CBSE Class 10 Science 2017 · Paper 531/2 · Question 8

Periodic Classification of Elements 3 marks Medium

Write the electronic configurations of the elements ‘X’, ‘Y’ and ‘Z’ having atomic numbers 8, 17 and 20 respectively, and answer the following questions :

(a)What is the period number of ‘Z’ ?
(b)What is the group number of ‘Y’ ?
(c)Write the formula of the compound formed by the combination of ‘X’ and ‘Z’.

Answer

Z is in period 4, Y is in group 17, and X and Z form the compound ZX.

Electronic configurations: X (atomic number 8): 2, 6; Y (17): 2, 8, 7; Z (20): 2, 8, 8, 2. (a) Z has electrons in four shells, so its period number is 4. (b) Y has 7 valence electrons, so its group number is 17. (c) Z loses its two valence electrons to form

Z^{2 +}

and X gains two electrons to form

X^{2 -}

; one of each balances the charge, so the formula is ZX. (X is oxygen and Z is calcium: the compound is calcium oxide, CaO.)

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