* The number of protons: Protons are located in the nucleus of an atom and determine its atomic number. This number remains constant during ion formation.

* The number of neutrons: Neutrons, also located in the nucleus, contribute to the atom's mass. Their number typically remains constant, although isotopes of the same element can have varying neutron numbers.

What changes when an ion forms:

* The number of electrons: An ion is formed when an atom gains or loses electrons. This change in the number of electrons results in a net positive or negative charge on the atom, creating a cation (positive ion) or an anion (negative ion).

* The overall charge: As mentioned, the gain or loss of electrons creates a net charge on the ion.

In summary, the identity of an element is determined by its number of protons. This identity remains the same when an atom gains or loses electrons to form an ion.