CBSE Class 10 Science 2017 · Paper 31/3/C · Question 9

Periodic Classification of Elements 3 marks Medium

Consider the following elements (atomic numbers are given in parenthesis) Ca(20) ; K(19) ; F(9) ; Be(4) (a) Select :

(i)the element having one electron in the outermost shell
(ii)two elements of the same group. Write the number of this group. (b) Write the formula of the compound formed by the union of Ca(20) and the element X(2, 8, 7).

Answer

Potassium has one outer electron; beryllium and calcium are in group 2; calcium and chlorine form $CaCl_{2}$ .

(a)(i) Potassium, K (2, 8, 8, 1), has one electron in its outermost shell. (ii) Beryllium, Be (2, 2), and calcium, Ca (2, 8, 8, 2), both have two electrons in their outermost shell, so they belong to the same group: group 2. (b) X (2, 8, 7) has atomic number 17, so it is chlorine, with valency 1; calcium has valency 2. The formula of the compound is

CaX_{2}

, that is,

CaCl_{2}

(calcium chloride).

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CBSE Class 10 Science 2017 · Paper 31/3/C · Question 9

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