Relative atomic mass - wikipedia Relative atomic mass Not to be confused with atomic mass. See also : standard atomic weight Relative atomic mass ( symbol : A ), or atomic weight, is a dimensionless ( number only ) physical quantity. In its modern definition, it is the ratio of the average mass of atoms of an element ( in a given sample ) to one unified atomic mass unit. The unified atomic mass unit, symbol u, is defined being ​ ⁄ of the mass of a carbon - 12 atom. Since both values in the ratio are expressed in the same unit ( u ), the resulting value is dimensionless ; hence the value is relative. For one sample, its relative atomic mass ( atomic weight ) is a straight average over the individual atom weights ( isotopes ) present. Between sources, the atomic weight can vary when the source 's origin ( radioactive history ) resulted in different isotopic concentrations. For example, a sample of elemental carbon from volcanic methane will have a different relative atomic mass than one collected from plant or animal tissues. The well - known standard atomic weight is an application of this relative atomic mass values from different samples. It is expected range of the relative atomic mass values, with the various sources being terrestrial ( taken from Earth ). These standard atomic weights are what chemists loosely and so incorrectly call `` atomic weights '' ( incorrectly, because they are not from a single sample ). They are the most published form of the relative atomic mass, because they are not sample - specific, but cover a broad range of expected Earth samples. The continued use of the term `` atomic weight '' ( of any element ), as opposed to `` relative atomic mass '' has attracted considerable controversy, since at least the 1960s, mainly due to the technical difference between weight and mass in physics. Both terms are officially sanctioned by IUPAC. The term `` relative atomic mass '' now seems to be gaining as the preferred term over `` atomic weight '', although in the case of `` standard atomic weight '', this shorter term ( as opposed to the more correct `` standard relative atomic mass '' ) continues to be used. Contents 1 Definition 1.1 Current definition 1.2 Historical amu 2 Standard atomic weight 3 Other definitions of the mass of atoms 4 Determination of relative atomic mass 5 See also 6 References 7 External links Definition Relative atomic mass is determined by the average atomic mass, or the weighted mean of the atomic masses of all the atoms of a particular chemical element found in a particular sample, which is then compared to the atomic mass of carbon - 12. This comparison is the quotient of the two weights, which makes the value dimensionless ( no unit appended ). This quotient also explains the word relative : the sample mass value is made relative to carbon - 12. It is a synonym for atomic weight ( and is not to be confused with relative isotopic mass ). Relative atomic mass is frequently used as a synonym for the standard atomic weight and these will have overlapping values if the relative atomic mass used is that for an element from Earth under defined conditions. However, relative atomic mass ( atomic weight ) covers more than standard atomic weights, and is more general term that may more broadly refer to non-terrestrial environments and highly specific terrestrial environments that deviate from Earth - average or have different certainties ( number of significant figures ) than do the standard atomic weights. Current definition Prevailing IUPAC definitions taken from the `` Gold Book '' are atomic weight -- See : relative atomic mass and relative atomic mass ( standard atomic weight ) -- The ratio of the average mass of the atom to the unified atomic mass unit. Here the `` unified atomic mass unit '' refers to ​ ⁄ of the mass of an atom of C in its ground state. The IUPAC definition of relative atomic mass is : An atomic weight ( relative atomic mass ) of an element from a specified source is the ratio of the average mass per atom of the element to 1 / 12 of the mass of an atom of C. The definition deliberately specifies `` An atomic weight... '', as an element will have different relative atomic masses depending on the source. For example, boron from Turkey has a lower relative atomic mass than boron from California, because of its different isotopic composition. Nevertheless, given the cost and difficulty of isotope analysis, it is usual to use the tabulated values of standard atomic weights which are ubiquitous in chemical laboratories. Historical amu Older ( pre-1961 ) historical relative scales ( based on the atomic mass unit, or a.m.u., or amu ) used either the oxygen - 16 relative isotopic mass for reference, or else the oxygen relative atomic mass ( i.e., atomic weight ) for reference. See the article on the history of the modern unified atomic mass unit for the resolution of these problems in 1961. Standard atomic weight Main article : Standard atomic weight The IUPAC commission CIAAW maintains a expectation - interval value for relative atomic mass ( or atomic weight ) on Earth, named standard atomic weight. Standard atomic weight requires the sources be terrestrial, natural, and stable with regard to radioactivity. Also there are requirements for the research process. For 84 stable elements CIAAW has determined this standard atomic weight. These values are widely published and referred to loosely as ' the ' atomic weight of elements for real life substances like pharmaceuticals and commercial trade. Also, CIAAW has published abridged ( rounded ) values, and simplified values ( for when the Earthly sources vary systematically ). Other definitions of the mass of atoms Atomic mass ( m ) is the mass of a single atom, with unit Da or u ( the unified atomic mass unit ). It defines the weight of a specific isotope, which is an input value for the determination of the relative atomic mass. An example for three silicon isotopes is given in determination of relative atomic mass From this mass, the relative isotopic mass is specifically the ratio of the mass of a single atom to the mass of a unified atomic mass unit. This value too is relative, and so dimensionless. Determination of relative atomic mass Main article : Isotope geochemistry Modern relative atomic masses ( a term specific to a given element sample ) are calculated from measured values of atomic mass ( for each nuclide ) and isotopic composition of a sample. Highly accurate atomic masses are available for virtually all non-radioactive nuclides, but isotopic compositions are both harder to measure to high precision and more subject to variation between samples. For this reason, the relative atomic masses of the 22 mononuclidic elements ( which are the same as the isotopic masses for each of the single naturally occurring nuclides of these elements ) are known to especially high accuracy. For example, there is an uncertainty of only one part in 38 million for the relative atomic mass of fluorine, a precision which is greater than the current best value for the Avogadro constant ( one part in 20 million ). Isotope Atomic mass Abundance Standard Range Si 7001279769265324600 ♠ 27.976 926 532 46 ( 194 ) 92.2297 ( 7 ) % 92.21 -- 92.25 % Si 7001289764947000000 ♠ 28.976 494 700 ( 22 ) 4.6832 ( 5 ) % 4.67 -- 4.69 % Si 7001299737701710000 ♠ 29.973 770 171 ( 32 ) 3.0872 ( 5 ) % 3.08 -- 3.10 % The calculation is exemplified for silicon, whose relative atomic mass is especially important in metrology. Silicon exists in nature as a mixture of three isotopes : Si, Si and Si. The atomic masses of these nuclides are known to a precision of one part in 14 billion for Si and about one part in one billion for the others. However the range of natural abundance for the isotopes is such that the standard abundance can only be given to about ± 0.001 % ( see table ). The calculation is A ( Si ) = ( 7001279769300000000 ♠ 27.976 93 × 0.922297 ) + ( 7001289764900000000 ♠ 28.976 49 × 0.046832 ) + ( 7001299737700000000 ♠ 29.973 77 × 0.030872 ) = 28.0854 The estimation of the uncertainty is complicated, especially as the sample distribution is not necessarily symmetrical : the IUPAC standard relative atomic masses are quoted with estimated symmetrical uncertainties, and the value for silicon is 28.0855 ( 3 ). The relative standard uncertainty in this value is 1 × 10 or 10 ppm. Apart from this uncertainty by measurement, some elements have variation over sources. That is, different sources ( ocean water, rocks ) have a different radioactive history, and so different isotopic composition. To reflect this natural variability, in 2010 IUPAC made the decision to list the standard relative atomic masses of 12 elements as an interval rather than a fixed number. See also International Union of Pure and Applied Chemistry ( IUPAC ) Commission on Isotopic Abundances and Atomic Weights References ^ Jump up to : International Union of Pure and Applied Chemistry ( 1980 ). `` Atomic Weights of the Elements 1979 ''. Pure Appl. Chem. 52 ( 10 ) : 2349 -- 84. doi : 10.1351 / pac198052102349. Jump up ^ International Union of Pure and Applied Chemistry ( 1993 ). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford : Blackwell Science. ISBN 0 - 632 - 03583 - 8. p. 41. Electronic version. Jump up ^ Definition of element sample Jump up ^ de Bièvre, P. ; Peiser, H.S. ( 1992 ). `` ' Atomic Weight ' -- The Name, Its History, Definition, and Units ''. Pure Appl. Chem. 64 ( 10 ) : 1535 -- 43. doi : 10.1351 / pac199264101535. Jump up ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. ( the `` Gold Book '' ) ( 1997 ). Online corrected version : ( 2006 -- ) `` relative atomic mass ''. Jump up ^ IUPAC Gold Book - atomic weight Jump up ^ IUPAC Gold Book - relative atomic mass ( atomic weight ), A r Jump up ^ IUPAC Gold Book - unified atomic mass unit Jump up ^ Greenwood, Norman N. ; Earnshaw, Alan ( 1984 ). Chemistry of the Elements. Oxford : Pergamon Press. pp. 21, 160. ISBN 0 - 08 - 022057 - 6. Jump up ^ International Union of Pure and Applied Chemistry ( 2003 ). `` Atomic Weights of the Elements : Review 2000 ''. Pure Appl. Chem. 75 ( 6 ) : 683 -- 800. doi : 10.1351 / pac200375060683. Jump up ^ National Institute of Standards and Technology. Atomic Weights and Isotopic Compositions for All Elements. ^ Jump up to : Wapstra, A.H. ; Audi, G. ; Thibault, C. ( 2003 ), The AME2003 Atomic Mass Evaluation ( Online ed. ), National Nuclear Data Center. Based on : Wapstra, A.H. ; Audi, G. ; Thibault, C. ( 2003 ), `` The AME2003 atomic mass evaluation ( I ) '', Nuclear Physics A, 729 : 129 -- 336, Bibcode : 2003NuPhA. 729... 129W, doi : 10.1016 / j. nuclphysa. 2003.11. 002 Audi, G. ; Wapstra, A.H. ; Thibault, C. ( 2003 ), `` The AME2003 atomic mass evaluation ( II ) '', Nuclear Physics A, 729 : 337 -- 676, Bibcode : 2003NuPhA. 729... 337A, doi : 10.1016 / j. nuclphysa. 2003.11. 003 ^ Jump up to : Rosman, K.J.R. ; Taylor, P.D.P. ( 1998 ), `` Isotopic Compositions of the Elements 1997 '' ( PDF ), Pure and Applied Chemistry, 70 ( 1 ) : 217 -- 35, doi : 10.1351 / pac199870010217 Jump up ^ Coplen, T.B. ; et al. ( 2002 ), `` Isotopic Abundance Variations of Selected Elements '' ( PDF ), Pure and Applied Chemistry, 74 ( 10 ) : 1987 -- 2017, doi : 10.1351 / pac200274101987 Jump up ^ Meija, Juris ; Mester, Zoltán ( 2008 ). `` Uncertainty propagation of atomic weight measurement results ''. Metrologia. 45 : 53 -- 62. Bibcode : 2008Metro... 45... 53M. doi : 10.1088 / 0026 - 1394 / 45 / 1 / 008. Jump up ^ Holden, Norman E. ( 2004 ). `` Atomic Weights and the International Committee -- A Historical Review ''. Chemistry International. 26 ( 1 ) : 4 -- 7. Jump up ^ IUPAC - International Union of Pure and Applied Chemistry : Atomic Weights of Ten Chemical Elements About to Change External links IUPAC Commission on Isotopic Abundances and Atomic Weights NIST relative atomic masses of all isotopes and the standard atomic weights of the elements Atomic Weights of the Elements 2011 Periodic table Periodic table forms Standard 18 - column 18 - column, large cells 32 - column, large cells Alternative Alternatives Chemical Galaxy Janet 's left step table Extended Extension beyond the 7th period Fricke model large cells Pyykkö model Sets of elements By periodic table structure Groups 1 ( Alkali metals ) 2 ( Alkaline earth metals ) 5 6 7 8 9 10 11 12 13 14 15 ( Pnictogens ) 16 ( Chalcogens ) 17 ( Halogens ) 18 ( Noble gases ) Periods 5 6 7 8 + Aufbau Fricke Pyykkö Blocks s - block p - block d - block f - block g - block Aufbau principle By metallicity Metals Alkali metals Alkaline earth metals Lanthanides Actinides Superactinides Eka - superactinides Transition metals Post-transition metals Metalloids Lists of metalloids by source Dividing line Nonmetals Polyatomic nonmetals Diatomic nonmetals Noble gases Other sets Nutritional elements Platinum - group metals ( PGM ) Rare - earth elements Refractory metals Precious metals Coinage metals Noble metal Heavy metals Native metals Transuranium elements Transactinide elements Major actinides Minor actinides Elements Lists By : Abundance ( in humans ) Atomic properties Nuclear stability Production Symbol Properties Atomic weight Crystal structure Electron affinity Electron configuration Electronegativity Allen Pauling Goldschmidt classification Maximum valence Data pages Abundance Atomic radius Boiling point Critical point Density Elasticity Electrical resistivity Electron affinity Electron configuration Electronegativity Hardness Heat capacity Heat of fusion Heat of vaporization Ionization energy Melting point Oxidation state Speed of sound Thermal conductivity Thermal expansion coefficient Vapor pressure History Element discoveries Mendeleev 's predictions Naming Etymology controversies places scientists in East Asia See also IUPAC nomenclature systematic element name Trivial name Dmitri Mendeleev Book Chemistry Portal LCCN : sh85009327 GND : 4143329 - 4 BNF : cb11978880x ( data ) NDL : 00562390 Retrieved from `` https://en.wikipedia.org/w/index.php?title=Relative_atomic_mass&oldid=805834230 '' Categories : Amount of substance Chemical properties Stoichiometry Periodic table Hidden categories : Wikipedia articles with LCCN identifiers Wikipedia articles with GND identifiers Wikipedia articles with BNF identifiers Talk Contents About Wikipedia Bân - lâm - gú Беларуская Беларуская ( тарашкевіца ) ‎ Català Čeština Español فارسی Fiji Hindi Gaeilge Gaelg 한국어 Հայերեն हिन्दी Hrvatski Ilokano Bahasa Indonesia Italiano עברית ಕನ್ನಡ Kreyòl ayisyen Kurdî Македонски Bahasa Melayu Norsk nynorsk Oʻzbekcha / ўзбекча ਪੰਜਾਬੀ Română Scots Simple English Slovenčina Soomaaliga Српски / srpski Tagalog ไทย Edit links This page was last edited on 17 October 2017, at 22 : 47. 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Question: which isotope is used as relative scale to find atomic mass
Answer: carbon-12