- = -
and, unlike the mass number, is usually not added to the chemical symbol .
The number of neutrons can vary for an element. These variations are called isotopes . The simplest example is element number 1, hydrogen , with the isotopes protium (no neutron), deuterium (one neutron), tritium (two neutrons), 4 H, 5 H, 6 H and 7 H.
Due to different numbers of neutrons, the physical properties of a substance can vary, chemical properties change only slightly. This is particularly noticeable in the case of elements with a low atomic number, since the change in mass relative to other isotopes is higher here than with heavier elements (see isotope effect ). For example, C-12 (6 neutrons) - ordinary carbon, the main component of organic molecules and with 98.9% the most common of all natural C isotopes - is a stable atom. The carbon isotope C-14 with 8 neutrons, which decays radioactively with a half-life of 5730 years, is used in the radiocarbon method to date dead organic material.
The number of neutrons in an atom has an impact on whether an atom is radioactive or not.