## Atomic Masses

The average weight of an element’s atoms is its atomic mass, which is measured in atomic mass units (amu, also known as daltons, D). The atomic mass is a weighted average of the masses of all of the isotopes of that element. This is done by multiplying the mass of each isotope by how many of them there are. Atomic mass is also called atomic weight, but “mass” is a better word for it.

Experiments have shown that neon has three isotopes: neon-20, which has 10 protons and 10 neutrons in its nucleus and has a mass of 19.992 amu and an abundance of 90.48%; neon-21, which has 10 protons and 11 neutrons in its nucleus and has a mass of 20.994 amu and an abundance of 0.27%; and neon-22, which has 10 protons and 12 neutrons in its nucleus and has a mass The average number of atoms in a piece of neon is:

When used with the idea of a mole, the atomic mass is useful in chemistry. The atomic mass of an element, which is measured in amu, is the same as the mass of one mole of an element, which is measured in grammes. So, since the atomic mass of iron is 55.847 amu, 55.847 grammes would be the weight of one mole of iron atoms. You can use the same idea to think about ionic compounds and molecules. One formula unit of sodium chloride (NaCl) would weigh 58.44 amu (22.98977 amu for Na and 35.453 amu for Cl), so a mole of sodium chloride would weigh 58.44 grammes. One molecule of water (H2O) weighs 18.02 amu (21.00797 amu for H + 15.9994 amu for O), and a mole of water molecules weighs 18.02 grammes.

Dimitri Mendeleev published the first periodic table of the elements in 1869. The elements that were known at the time were listed in order of increasing atomic weight, since the nucleus and the inside structure of the atom had not yet been discovered. Instead, the modern periodic table is organised by the number of atoms.

## Elements sorted by Atomic Mass

Atomic NumberSymbolNameAtomic Weight (amu, g/mol)
1HHydrogen1.00797
2HeHelium4.00260
3LiLithium6.941
4BeBeryllium9.01218
5BBoron10.81
6CCarbon12.011
7NNitrogen14.0067
8OOxygen15.9994
9FFluorine18.998403
10NeNeon20.179
11NaSodium22.98977
12MgMagnesium24.305
13AlAluminum26.98154
14SiSilicon28.0855
15PPhosphorus30.97376
16SSulfur32.06
17ClChlorine35.453
19KPotassium39.0983
18ArArgon39.948
20CaCalcium40.08
21ScScandium44.9559
22TiTitanium47.90
24CrChromium51.996
25MnManganese54.9380
26FeIron55.847
28NiNickel58.70
27CoCobalt58.9332
29CuCopper63.546
30ZnZinc65.38
31GaGallium69.72
32GeGermanium72.59
33AsArsenic74.9216
34SeSelenium78.96
35BrBromine79.904
36KrKrypton83.80
37RbRubidium85.4678
38SrStrontium87.62
39YYttrium88.9059
40ZrZirconium91.22
41NbNiobium92.9064
42MoMolybdenum95.94
43TcTechnetium(98)
44RuRuthenium101.07
45RhRhodium102.9055
47AgSilver107.868
49InIndium114.82
50SnTin118.69
51SbAntimony121.75
53IIodine126.9045
52TeTellurium127.60
54XeXenon131.30
55CsCesium132.9054
56BaBarium137.33
57LaLanthanum138.9055
58CeCerium140.12
59PrPraseodymium140.9077
60NdNeodymium144.24
61PmPromethium(145)
62SmSamarium150.4
63EuEuropium151.96
65TbTerbium158.9254
66DyDysprosium162.50
67HoHolmium164.9304
68ErErbium167.26
69TmThulium168.9342
70YbYtterbium173.04
71LuLutetium174.967
72HfHafnium178.49
73TaTantalum180.9479
74WTungsten183.85
75ReRhenium186.207
76OsOsmium190.2
77IrIridium192.22
78PtPlatinum195.09
79AuGold196.9665
80HgMercury200.59
81TlThallium204.37
83BiBismuth208.9804
84PoPolonium(209)
85AtAstatine(210)
87FrFrancium(223)
89AcActinium227.0278
91PaProtactinium231.0359
90ThThorium232.0381
93NpNeptunium237.0482
92UUranium238.029
94PuPlutonium(242)
95AmAmericium(243)
97BkBerkelium(247)
96CmCurium(247)
102NoNobelium(250)
98CfCalifornium(251)
99EsEinsteinium(252)
108HsHassium(255)
109MtMeitnerium(256)
100FmFermium(257)
101MdMendelevium(258)
103LrLawrencium(260)
104RfRutherfordium(261)
107BhBohrium(262)
105DbDubnium(262)
106SgSeaborgium(263)
110UunUnunnilium(269)
111UuuUnununium(272)
112UubUnunbiium(277)
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