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.
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