CBSE Class 10 Science 2017 · Paper 531/1 · Question 8

Periodic Classification of Elements 3 marks Medium

Consider the following elements of the Modern Periodic Table : Be(4); O(8); Cl(17); K(19); Ca(20)

(a)Select from these an element having only one valence electron. Write the electronic configuration of this element.
(b)Select from these, two elements of the same group and state the reason for your answer.
(c)Select from these, two elements of the same period and state the reason for your answer.

Answer

Potassium has one valence electron ( $2, 8, 8, 1$ ); Be and Ca are in group 2; K and Ca are in period 4.

(a) Potassium, K (atomic number 19). Its electronic configuration is 2, 8, 8, 1, with only one electron in its outermost shell. (b) Beryllium (Be) and calcium (Ca). Be (2, 2) and Ca (2, 8, 8, 2) both have two valence electrons, and among the first twenty elements (helium apart) elements with the same number of valence electrons are in the same group; so both are in group 2. (c) Potassium (K) and calcium (Ca). K (2, 8, 8, 1) and Ca (2, 8, 8, 2) both have four shells; elements with the same number of shells belong to the same period, so both are in period 4. (Beryllium and oxygen, with two shells each, are also a correct pair: both are in period 2.)

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CBSE Class 10 Science 2017 · Paper 531/1 · Question 8

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