What Is an Atomic Number? Definition and Examples


Atomic Number vs Mass Number
Atomic Number vs Mass Number

The atomic number is the number of protons found in the nucleus of an atom, which uniquely identifies its element. The atomic number is also called the proton number. It is denoted by the symbol Z and is the subscript in atomic notation. The symbol Z comes from the German word zahl, which means numeral, or atomzahl, which means atomic number.

Because neutrons are neutral, the atomic number equals the electric charge of the atomic nucleus. In a neutral atom, the atomic number is equal to the number of electrons.

Importance of the Atomic Number

The atomic number is important because it identifies the element. Also, the periodic table is arranged in order of increasing atomic number. The atomic number determines an element’s properties because its is the number of electrons in a neutral atom. This defines the atom’s electron configuration and the nature of its valence electron shell. The valence electrons determine how readily an atom forms chemical bonds and the type of bonds it forms.

List of Atomic Numbers

Atomic numbers are always whole positive numbers. All atoms with atomic number 1 are hydrogen atoms; all atoms with atomic number 118 are oganesson atoms. The number of neutrons and electrons doesn’t affect an atom’s identity, only its isotope and electrical charge, respectively. The periodic table currently has 118 atomic numbers. When a new element is discovered, its atomic number will be the number of protons in its atomic nucleus.

 ATOMIC NUMBER SYMBOLELEMENT
1HHydrogen
2HeHelium
3LiLithium
4BeBeryllium
5BBoron
6CCarbon
7NNitrogen
8OOxygen
9FFluorine
10NeNeon
11NaSodium
12MgMagnesium
13AlAluminum
14SiSilicon
15PPhosphorus
16SSulfur
17ClChlorine
18ArArgon
19KPotassium
20CaCalcium
21ScScandium
22TiTitanium
23VVanadium
24CrChromium
25MnManganese
26FeIron
27CoCobalt
28NiNickel
29CuCopper
30ZnZinc
31GaGallium
32GeGermanium
33AsArsenic
34SeSelenium
35BrBromine
36KrKrypton
37RbRubidium
38SrStrontium
39YYttrium
40ZrZirconium
41NbNiobium
42MoMolybdenum
43TcTechnetium
44RuRuthenium
45RhRhodium
46PdPalladium
47AgSilver
48CdCadmium
49InIndium
50SnTin
51SbAntimony
52TeTellurium
53IIodine
54XeXenon
55CsCesium
56BaBarium
57LaLanthanum
58CeCerium
59PrPraseodymium
60NdNeodymium
61PmPromethium
62SmSamarium
63EuEuropium
64GdGadolinium
65TbTerbium
66DyDysprosium
67HoHolmium
68ErErbium
69TmThulium
70YbYtterbium
71LuLutetium
72HfHafnium
73TaTantalum
74WTungsten
75ReRhenium
76OsOsmium
77IrIridium
78PtPlatinum
79AuGold
80HgMercury
81TlThallium
82PbLead
83BiBismuth
84PoPolonium
85AtAstatine
86RnRadon
87FrFrancium
88RaRadium
89AcActinium
90ThThorium
91PaProtactinium
92UUranium
93NpNeptunium
94PuPlutonium
95AmAmericium
96CmCurium
97BkBerkelium
98CfCalifornium
99EsEinsteinium
100FmFermium
101MdMendelevium
102NoNobelium
103LrLawrencium
104RfRutherfordium
105DbDubnium
106SgSeaborgium
107BhBohrium
108HsHassium
109MtMeitnerium
110DsDarmstadtium
111RgRoentgenium
112CnCopernicium
113NhNihonium
114FlFlerovium
115McMoscovium
116LvLivermorium
117TsTennessine
118OgOganesson
Atomic numbers and symbols of all 118 elements

Atomic Number vs Mass Number

While the atomic number is the number of protons in an atom, the mass number is the sum of the number of protons and neutrons (the nucleons). The symbol for mass number is A, which comes from the German word Atomgewcht (atomic weight).

The mass number identifies the isotope of an element. Isotopes of an element have the same atomic number, but different mass numbers. Mass number may be written after an element name or symbol (e.g., carbon-14) or as superscript above or to the left of an element symbol (e.g., 14C). A full isotope symbol (A/Z format) includes both the atomic mass and atomic number (e.g., 146C, 126C).

Atomic Number and Mass Number
Atomic number (A) is the number of protons plus neutrons, while mass number (Z) is the number of protons.

Mass number does not include the mass of electrons because they are negligible compared to the mass of protons or neutrons. Protons and neutrons each weigh about one atomic mass unit (amu), while the mass of an electron is only 0.000549 amu.

How to Find the Atomic Number

How you find the atomic number of an element depends on the information you’re given.

  • If you know the name or symbol of the element, you can look up the atomic number on any periodic table. (The exception is Mendeleev’s periodic table, which arranged elements by atomic weight rather than atomic number.) There may be many numbers associated with each element, but the atomic number is always a positive whole number.
  • Find the atomic number from the isotope symbol the same way. For example, if the symbol is 14C, you know the element symbol is C. Look for the symbol “C” on the periodic table to get the atomic number.
  • Usually, both the mass number and atomic number are given in an isotope symbol. For example, if the symbol is 146C, the number “6” is listed. The atomic number is the smaller of the two numbers in the symbol. It is typically located as a subscript to the left of the element symbol.

References

  • IUPAC (1997). “Atomic Number (Proton Number) Z”. Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). Blackwell Scientific Publications: Oxford. doi:10.1351/goldbook
  • Jensen, William B. (2005). “The Origins of the Symbols A and Z for Atomic Weight and Number”. J. Chem. Educ. 82: 1764.
  • Scerri, Eric (2013). A Tale of Seven Elements. Oxford University Press. ISBN 978-0-19-539131-2.

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