Element
A pure substance composed entirely of atoms that share the same atomic number and therefore cannot be split into simpler substances by chemical means.
Unit
Dimensionless
In depth
There are 118 confirmed elements, each with its own symbol and position on the periodic table. An element may still contain several isotopes, since the neutron count can vary while the proton count cannot. Only nuclear processes — not chemical reactions — can convert one element into another.
Examples in the real world
Gold (Au), oxygen (O₂), helium (He) and carbon, which occurs as the allotropes graphite, diamond and graphene.