What is the one thing that determines the identity of an atom?

Atomic Number

Explanation:

The atomic number tells you the placement of an atom on the periodic table. It is the number of protons found in the nucleus of an atom. All atoms have a different number of protons, thus different atomic number which makes it a good identifier of an atom.  

132

explanation:

12

halogen, any of the six nonmetallic elements that constitute group 17 (group viia) of the periodic table. the halogen elements are fluorine (f), chlorine (cl), bromine (br), iodine (i), astatine (at), and tennessine (ts).