What Is an Atomic Number? Definition and Examples

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 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).

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.