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!!
