The mass number is the sum of the number of protons and neutrons of an element. So, to determine the element "X", we have to subtract 18 from 35, which makes it 17.
From the periodic table, the element with 17 protons is chlorine. Chlorine is in the Group VII, as known as halogens. It is the 3rd period in the periodic table.
Halogens very reactive. One thing they have in common is that they have 7 outmost shell electrons. Chlorine is the second in that group.
Period 3 means that it has 3 occupied shells. The arrangements of the electron shells of chlorine should be 2,8,7
