Question

an element which has mass number 35 and the number of neutrons, 18

Answer 1

atomic no = mass no. - no. of neutrons

= 35-18= 17

group no. = 17

period = 3

Answer 2

Mass number = No of protons+ No of neutrons
35 = No of protons + 18
No of protons = 17.
No of protons = Atomic number = 17.
So Chlorine is the element having atomic number 17 .