Which value changes when a Cu atom becomes a Cu2+ ion? 1.
mass number
2.
oxidation number
3
number of protons
4.
number of neutrons

2. Oxidation number

Explanation:

In an atom, M, nuclide notation is expressed as:

ₓᵃMⁿ

Where a is mass number (Protons + neutrons)

x is atomic number (Protons)

And n is the charge of the atom (Protons - Electrons ; Oxidation number)

The atom Cu²⁺ has lost 2 electrons, changing, thus, its oxidation number.

Right answer is:

answer; protein;