What is an atomic number

An atomic number is the number of protons in the nucleus of an atom. It is used to identify an element. For example: Hydrogen has atomic number 1 because it has 1 proton. Carbon has atomic number 6 because it has 6 protons. Oxygen has atomic number 8 because it has 8 protons. So, the simple definition is: Atomic number = number of protons in an atom.