However, most atoms can exist in ionized states where there are more, or less, electrons than protons. The atomic number of an atom, and it identity as an element, is given by the number of protons. Hence, all Fe atoms have 26 protons. The number of neutrons that an atom has determines which isotope of the element it is. The sum of the number of protons and neutrons is the mass number.
Different isotopes have different mass numbers.
The Periodic Table of the Elements There are 93 different stable atoms that occur in nature. We have managed to make new elements with atomic numbers greater than 93 in nuclear reactions in the laboratory. Of these 93 elements, only a few are very abundant in the universe. Most elements are quite rare. At first, there were only hydrogen atoms.