Question

an element "X" has mass number 35and the number of neutron, is 18.identify the group number and period of "X"

Answer 1

Since at. No. =mass no.-no. Of neutron
Therefore,at. No.=35-18=17
And the its electronic configuration =2,8,7
Therefore it belongs to 17th grp.
And
3 period

Answer 2

Mass number = no. Of electrons + no. Of neutrons
35= electrons + 18
Electrons = 17
So the atomic number is 17
it is chlorine
configuration of chlorine is 2,8,7
So it lies in 3 period
and 17 group