noun
atomic number
uh-TOM-ik NUM-ber
Atomic number (noun) means the number of protons in the nucleus of an atom, which determines what element the atom is and where it sits on the periodic table. Example: “Oxygen has an atomic number of 8, so every oxygen atom holds eight protons.”
noun
1
The number of protons in the nucleus of an atom, which determines what element the atom is and where it sits on the periodic table.
"Oxygen has an atomic number of 8, so every oxygen atom holds eight protons."
"The elements are arranged in order of increasing atomic number."
How to use atomic number
Learner’s notesIn plain EnglishHow many protons an atom has, which is what makes it a particular element.
When to use it
Chemistry and physics. It is written as a subscript before the element symbol and often labelled Z.
Memory tip
Change the atomic number and you change the element itself. Change the neutron count and you only get a different isotope of the same element.
Easily confused with
Common pairings
increasing atomic number
atomic number 26
proton number
in order of atomic number
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Oxygen has an _____ of 8, so every oxygen atom holds eight protons.
Synonyms
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proton number
Z
Related words
Rhymes for atomic number
See all rhymes for atomic number →Frequently asked questions
What does the atomic number tell you?
It tells you how many protons are in the nucleus, and therefore which element you are dealing with. In a neutral atom it also gives the number of electrons, which sets the element's chemical behaviour.
Why are elements ordered by atomic number rather than atomic mass?
Ordering by mass puts a few pairs, such as argon and potassium, in the wrong chemical groups. Sorting by proton count fixes those anomalies because it reflects the underlying structure that actually governs how elements react.
What is the meaning of atomic number?
"atomic number" (noun) means the number of protons in the nucleus of an atom, which determines what element the atom is and where it sits on the periodic table.
What is another word for atomic number?
Other words for "atomic number" include proton number, Z.
