Which value changes when a Cu atom becomes a Cu^2+ ion?

August 2018 NY Regents Chemistry Exam

25 Which value changes when a Cu atom becomes a Cu^2+ ion?
(1) mass number
(2) oxidation number

(3) number of protons

(4) number of neutrons

Solution: All free, neutral elements have an oxidation number of zero. Cu atom has an oxidation number of zero while Cu^2+ ion has an oxidation number of 2+

Answer:2

Prepare for the Chemistry Regents Exam: HERE