Which of these properties is the best one to use for identification of an element? 1.the number of neutrons in the atomic nucleus
2. the number of protons in the atomic nucleus
3.the number of isotopes that are formed by the element
4. the number of charges on ions of the elemen

2) the number of protons in the atomic nucleus ( also known as the atomic number ).

Elements will permanently have the same number of protons in their nuclei. Changing the proton number, by even just 1 proton, means a whole new element.
-In chemical reactions, the number of electrons will constantly change. -The number of neutrons can change too (the definition of an isotope is: an atom of the same element with the same number of protons but different number of neutrons).
-The number of isotopes doesn't necessarily identify an element. More than one element can have the same number of isotopes.
-The number of charges only tells us what group the element is in the periodic table, not the specific element itself.

Hope it helps!!