Editing Mass number (section)

==Mass number and isotopic mass==
The mass number gives an estimate of the [[isotopic mass]] measured in [[atomic mass unit]]s (u). For <sup>12</sup>C, the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of <sup>12</sup>C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, <sup>35</sup>Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885.{{AME2016 II|ref}} The difference of the actual isotopic mass minus the mass number of an atom is known as the [[mass excess]],<ref>{{cite book |doi=10.1351/goldbook.M03719 |doi-access=free  |chapter=Mass excess, Δ |title=The IUPAC Compendium of Chemical Terminology |year=2014 }}
</ref> which for <sup>35</sup>Cl is –0.03115. Mass excess should not be confused with [[mass defect]] which is the difference between the mass of an atom and its constituent particles (namely [[Proton|protons]], [[Neutron|neutrons]] and [[Electron|electrons]]).

There are two reasons for mass 
excess:
# The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on <sup>12</sup>C with equal numbers of protons and neutrons.
# Nuclear [[binding energy]] varies between nuclei. A nucleus with greater binding energy has a lower total energy, and therefore a lower mass according to Einstein's [[mass–energy equivalence]] relation ''E'' = ''mc''<sup>2</sup>. For <sup>35</sup>Cl, the isotopic mass is less than 35, so this must be the dominant factor.