Q17 of 24 Page 5

An element 'X' has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of 'X'. Also write group number, period number and valency of 'X'. (CBSE 2016)

Given: mass of number of X = 35


No. of neutrons = 18


Atomic number of element X = Mass number – no. of neutrons present


Atomic no. = 35 -18 = 17


The electronic configuration of X will be 2, 8, 7


X will belong to group no. 10 + 7= 17th


X will belong to period 3rd as it has three shells.


Since it requires one more electron to get stable gas electron configuration, so the valency is 1.


'X'

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Three elements 'X', 'Y' and 'Z' have atomic numbers 7, 8 and 9 respectively.

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