Why are halogens kept in the group 17 of modern peroidic table?

The halogens are all elements that are found in group 17 of the periodic table. The halogens include fluorine, chlorine, bromine, iodine, and astatine. All of these elements are considered to be reactive nonmetals. Atoms of an element react with other atoms based on the number of electrons found in their outer orbit.

The molecules contain like 54 of water 

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.