A set of atoms of the same chemical element, taken individually have the same number and the same disposition of the electrons but can differ, without prejudice to the number of protons, for the atomic mass, or the number of neutrons.
Two or more atoms are said to belong to the same chemical element if they are characterized by the same atomic number (Z), that is by the same number of protons. Atoms of the same element may differ instead by the number of neutrons (mass); in particular atoms of the same chemical element but with different neutron numbers are called isotopes.
Origin of chemical elements
The origin of the chemical elements present in the universe is usually derived from the astrophysical theory of the life cycle of the stars.
Large stars (supernovae) in fact end their existence by continuing to burn nuclear fuel: starting from the first nuclear fusion reactions that involve hydrogen to form helium, at the end of the fuel represented by hydrogen and the subsequent gravitational collapse of the star (due to lack of balance due to the absence of reactions) the consequent increase in temperature triggers successive nuclear reactions involving helium to form other heavier and more complex elements and so on in a long chain of nuclear reactions that lead to the formation of all other chemical elements.
The final explosion of the supernova leads to the dispersion of various chemical elements in the universe.