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

Recent Examples on the Web

In this photo illustration, metal cubes representing rare earth elements are stacked in a pyramid formation in front of the national flag of China, with each cube labeled with its chemical symbol and atomic number, on July 26, 2025 in Chongqing, China. —Edward Fishman, Time, 21 Nov. 2025

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

Word History

First Known Use

1913, in the meaning defined above

Rhymes for atomic number

• cumber
• lumber
• slumber
• umber
• encumber
• outnumber
• renumber

Kids Definition

atomic number

noun

: a number that is characteristic of a chemical element and represents the number of protons in the nucleus

Medical 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