出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2013/10/19 00:39:40」(JST)
Relative atomic mass (symbol: Ar) is a dimensionless physical quantity, the ratio of the average mass of atoms of an element (from a single given sample or source) to 1/12 of the mass of an atom of carbon-12 (known as the unified atomic mass unit).[1][2] The term is equivalent to atomic weight, which is the older term, and which is also sample (source) specific. Thus, two samples of a chemical element that normally consists of more than one isotope, collected from two widely spaced natural solid sources on Earth, are expected to have slightly different relative atomic masses (atomic weights).
Both the terms relative atomic mass and atomic weight are sometimes loosely used to refer to a technically different standardized expectation value, called the standard atomic weight. This value is the mean value of atomic weights of a number of "normal samples" of the element in question. For this definition, "[a] normal sample is any reasonably possible source of the element or its compounds in commerce for industry and science and has not been subject to significant modification of isotopic composition within a geologically brief period."[3] These standard atomic weights are published at regular intervals by the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry (IUPAC)[4][5] The "standard" values are intended as mean values that compensate for small variances in the isotopic composition of the chemical elements across a range of ordinary samples on Earth, and thus to be applicable to normal laboratory materials. However, they may not accurately reflect values from samples from unusual locations or extraterrestrial objects, which often have more widely variant isotopic compositions.
The standard atomic weights are reprinted in a wide variety of textbooks, commercial catalogues, wallcharts etc., and in the table below.
The continued use of the terms "standard atomic weight" and "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.[6] (see below).
Lithium represents a unique case where the natural abundances of the isotopes have in some cases been found to have been perturbed by human isotopic separation activities to the point of affecting the uncertainty in its standard atomic weight, even in samples obtained from natural sources, such as rivers.
For synthetic elements the isotope formed depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore, for synthetic elements the total nucleon count of the most stable isotope (i.e., the isotope with the longest half-life) is listed in brackets, in place of the standard atomic weight.
When the term "atomic weight" is used in chemistry, usually it is the more specific standard atomic weight that is implied. It is standard atomic weights that are used in periodic tables and many standard references in ordinary terrestrial chemistry.
The IUPAC definition[1] 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 12C.
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.[11][12] 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.
Prevailing IUPAC definitions taken from the "Gold Book" are
and
Here the "unified atomic mass unit" refers to 1/12 of the mass of an atom of 12C in its ground state.[15]
The use of the name "atomic weight" has attracted a great deal of controversy among scientists.[6] Objectors to the name usually prefer the term "relative atomic mass" (not to be confused with atomic mass). The basic objection is that atomic weight is not a weight, that is the force exerted on an object in a gravitational field, measured in units of force such as the newton or poundal.
In reply, supporters of the term "atomic weight" point out (among other arguments)[6] that
It could be added that atomic weight is often not truly "atomic" either, as it does not correspond to the property of any individual atom. The same argument could be made against "relative atomic mass" used in this sense.
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[17][18] for virtually all non-radioactive nuclides, but isotopic compositions are both harder to measure to high precision and more subject to variation between samples.[19][20] 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[18] | Abundance[19] | |
---|---|---|---|
Standard | Range | ||
28Si | 27.976 926 532 46(194) | 92.2297(7)% | 92.21–92.25% |
29Si | 28.976 494 700(22) | 4.6832(5)% | 4.69–4.67% |
30Si | 29.973 770 171(32) | 3.0872(5)% | 3.10–3.08% |
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: 28Si, 29Si and 30Si. The atomic masses of these nuclides are known to a precision of one part in 14 billion for 28Si 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
The estimation of the uncertainty is complicated,[21] especially as the sample distribution is not necessarily symmetrical: the IUPAC standard relative atomic masses are quoted with estimated symmetrical uncertainties,[22] and the value for silicon is 28.0855(3). The relative standard uncertainty in this value is 1×10–5 or 10 ppm. To further reflect this natural variability, in 2010, IUPAC made the decision to list the relative atomic masses of 10 elements as an interval rather than a fixed number.[23]
Atomic weight
|
|||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||||
Group → | |||||||||||||||||||||
↓ Period | |||||||||||||||||||||
1 | H 1.008 |
He 4.003 |
|||||||||||||||||||
2 | Li 6.941 |
Be 9.012 |
B 10.81 |
C 12.01 |
N 14.01 |
O 16.00 |
F 19.00 |
Ne 20.18 |
|||||||||||||
3 | Na 22.99 |
Mg 24.31 |
Al 26.98 |
Si 28.09 |
P 30.97 |
S 32.07 |
Cl 35.45 |
Ar 39.95 |
|||||||||||||
4 | K 39.10 |
Ca 40.08 |
Sc 44.96 |
Ti 47.87 |
V 50.94 |
Cr 52.00 |
Mn 54.94 |
Fe 55.84 |
Co 58.93 |
Ni 58.69 |
Cu 63.55 |
Zn 65.39 |
Ga 69.72 |
Ge 72.63 |
As 74.92 |
Se 78.96 |
Br 79.90 |
Kr 83.80 |
|||
5 | Rb 85.47 |
Sr 87.62 |
Y 88.91 |
Zr 91.22 |
Nb 92.91 |
Mo 95.94 |
Tc [97.91] |
Ru 101.07 |
Rh 102.91 |
Pd 106.42 |
Ag 107.87 |
Cd 112.41 |
In 114.82 |
Sn 118.71 |
Sb 121.76 |
Te 127.60 |
I 126.90 |
Xe 131.29 |
|||
6 | Cs 132.91 |
Ba 137.33 |
* |
Hf 178.49 |
Ta 180.95 |
W 183.84 |
Re 186.21 |
Os 190.23 |
Ir 192.22 |
Pt 195.08 |
Au 196.97 |
Hg 200.59 |
Tl 204.38 |
Pb 207.2 |
Bi 208.98 |
Po [208.98] |
At [209.99] |
Rn [222.02] |
|||
7 | Fr [223.02] |
Ra 226.03 |
** |
Rf [267.12] |
Db [268.13] |
Sg [269.13] |
Bh [270.14] |
Hs [269.15] |
Mt [278.15] |
Ds [281.16] |
Rg [281.16] |
Cn [285.17] |
Uut [286.18] |
Fl [289.18] |
Uup [289.19] |
Lv [293.15] |
Uus [294.13] |
Uuo [294.17] |
|||
* Lanthanides | La 138.91 |
Ce 140.12 |
Pr 140.91 |
Nd 144.24 |
Pm [144.92] |
Sm 150.36 |
Eu 151.96 |
Gd 157.25 |
Tb 158.93 |
Dy 162.50 |
Ho 164.93 |
Er 167.26 |
Tm 168.93 |
Yb 173.04 |
Lu 174.97 |
||||||
** Actinides | Ac [227.03] |
Th 232.04 |
Pa 231.04 |
U 238.03 |
Np [237.05] |
Pu 244.06 |
Am [243.06] |
Cm [247.07] |
Bk [247.07] |
Cf [251.08] |
Es [252.08] |
Fm [257.10] |
Md [258.10] |
No [259.10] |
Lr [262.12] |
||||||
Legend
|
||||||||||||||||||||||
|
||||||||||||||||||||||
|
|
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
関連記事 | 「weigh」「weighted」「atomic」「weight」「atom」 |
.