An element is a substance composed only of atoms having the same proton count (atomic number), Z. Neutron count may differ between atoms of an element. Atoms with the same Z but differing total particle count, A, are termed isotopes of element Z. With the exception of hydrogen, all isotopes of an element are chemically and physically almost identical; delicate physical and chemical techniques are required to separate them.

Elements may be effectively stable, which means their abundance does not change within time spans less than a hundred times the age of the universe, or they may be radioactive. Nature itself makes the distinction. 152Gd has a half-life around 8000 times the age of the universe, 190Pt has a half-life only 47 times the universe's age, and the chance of an undiscovered nuclide whose half-life lies between the two is essentially zero. 152Gd may emit radiation, but the fact is of purely academic importance. Effective stability is better than the currently-used category of "observationally stable". What is observationally stable changes every time radiation is detected from a nuclide thought to be stable. Effective stability does not change; there are 81 effectively stable elements and always will be.

There are at least 10000, and possibly as many as 80000 radioactive nuclides. Most of these vanish within seconds of an event which led to their formation. In addition, most cannot form except one atom at a time in charged-particle reactions. This means they exist only within supernovae; neutron star mergers; stellar electromagnetic storms; polar jets from stars, neutron stars, or black holes; and in laboratories. This causes the fundamental question of how many elements exist to depend on why the question is asked. If the question is how many combinations of protons and neutrons meet the stability condition for being nuclides; there could, in principle, be as many as 324. If the question is how many combinations actually exist as part of the universe, the number is probably under 220. If the question is how many combinations are persistent enough to remain after a supernova or other transient event is over, the number is probably less than 130. (But there may be a few more in the vicinity of 396Uto.)

Elements retain their chemical identity only at low temperature. At higher temperature, all elements are positively ionized. The chemistry of such ions, if it is possible at all, will be radically different from the corresponding element's low-temperature behavior. (Although not due to a chemical reaction, changes in ionization with temperature is thought to be responsible for variability in a number of different types of star.)