1; Na: 2, 8, 1; K: 2, 8, 8, 1). (ii) Lithium is the least reactive. These metals react by losing their one valence electron; lithium has only two shells, so its valence electron is closest to the nucleus and held most strongly, and it is lost least easily. (iii) Potassium has the largest atomic radius. Atomic size increases down a group because a new shell is added in each period, and potassium, in period 4, has four shells, the most of the three.

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CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 8

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