atomic number
noun
: an experimentally determined number characteristic of a chemical element that represents the number of protons in the nucleus which in a neutral atom equals the number of electrons outside the nucleus and that determines the place of the element in the periodic table see Chemical Elements Table
Examples of atomic number in a Sentence
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The two metals appear on the same line in the periodic table with their proton count and also their atomic number separated only by mercury (80) and thallium (81).
—
Tim Treadgold, Forbes.com, 13 May 2025
Soil testing: In the May 4 A Section, an article about toxic substances found in the Palisades and Eaton fire zones included an illustration that gave the atomic number of molybdenum as 40.
—Los Angeles Times, 5 May 2025
Because 79 is the atomic number of the chemical element of gold.
—Devorah Lev-Tov, Architectural Digest, 17 Oct. 2024
The periodic table arranges elements based on their atomic number, or the number of protons each element has in its nucleus.
—Michael Irving, New Atlas, 25 July 2024
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Tim Treadgold, Forbes.com, 13 May 2025
Soil testing: In the May 4 A Section, an article about toxic substances found in the Palisades and Eaton fire zones included an illustration that gave the atomic number of molybdenum as 40.
—Los Angeles Times, 5 May 2025
Because 79 is the atomic number of the chemical element of gold.
—Devorah Lev-Tov, Architectural Digest, 17 Oct. 2024
The periodic table arranges elements based on their atomic number, or the number of protons each element has in its nucleus.
—Michael Irving, New Atlas, 25 July 2024
See All Example Sentences for atomic number
Cite this Entry
“Atomic number.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/atomic%20number. Accessed 31 Oct. 2025.
Copy CitationMedical Definition
atomic number
noun
atomic num·ber
-ˈnəm-bər 
: an experimentally determined number characteristic of a chemical element that represents the number of protons in the nucleus which in a neutral atom equals the number of electrons outside the nucleus and that determines the place of the element in the periodic table see Chemical Elements Table
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