Atomic mass

In chemistry, the atomic mass (formerly atomic weight) is the mass of an atom expressed in unified atomic mass units (u).

Different isotopes of an atom are characterized by a different numbers of neutrons in the atomic nucleus. Hence, different isotopes of the same atom have different masses. For instance, carbon has two stable isotopes and one radioactive—but long-lived—isotope. Their respective atomic masses are, ¹²C: 12 u, ¹³C: 13.0033548378 u, and ¹⁴C: 14.003241988 u. The atomic mass of ¹²C is by definition the integral number 12.

In high resolution spectroscopy masses of different isotopes are observed in the spectra, and in this field computations are usually done for molecules consisting of well defined isotopes. In chemistry this is different. Chemicals used in the laboratory are in general isotopic mixtures: their molecules consist of different isotopes of one and the same element. The proportion of different isotopes in the molecule is determined by the natural abundance of the isotope. Take chlorine as an example. This element has two stable isotopes:  ³⁵Cl (with a mass of 34.96885271 u) and ³⁷Cl (with a mass of 36.96590260 u). Of all the chlorine atoms occurring on earth 75.78 % is of the lighter kind, while 24.22 % is the heavier isotope. The average mass of the Cl atom is thus (34.96885271*75.78 + 36.96590260*24.22)/100 = 35.453 u.

The atomic mass averaged over isotopic abundances is called the relative atomic mass or standard atomic weight. (For historical reasons the term "weight" is used here. Instead of average atomic mass, as one would expect, one uses relative atomic mass, also for historical reasons). For the standard atomic weight (= relative atomic mass), the symbol A_r(X) is used, where X is an isotope.

Below a table is given for the standard atomic weights. Brackets [ ] indicate the mass number of the most stable isotope. CS stands for chemical symbol. Z is the atomic number. The numbers in this table are from the web site of NIST: physics reference data.

External link

Article about Atomic Weight

---

Standard Atomic Weights of the Elements

Z	CS	Mass		Z	CS	Mass		Z	CS	Mass
1	H	1.00794(7)		38	Sr	87.62(1)		75	Re	186.207(1)
2	He	4.002602(2)		39	Y	88.90585(2)		76	Os	190.23(3)
3	Li	6.941(2)		40	Zr	91.224(2)		77	Ir	192.217(3)
4	Be	9.012182(3)		41	Nb	92.90638(2)		78	Pt	195.078(2)
5	B	10.811(7)		42	Mo	95.94(2)		79	Au	196.96655(2)
6	C	12.0107(8)		43	Tc	[98]		80	Hg	200.59(2)
7	N	14.0067(2)		44	Ru	101.07(2)		81	Tl	204.3833(2)
8	O	15.9994(3)		45	Rh	102.90550(2)		82	Pb	207.2(1)
9	F	18.9984032(5)		46	Pd	106.42(1)		83	Bi	208.98038(2)
10	Ne	20.1797(6)		47	Ag	107.8682(2)		84	Po	[209]
11	Na	22.989770(2)		48	Cd	112.411(8)		85	At	[210]
12	Mg	24.3050(6)		49	In	114.818(3)		86	Rn	[222]
13	Al	26.981538(2)		50	Sn	118.710(7)		87	Fr	[223]
14	Si	28.0855(3)		51	Sb	121.760(1)		88	Ra	[226]
15	P	30.973761(2)		52	Te	127.60(3)		89	Ac	[227]
16	S	32.065(5)		53	I	126.90447(3)		90	Th	232.0381(1)
17	Cl	35.453(2)		54	Xe	131.293(6)		91	Pa	231.03588(2)
18	Ar	39.948(1)		55	Cs	132.90545(2)		92	U	238.02891(3)
19	K	39.0983(1)		56	Ba	137.327(7)		93	Np	[237]
20	Ca	40.078(4)		57	La	138.9055(2)		94	Pu	[244]
21	Sc	44.955910(8)		58	Ce	140.116(1)		95	Am	[243]
22	Ti	47.867(1)		59	Pr	140.90765(2)		96	Cm	[247]
23	V	50.9415(1)		60	Nd	144.24(3)		97	Bk	[247]
24	Cr	51.9961(6)		61	Pm	[145]		98	Cf	[251]
25	Mn	54.938049(9)		62	Sm	150.36(3)		99	Es	[252]
26	Fe	55.845(2)		63	Eu	151.964(1)		100	Fm	[257]
27	Co	58.933200(9)		64	Gd	157.25(3)		101	Md	[258]
28	Ni	58.6934(2)		65	Tb	158.92534(2)		102	No	[259]
29	Cu	63.546(3)		66	Dy	162.500(1)		103	Lr	[262]
30	Zn	65.409(4)		67	Ho	164.93032(2)		104	Rf	[261]
31	Ga	69.723(1)		68	Er	167.259(3)		105	Db	[262]
32	Ge	72.64(1)		69	Tm	168.93421(2)		106	Sg	[266]
33	As	74.92160(2)		70	Yb	173.04(3)		107	Bh	[264]
34	Se	78.96(3)		71	Lu	174.967(1)		108	Hs	[277]
35	Br	79.904(1)		72	Hf	178.49(2)		109	Mt	[268]
36	Kr	83.798(2)		73	Ta	180.9479(1)		110	Ds	[281]
37	Rb	85.4678(3)		74	W	183.84(1)		111	Rg	[272]