What is Mass number?
The '''mass number''' (symbol: '''A''') of an [[atom]] is the sum of the number of [[proton]]s and the number of [[neutron]]s in the [[atomic nucleus|nucleus]].<ref name=moore>{{cite book|first=John T.|last=Moore|title=Chemistry Essentials For Dummies|url=https://archive.org/details/chemistryessenti0000moor|publisher=Wiley|date=2010|isbn=978-0-470-61836-3}}</ref>{{rp|20}} The mass number is different for each [[isotope]] of a [[chemical element]]. We write a mass number after an element's name or as a superscript to the left of an element's symbol. For example, the most common isotope of [[carbon]] has 6 protons and 6 neutrons. We write it as carbon-12 or <sup>12</sup>C. Mass number is not the same as: * '''[[atomic number]]''' (symbol: '''Z'''), which is the number of protons in a nucleus of an atom * '''[[atomic mass]]''' (symbol: ''m''<sub>a</sub>), which is the mass of a single atom, commonly expressed in [[atomic mass unit|unified atomic mass units]] * '''[[relative atomic mass]]''' (symbol: ''A''<sub>r</sub>, also called ''atomic weight''), which is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom. The difference between the mass number and the atomic number gives the number of neutrons (''N'') in a given nucleus: ''N ''= ''A ''− ''Z''.<ref name=moore />{{rp|44}} The mass number is not shown on the [[periodic table]].<ref name=moore />{{rp|21}} For each element, what is shown is the ''standard atomic weight'' and the ''atomic number''. == References == <references /> {{chem-stub}} [[Category:Chemistry]] [[Category:Nuclear physics]]