What is the atomic number of helium?

Helium has atomic number 2 because every helium atom contains two protons in its nucleus. Atomic number is defined by proton count, so it uniquely identifies an element. A neutral helium atom also contains two electrons, arranged as 1s². This completely filled first shell makes helium chemically stable and places it in Group 18 with the noble gases. The most common isotope, helium-4, has two protons and two neutrons, giving it a mass number of 4. Atomic number and mass number are different: atomic number counts only protons, whereas mass number counts protons plus neutrons. Concept depth: Changing the number of neutrons creates an isotope, but changing the number of protons creates a different element.