Atomic number: definition in chemistry
Atomic number: The number of protons in the nucleus of an atom, given the symbol Z. It identifies the element: every atom with 6 protons is carbon.
Atoms of one element can have different numbers of neutrons or electrons, but never a different number of protons. In a neutral atom the atomic number also equals the number of electrons, which is why it predicts chemical behavior. In 1913 Henry Moseley used X-rays to show that atomic number, rather than atomic mass, is the true basis for ordering the elements.
Examples
- Hydrogen — atomic number 1
- Iron — atomic number 26
- Uranium — atomic number 92
- Written as a subscript at lower left of the symbol, as in ¹⁴₆C
On the periodic table
The atomic number is usually the whole number at the top of each element's box, and the table runs in order from 1 (hydrogen) to 118 (oganesson).