Atomic Numbers

The elements on the modern periodic table are listed in order of increasing atomic number. The number of protons in the nucleus of an atom is its atomic number. The number of protons tells us what an element is (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present). The number of protons determines how many electrons are around the nucleus, and how these electrons are arranged determines most of an element’s chemical behaviour.

1A2A3A4A5A6A7A8A
(1)(2)(13)(14)(15)(16)(17)(18)
3B4B5B6B7B8B1B2B
(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)
112
2345678910
31112131415161718
4192021222324252627282930313233343536
5373839404142434445464748495051525354
6555657 727374757677787980818283848586
7878889 104105106107108109110111112114
6 5859606162636465666768697071
7 90919293949596979899100101102103

In an atomic number-based periodic table, elements with similar chemical properties tend to fall into the same column (group). For example, all of the elements in Group 1A are soft metals that react violently with water and make 1+ charges. All of the elements in Group 8A are nonreactive monatomic gases at room temperature. In other words, the properties of chemical elements tend to repeat themselves over and over again as their mass goes up.

In Dimitri Mendeleev’s first periodic table, published in 1869, the elements were arranged in order of increasing atomic mass. At the time, the nucleus had not yet been found, and no one knew how the inside of an atom was put together, so atomic mass was the only way to organise the elements. Once the structure of the nucleus was understood, it was clear that the atomic number was what controlled the properties of the elements.

Atomic Number Symbol Name Atomic Mass (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
18ArArgon39.948
19KPotassium39.0983
20CaCalcium40.08
21ScScandium44.9559
22TiTitanium47.90
23VVanadium50.9415
24CrChromium51.996
25MnManganese54.9380
26FeIron55.847
27CoCobalt58.9332
28NiNickel58.70
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
46PdPalladium106.4
47AgSilver107.868
48CdCadmium112.41
49InIndium114.82
50SnTin118.69
51SbAntimony121.75
52TeTellurium127.60
53IIodine126.9045
54XeXenon131.30
55CsCesium132.9054
56BaBarium137.33
57LaLanthanum138.9055
58CeCerium140.12
59PrPraseodymium140.9077
60NdNeodymium144.24
61PmPromethium(145)
62SmSamarium150.4
63EuEuropium151.96
64GdGadolinium157.25
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
82PbLead207.2
83BiBismuth208.9804
84PoPolonium(209)
85AtAstatine(210)
86RnRadon(222)
87FrFrancium(223)
88RaRadium226.0254
89AcActinium227.0278
90ThThorium232.0381
91PaProtactinium231.0359
92UUranium238.029
93NpNeptunium237.0482
94PuPlutonium(242)
95AmAmericium(243)
96CmCurium(247)
97BkBerkelium(247)
98CfCalifornium(251)
99EsEinsteinium(252)
100FmFermium(257)
101MdMendelevium(258)
102NoNobelium(250)
103LrLawrencium(260)
104RfRutherfordium(261)
105DbDubnium(262)
106SgSeaborgium(263)
107BhBohrium(262)
108HsHassium(255)
109MtMeitnerium(256)
110DsDarmstadtium(269)
111RgRoentgenium(272)
112UubUnunbiium(277)
113—————
114UuqUnunquadium