Mass number 659068 224518929 2008-07-09T05:52:41Z XDanielx 4860300 adding general reference {{refimprove|date=February 2008}} The '''mass number''' ('''A'''), also called '''atomic mass number''' or '''nucleon number''', is the number of [[proton]]s and [[neutron]]s (together known as [[nucleon]]s) in an [[atomic nucleus]]. It is not the same as the [[atomic number]] ('''Z''') which denotes the number of protons in a nucleus. The mass number is unique for each [[isotope]] of an element and is written either after the element name or as a superscript to the left of an element's symbol. For example, [[carbon-12]] (<sup>12</sup>C) has 6 protons and 6 neutrons. The full isotope symbol would also have the [[atomic number]] ('''''Z''''') as a subscript to the left of the element symbol directly below the mass number: <math>{}_{6}^{12}</math>. Note that this is redundant, as there is a one-to-one mapping between atomic number and element symbol, so it is rarely used, except when we want to clarify the number of protons in a nucleus, such as in atomic reactions. The difference between the mass number and the atomic number gives the number of neutrons (''N'') in a given nucleus: ''N=A&minus;Z''. For example: [[Carbon-14]] is created from [[Nitrogen-14]] with seven protons (''p'') and seven neutrons via a [[cosmic ray]] interaction which transmutes 1 proton into 1 neutron. Thus the atomic number decreases by 1 (''Z'': 7→6) and the mass number remains the same (''A'' = 14), however the number of neutrons increases by 1 (''n'': 7→8). :Before: Nitrogen-14 (7p, 7n) :After: Carbon-14 (6p, 8n). This should not be confused with the [[atomic mass|relative atomic mass]] which is the average abundance atomic mass number of the differing isotopes found. For instance, there are two isotopes of chlorine: Chlorine-35 and Chlorine-37. In any given sample of chlorine that has not had any mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are Chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/[[Mole (unit)|mol]]). ==References== * {{cite book |last=Bishop |first=Mark |title=An Introduction to Chemistry |url=http://preparatorychemistry.com |accessdate=2008-07-08 |publisher=Chiral Publishing |isbn=978-0-9778105-4-3 |pages=p. 93 |chapter=The Structure of Matter and Chemical Elements (ch. 3) |chapterurl=http://preparatorychemistry.com/Bishop_Book_atoms_3.pdf}} [[Category:Nuclear chemistry]] [[Category:Chemical properties]] [[ar:عدد الكتلة]] [[bs:Maseni broj]] [[br:Niver mas]] [[ca:Nombre màssic]] [[cs:Nukleonové číslo]] [[cy:Rhif màs]] [[de:Massenzahl]] [[et:Massiarv]] [[el:Μαζικός αριθμός]] [[es:Número másico]] [[eo:Masnumero]] [[fa:عدد جرمی]] [[fr:Nombre de masse]] [[fur:Numar di masse]] [[gl:Número másico]] [[is:Massatala]] [[it:Numero di massa]] [[he:מספר מסה]] [[ku:Hijmara baristeyî]] [[hu:Tömegszám]] [[mn:Атом масс]] [[nl:Massagetal]] [[ja:質量数]] [[nds:Massentall]] [[pl:Liczba masowa]] [[pt:Número de massa]] [[ru:Массовое число]] [[sk:Nukleónové číslo]] [[sl:Masno število]] [[sr:Масени број]] [[fi:Massaluku]] [[sv:Masstal]] [[ta:திணிவெண்]] [[vi:Số khối]] [[uk:Масове число]] [[vec:Nùmaro de masa]] [[zh:質量數]]