atomic number

American  

[uh-tom-ik nuhm-ber] / əˈtɒm ɪk ˈnʌm bər /

noun

1. the number of positive charges or protons in the nucleus of an atom of a given element, and therefore also the number of electrons normally surrounding the nucleus. at. no.; Z

---

atomic number British  

noun

1.  Z.  Also called: proton number.   at. no..  the number of protons in the nucleus of an atom of an element

---

atomic number Scientific  

1. The number of protons in the nucleus of an atom. In electrically neutral atoms, this number is also equal to the number of electrons orbiting about the atom's nucleus. The atomic number of an element determines its position in the Periodic Table, and is usually denoted by the letter Z and written as a subscript before an element's symbol, as in ₉₂ U.

---

atomic number Cultural  

1. The number of protons or electrons normally found in an atom of a given chemical element. The higher the atomic number, the heavier the atom is. In a neutral atom, the number of protons and electrons is the same. (See atomic weight and periodic table of the elements.)

---

Etymology

Origin of atomic number

First recorded in 1815–25

Compare meaning

How does atomic-number compare to similar and commonly confused words? Explore the most common comparisons:

• atomic mass vs. atomic number
• atomic number vs. mass number
• atomic number vs. atomic weight

Example Sentences

One of the most critical rare earths is dysprosium, atomic number 66 on the periodic table.

From The Wall Street Journal • Oct. 11, 2025

Lighter elements -- generally those with an atomic number of 20 or lower on the periodic table -- often have an equal number of protons and neutrons.

From Science Daily • Feb. 26, 2024

Katie Hafner: Uranium is the last of the naturally occurring elements on the periodic table, with an atomic number of 92.

From Scientific American • Sep. 7, 2023

For in truth, how does it really benefit most of us to know why sodium’s atomic number is 11 and magnesium’s 12?

From Washington Post • Apr. 28, 2023

The chemistry of an atom depends only on the number of electrons, which equals the number of protons, and which is called the atomic number.

From "Cosmos" by Carl Sagan