الوزن الذري القياسي

يجب ألا يـُخلـَط بينها وبين كتلة ذرية.

الوزن الذري العياري of a chemical element (symbol A_r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, isotope ⁶³Cu (A_r = 62.929) constitutes 69% of the copper on Earth, the rest being ⁶⁵Cu (A_r = 64.927), so

Example: copper in terrestrial sources. Two isotopes are present: copper-63 (62.9) and copper-65 (64.9), in abundances 69% + 31%. The standard atomic weight (A_r°(Cu)) for copper is the average, weighted by their natural abundance, and then divided by the atomic mass constant m_u.

$A_{\text{r}}{\text{°}}(_{\text{29}}{\text{Cu}})=0.69\times 62.929+0.31\times 64.927=63.55.$

Because relative isotopic masses are dimensionless quantities, this weighted mean is also dimensionless. It can be converted into a measure of mass (with dimension قالب:Dimanalysis) by multiplying it with the dalton, also known as the atomic mass constant.

Among various variants of the notion of atomic weight (A_r, also known as relative atomic mass) used by scientists, the standard atomic weight (A_r°) is the most common and practical. The standard atomic weight of each chemical element is determined and published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW) of the International Union of Pure and Applied Chemistry (IUPAC) based on natural, stable, terrestrial sources of the element. The definition specifies the use of samples from many representative sources from the Earth, so that the value can widely be used as "the" atomic weight for substances as they are encountered in reality—for example, in pharmaceuticals and scientific research. Non-standardized atomic weights of an element are specific to sources and samples, such as the atomic weight of carbon in a particular bone from a particular archeological site. Standard atomic weight averages such values to the range of atomic weights that a chemist might expect to derive from many random samples from Earth. This range is the rationale for the interval notation given for some standard atomic weight values.

Of the 118 known chemical elements, 80 have stable isotopes and 84 have this Earth-environment based value. Typically, such a value is, for example helium: A_r°(He) = 4.002602(2). The "(2)" indicates the uncertainty in the last digit shown, to read 4.002602±0.000002. IUPAC also publishes abridged values, rounded to five significant figures. For helium, A_{r, abridged}°(He) = 4.0026.

For fourteen elements the samples diverge on this value, because their sample sources have had a different decay history. For example, thallium (Tl) in sedimentary rocks has a different isotopic composition than in igneous rocks and volcanic gases. For these elements, the standard atomic weight is noted as an interval: A_r°(Tl) = [204.38, 204.39]. With such an interval, for less demanding situations, IUPAC also publishes a conventional value. For thallium, A_{r, conventional}°(Tl) = 204.38.

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 التعريف

Excerpt of an IUPAC Periodic Table showing the interval notation of the standard atomic weights of boron, carbon, and nitrogen (Chemistry International, IUPAC). Example: the pie chart for boron shows it to be composed of about 20% ¹⁰B and 80% ¹¹B. This isotope mix causes the atomic weight of ordinary Earthly boron samples to be expected to fall within the interval 10.806 to 10.821. and this interval is the standard atomic weight. Boron samples from unusual sources, particularly non-terrestrial sources, might have measured atomic weights that fall outside this range. Atomic weight and relative atomic mass are synonyms.

The standard atomic weight is a special value of the relative atomic mass. It is defined as the "recommended values" of relative atomic masses of sources in the local environment of the Earth's crust and atmosphere as determined by the IUPAC Commission on Atomic Weights and Isotopic Abundances (CIAAW). In general, values from different sources are subject to natural variation due to a different radioactive history of sources. Thus, standard atomic weights are an expectation range of atomic weights from a range of samples or sources. By limiting the sources to terrestrial origin only, the CIAAW-determined values have less variance, and are a more precise value for relative atomic masses (atomic weights) actually found and used in worldly materials.

The CIAAW-published values are used and sometimes lawfully required in mass calculations. The values have an uncertainty (noted in brackets), or are an expectation interval (see example in illustration immediately above). This uncertainty reflects natural variability in isotopic distribution for an element, rather than uncertainty in measurement (which is much smaller with quality instruments).

Although there is an attempt to cover the range of variability on Earth with standard atomic weight figures, there are known cases of mineral samples which contain elements with atomic weights that are outliers from the standard atomic weight range.

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.

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.^{[محل شك]}

 التعريف الأرضي

An example of why "conventional terrestrial sources" must be specified in giving standard atomic weight values is the element argon. Between locations in the Solar System, the atomic weight of argon varies as much as 10%, due to extreme variance in isotopic composition. Where the major source of argon is the decay of 40 K in rocks, 40 Ar will be the dominant isotope. Such locations include the planets Mercury and Mars, and the moon Titan. On Earth, the ratios of the three isotopes ³⁶Ar : ³⁸Ar : ⁴⁰Ar are approximately 5 : 1 : 1600, giving terrestrial argon a standard atomic weight of 39.948(1).

However, such is not the case in the rest of the universe. Argon produced directly, by stellar nucleosynthesis, is dominated by the alpha-process nuclide 36 Ar. Correspondingly, solar argon contains 84.6% 36 Ar (according to solar wind measurements), and the ratio of the three isotopes ³⁶Ar : ³⁸Ar : ⁴⁰Ar in the atmospheres of the outer planets is 8400 : 1600 : 1. The atomic weight of argon in the Sun and most of the universe, therefore, would be only approximately 36.3.

 مسببات عدم اليقين على الأرض

Famously, the published atomic weight value comes with an uncertainty. This uncertainty (and related: precision) follows from its definition, the source being "terrestrial and stable". Systematic causes for uncertainty are:

1. Measurement limits. As always, the physical measurement is never finite. There is always more detail to be found and read. This applies to every single, pure isotope found. For example, today the mass of the main natural fluorine isotope (fluorine-19) can be measured to the accuracy of eleven decimal places: 18.998403163(6). But a still more precise measurement system could become available, producing more decimals.
2. Imperfect mixtures of isotopes. In the samples taken and measured the mix (relative abundance) of those isotopes may vary. For example, copper. While in general its two isotopes make out 69.15% and 30.85% each of all copper found, the natural sample being measured can have had an incomplete 'stirring' and so the percentages are different. The precision is improved by measuring more samples of course, but there remains this cause of uncertainty. (Example: lead samples vary so much, it can not be noted more precise than four figures: 207.2)
3. Earthly sources with a different history. A source is the greater area being researched, for example 'ocean water' or 'volcanic rock' (as opposed to a 'sample': the single heap of material being investigated). It appears that some elements have a different isotopic mix per source. For example, thallium in igneous rock has more lighter isotopes, while in sedimentary rock it has more heavy isotopes. There is no Earthly mean number. These elements show the interval notation: A_r°(Tl) = [204.38, 204.39]. For practical reasons, a simplified 'conventional' number is published too (for Tl: 204.38).

These three uncertainties are accumulative. The published value is a result of all these.

 تحديد الكتلة الذرية النسبية

المقالة الرئيسية: 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.

النظير	Atomic mass	Abundance
		المعيار	النطاق
28 Si	27.976 926 532 46(194)	92.2297(7)%	92.21–92.25%
29 Si	28.976 494 700(22)	4.6832(5)%	4.67–4.69%
30 Si	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: 28 Si, 29 Si and 30 Si. The atomic masses of these nuclides are known to a precision of one part in 14 billion for 28 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_r(Si) = (27.97693 × 0.922297) + (28.97649 × 0.046832) + (29.97377 × 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^–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.

 جدال التسمية

The use of the name "atomic weight" has attracted a great deal of controversy among scientists. 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.^{[بحاجة لمصدر]}

In reply, supporters of the term "atomic weight" point out (among other arguments) that:

• the name has been in continuous use for the same quantity since it was first conceptualized in 1808;
• for most of that time, atomic weights really were measured by weighing (that is by gravimetric analysis) and the name of a physical quantity should not change simply because the method of its determination has changed;
• the term "relative atomic mass" should be reserved for the mass of a specific nuclide (or isotope), while "atomic weight" be used for the weighted mean of the atomic masses over all the atoms in the sample;
• it is not uncommon to have misleading names of physical quantities which are retained for historical reasons, such as
  • electromotive force, which is not a force
  • resolving power, which is not a power quantity
  • molar concentration, which is not a molar quantity (a quantity expressed per unit amount of substance).

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.

 القيم المنشورة

قالب:Update section IUPAC publishes one formal value for each stable chemical element, called the standard atomic weight. Any updates are published biannually (in uneven years). In 2015, the atomic weight of ytterbium was updated. Per 2017, 14 atomic weights were changed, including argon changing from single number to interval value.

The value published can have an uncertainty, like for neon: 20.1797(6), or can be an interval, like for boron: [10.806, 10.821].

Next to these 84 values, IUPAC also publishes abridged values (up to five digits per number only), and for the twelve interval values, conventional values (single number values).

Symbol A_r is a relative atomic mass, for example from a specific sample. To be specific, the standard atomic weight can be noted as A_r°(E), where (E) is the element symbol.

 Abridged atomic weight

The abridged atomic weight, also published by CIAAW, is derived from the standard atomic weight reducing the numbers to five digits (five significant figures). The name does not say 'rounded'.

Interval borders are rounded downwards for the first (lowmost) border, and upwards for the upward (upmost) border. This way, the more precise original interval is fully covered.

أمثلة:

• Calcium: A_r°(Ca) = 40.078(4) → A_{r, abridged}°(Ca) = 40.078
• Helium: A_r°(He) = 4.002602(2) → A_{r, abridged}°(He) = 4.0026
• Hydrogen: A_r°(H) = [1.00784, 1.00811] → A_{r, abridged}°(H) = [1.0078, 1.0082]

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 Conventional atomic weight

Fourteen chemical elements – hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, argon, bromine, thallium, and lead – have a standard atomic weight that is defined not as a single number, but as an interval. For example, hydrogen has A_r°(H) = [1.00 784, 1.00811]. This notation states that the various sources on Earth have substantially different isotopic constitutions, and uncertainties are incorporated in the two numbers. For these elements, there is not an 'Earth average' constitution, and the 'right' value is not its middle (that would be 1.007975 for hydrogen, with an uncertainty of (±0.000135) that would make it just cover the interval). However, for situations where a less precise value is acceptable, CIAAW has published a single-number conventional atomic weight that can be used for example in trade. For hydrogen, A_{r, conventional}°(H) = 1.008.

 A formal short atomic weight

By using the abridged value, and the conventional value for the fourteen interval values, a short IUPAC-defined value (5 digits plus uncertainty) can be given for all stable elements. In many situations, and in periodic tables, this may be sufficiently detailed.

Overview: formal values of the standard atomic weight
Element (E) ​		Ar°(E)	نوع القيمة ​	Ar°(E)، مختصر أو تقليدي	عدد الكتلة [most stable isotope]
هيدروجين	1H		Interval	1.0080±0.0002
نيتروجين	7N		Interval	14.007±0.001
فلور	9F		Value ± uncertainty	18.998±0.001
كالسيوم	20Ca		Value ± uncertainty	40.078±0.004
تكنتيوم	43Tc	(none)	Most stable isotope

 قائمة الأوزان الذرية

الأوزان الذرية القياسية للعناصر (IUPAC 2009–2021)
Z	الرمز	الاسم	Ar, standard	مختصر	سنة التغيير
1	H	hydrogen	-	1.0080±0.0002	2009
2	He	helium	-	4.0026±0.0001	1983
3	Li	lithium	-	6.94±0.06	2009
4	Be	beryllium	-	9.0122±0.0001	2013
5	B	boron	-	10.81±0.02	2009
6	C	carbon	-	12.011±0.002	2009
7	N	nitrogen	-	14.007±0.001	2009
8	O	oxygen	-	15.999±0.001	2009
9	F	fluorine	-	18.998±0.001	2021
10	Ne	neon	-	20.180±0.001	1985
11	Na	sodium	-	22.990±0.001	2005
12	Mg	magnesium	-	24.305±0.002	2011
13	Al	aluminium	-	26.982±0.001	2017
14	Si	silicon	-	28.085±0.001	2009
15	P	phosphorus	-	30.974±0.001	2013
16	S	sulfur	-	32.06±0.02	2009
17	Cl	chlorine	-	35.45±0.01	2009
18	Ar	argon	-	39.95±0.16	2017
19	K	potassium	-	39.098±0.001	1979
20	Ca	calcium	-	40.078±0.004	1983
21	Sc	scandium	-	44.956±0.001	2021
22	Ti	titanium	-	47.867±0.001	1993
23	V	vanadium	-	50.942±0.001	1977
24	Cr	chromium	-	51.996±0.001	1983
25	Mn	manganese	-	54.938±0.001	2017
26	Fe	iron	-	55.845±0.002	1993
27	Co	cobalt	-	58.933±0.001	2017
28	Ni	nickel	-	58.693±0.001	2007
29	Cu	copper	-	63.546±0.003	1969
30	Zn	zinc	-	65.38±0.02	2007
31	Ga	gallium	-	69.723±0.001	1987
32	Ge	germanium	-	72.630±0.008	2009
33	As	arsenic	-	74.922±0.001	2013
34	Se	selenium	-	78.971±0.008	2013
35	Br	bromine	-	79.904±0.003	2011
36	Kr	krypton	-	83.798±0.002	2001
37	Rb	rubidium	-	85.468±0.001	1969
38	Sr	strontium	-	87.62±0.01	1969
39	Y	yttrium	-	88.906±0.001	2021
40	Zr	zirconium	-	91.224±0.002	1983
41	Nb	niobium	-	92.906±0.001	2017
42	Mo	molybdenum	-	95.95±0.01	2013
43	Tc	technetium	-
44	Ru	ruthenium	-	101.07±0.02	1983
45	Rh	rhodium	-	102.91±0.01	2017
46	Pd	palladium	-	106.42±0.01	1979
47	Ag	silver	-	107.87±0.01	1985
48	Cd	cadmium	-	112.41±0.01	2013
49	In	indium	-	114.82±0.01	2011
50	Sn	tin	-	118.71±0.01	1983
51	Sb	antimony	-	121.76±0.01	1993
52	Te	tellurium	-	127.60±0.03	1969
53	I	iodine	-	126.90±0.01	1985
54	Xe	xenon	-	131.29±0.01	1999
55	Cs	caesium	-	132.91±0.01	2013
56	Ba	barium	-	137.33±0.01	1985
57	La	lanthanum	-	138.91±0.01	2005
58	Ce	cerium	-	140.12±0.01	1995
59	Pr	praseodymium	-	140.91±0.01	2017
60	Nd	neodymium	-	144.24±0.01	2005
61	Pm	promethium	-
62	Sm	samarium	-	150.36±0.02	2005
63	Eu	europium	-	151.96±0.01	1995
64	Gd	gadolinium	-	157.25±0.03	1969
65	Tb	terbium	-	158.93±0.01	2021
66	Dy	dysprosium	-	162.50±0.01	2001
67	Ho	holmium	-	164.93±0.01	2021
68	Er	erbium	-	167.26±0.01	1999
69	Tm	thulium	-	168.93±0.01	2021
70	Yb	ytterbium	-	173.05±0.02	2015
71	Lu	lutetium	-	174.97±0.01	2007
72	Hf	hafnium	-	178.49±0.01	2019
73	Ta	tantalum	-	180.95±0.01	2005
74	W	tungsten	-	183.84±0.01	1991
75	Re	rhenium	-	186.21±0.01	1973
76	Os	osmium	-	190.23±0.03	1991
77	Ir	iridium	-	192.22±0.01	2017
78	Pt	platinum	-	195.08±0.02	2005
79	Au	gold	-	196.97±0.01	2017
80	Hg	mercury	-	200.59±0.01	2011
81	Tl	thallium	-	204.38±0.01	2009
82	Pb	lead	-	207.2±1.1	2020
83	Bi	bismuth	-	208.98±0.01	2005
84	Po	polonium	-
85	At	astatine	-
86	Rn	radon	-
87	Fr	francium	-
88	Ra	radium	-
89	Ac	actinium	-
90	Th	thorium	-	232.04±0.01	2013
91	Pa	protactinium	-	231.04±0.01	2017
92	U	uranium	-	238.03±0.01	1999
93	Np	neptunium	-
94	Pu	plutonium	-
95	Am	americium	-
96	Cm	curium	-
97	Bk	berkelium	-
98	Cf	californium	-
99	Es	einsteinium	-
100	Fm	fermium	-
101	Md	mendelevium	-
102	No	nobelium	-
103	Lr	lawrencium	-
104	Rf	rutherfordium	-
105	Db	dubnium	-
106	Sg	seaborgium	-
107	Bh	bohrium	-
108	Hs	hassium	-
109	Mt	meitnerium	-
110	Ds	darmstadtium	-
111	Rg	roentgenium	-
112	Cn	copernicium	-
113	Nh	nihonium	-
114	Fl	flerovium	-
115	Mc	moscovium	-
116	Lv	livermorium	-
117	Ts	tennessine	-
118	Og	oganesson	-

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 في الجدول الدوري

الجدول الدوري

المجموعة	1	2		3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
	الهيدروجين والفلزات القلوية	الفلزات الأرضية القلوية														Pnicto­gens	Chal­co­gens	Halo­gens	Noble gases
الدورة 1	Hydro­gen 1 H																		He­lium 2 He
2	Lith­ium 3 Li	Beryl­lium 4 Be												Boron 5 B	Carbon 6 C	Nitro­gen 7 N	Oxy­gen 8 O	Fluor­ine 9 F	Neon 10 Ne
3	So­dium 11 Na	Magne­sium 12 Mg												Alumin­ium 13 Al	Sili­con 14 Si	Phos­phorus 15 P	Sulfur 16 S	Chlor­ine 17 Cl	Argon 18 Ar
4	Potas­sium 19 K	Cal­cium 20 Ca		Scan­dium 21 Sc	Tita­nium 22 Ti	Vana­dium 23 V	Chrom­ium 24 Cr	Manga­nese 25 Mn	Iron 26 Fe	Cobalt 27 Co	Nickel 28 Ni	Copper 29 Cu	Zinc 30 Zn	Gallium 31 Ga	Germa­nium 32 Ge	Arsenic 33 As	Sele­nium 34 Se	Bromine 35 Br	Kryp­ton 36 Kr
5	Rubid­ium 37 Rb	Stront­ium 38 Sr		Yttrium 39 Y	Zirco­nium 40 Zr	Nio­bium 41 Nb	Molyb­denum 42 Mo	Tech­netium 43 Tc	Ruthe­nium 44 Ru	Rho­dium 45 Rh	Pallad­ium 46 Pd	Silver 47 Ag	Cad­mium 48 Cd	Indium 49 In	Tin 50 Sn	Anti­mony 51 Sb	Tellur­ium 52 Te	Iodine 53 I	Xenon 54 Xe
6	Cae­sium 55 Cs	Barium 56 Ba		Lute­tium 71 Lu	Haf­nium 72 Hf	Tanta­lum 73 Ta	Tung­sten 74 W	Rhe­nium 75 Re	Os­mium 76 Os	Iridium 77 Ir	Plat­inum 78 Pt	Gold 79 Au	Mer­cury 80 Hg	Thallium 81 Tl	Lead 82 Pb	Bis­muth 83 Bi	Polo­nium 84 Po	Asta­tine 85 At	Radon 86 Rn
7	Fran­cium 87 Fr	Ra­dium 88 Ra		Lawren­cium 103 Lr	Ruther­fordium 104 Rf	Dub­nium 105 Db	Sea­borgium 106 Sg	Bohr­ium 107 Bh	Has­sium 108 Hs	Meit­nerium 109 Mt	Darm­stadtium 110 Ds	Roent­genium 111 Rg	Coper­nicium 112 Cn	Unun­trium 113 Nh	Flerov­ium 114 Fl	Unun­pentium 115 Mc	Liver­morium 116 Lv	Unun­septium 117 Ts	Unun­octium 118 Og
				Lan­thanum 57 La	Cerium 58 Ce	Praseo­dymium 59 Pr	Neo­dymium 60 Nd	Prome­thium 61 Pm	Sama­rium 62 Sm	Europ­ium 63 Eu	Gadolin­ium 64 Gd	Ter­bium 65 Tb	Dyspro­sium 66 Dy	Hol­mium 67 Ho	Erbium 68 Er	Thulium 69 Tm	Ytter­bium 70 Yb
				Actin­ium 89 Ac	Thor­ium 90 Th	Protac­tinium 91 Pa	Ura­nium 92 U	Neptu­nium 93 Np	Pluto­nium 94 Pu	Ameri­cium 95 Am	Curium 96 Cm	Berkel­ium 97 Bk	Califor­nium 98 Cf	Einstei­nium 99 Es	Fer­mium 100 Fm	Mende­levium 101 Md	Nobel­ium 102 No

1 (أحمر)=غاز

3 (أسود)=صلب

80 (أخضر)=سائل

109 (رمادي)=غير معروفة

لون الرقم الذري يبين حالة المادة (عند 0 °س و 1 atm)

بدائي

من اضمحلال

اصطناعي

الإطار يبين التوافر الطبيعي للعنصر

Standard atomic weight A_{r, std}(E)

• Ca: 40.078 — Abridged value (uncertainty omitted here)
• Po: [209] — mass number of the most stable isotope

s-block

p-block

d-block

f-block

لون الخلفية يبين مستوى فرعي بالجدول الدوري

 انظر أيضاً

• International Union of Pure and Applied Chemistry (IUPAC)
• Commission on Isotopic Abundances and Atomic Weights (CIAAW)

 وصلات خارجية

• IUPAC Commission on Isotopic Abundances and Atomic Weights
• Atomic Weights of the Elements 2011