What does the atomic number of an element mean?
The number of protons or electrons an atom has.
The periodic table is organised by the atomic number. If you look at it, you will see it begin at hydrogen with
In a scientific sense, the atomic number is the number of protons you find in the nucleus of the atom, or the number of electrons floating around the outside. Protons and electrons come in the same number in atoms. (Ions are where the electron number differs from protons.)
For example, in lithium, the atomic number is