Atomic Number Definition

Glossary Definition of Atomic Number

protons in an atom.
The atomic number identifies the number of protons in an atom. ALFRED PASIEKA/SCIENCE PHOTO LIBRARY / Getty Images

The atomic number of a chemical element is the number of protons in the nucleus of an atom of the element. It is the charge number of the nucleus since neutrons carry no net electrical charge. The atomic number determines the identity of an element and many of its chemical properties. The modern periodic table is ordered by increasing atomic number.

Atomic Number Examples

The atomic number of hydrogen is 1; the atomic number of carbon is 6, and the atomic number of silver is 47: any atom with 47 protons is an atom of silver. Varying the number of neutrons in an element changes its isotopes while changing the numbers of electrons makes it an ion.

Also Known As: The atomic number is also known as the proton number. It may be represented by the capital letter Z. The use of capital letter Z comes from the German word Atomzahl, which means "atomic number." Before the year 1915, the word Zahl (number) was used to describe an element's position on the periodic table.

Relationship Between Atomic Number and Chemical Properties

The reason the atomic number determines the chemical properties of an element is that the number of protons also determines the number of electrons in an electrically neutral atom. This, in turn, defines the electron configuration of the atom and the nature of its outermost or valence shell. The behavior of the valence shell determines how readily an atom will form chemical bonds and participate in chemical reactions.

New Elements and Atomic Numbers

At the time of this writing, elements with atomic numbers 1 through 118 have been identified. Scientists typically talk about discovering new elements with higher atomic numbers. Some researchers believe there may be an "island of stability," where the configuration of protons and neutrons of superheavy atoms will be less susceptible to the quick radioactive decay seen in known heavy elements.

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Helmenstine, Anne Marie, Ph.D. "Atomic Number Definition." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). Atomic Number Definition. Retrieved from Helmenstine, Anne Marie, Ph.D. "Atomic Number Definition." ThoughtCo. (accessed June 7, 2023).