Question

An element "x" has mass no 35 and the no of newtron is 18 . Identify the group no and period of "x"

Answer 1

Given, Mass Number = 35
Number of neutrons = 18
Number of electrons = 35 - 18 = 17 .

So, The Atomic Number of the element is 17 .

The element is Chlorine.

Group number VIIA ( 7 A) or 17 ( Halogens)
Period number 3

Answer 2

mass number = 35 
number of neutron  = 18

as we know 
Mass number = number of protons + number of neutrons.
let number of protons = x
so
35=× +18
×=35 - 18
X=17.

as we know 
number of protons in a atom = number of electrons in a atom 
17 = number of electrons
hence element is chlorine as chlorine has atomic number 17 

electronic configuration 
k shell = 2
l shell = 8 
m shell = 7 
 so valency = 1 
and period is 3