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−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]]
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[[ru:Массовое число]]
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