Isotope electrons in the neutral (non-ionized) atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that
Atomic mass unit of exactly 16 amu in physics. The divergence of these values could result in errors in computations, and was unwieldy. The chemistry amu, based on the relative atomic mass (atomic weight) of natural oxygen (including the heavy naturally-occurring isotopes O and O), was about as massive as the physics amu, based on pure isotopic O. For these and other reasons, the reference standard for both physics and chemistry was changed to carbon-12 in 1961. The choice of carbon-12 was made to minimise further divergence with prior literature. The new and current unit was referred to as the "unified atomic mass