This means that every atom of helium has two protons in its nucleus and two electrons in its outer shell. The atomic number is a fundamental property of an element that defines its identity and place in the periodic table. It also determines the number of electrons that an atom can have in its ground state, and thus its chemical behavior and bonding characteristics. ^{2 3}

Some other properties of helium that are related to its atomic number are:

• Group and period: It belongs to group 18 (the noble gases) and period 1 of the periodic table. It is the second lightest and the first element in the s-block. ^{1 3}
• Stability and reactivity: It has a very low electron affinity and ionization energy, which makes it very stable and inert. It does not form compounds with other elements under normal conditions, and only forms weak and unstable excimers or van der Waals compounds under extreme conditions. ^{2 4}
• Isotopes: It has two stable isotopes: helium-3 and helium-4, which differ in the number of neutrons in their nuclei. Helium-4 is the most abundant isotope, accounting for nearly all of the natural helium. Helium-3 is rare and has applications in nuclear fusion and low-temperature physics. ^{2 4}
• Physical properties: It has the lowest boiling point and melting point of any element. It remains liquid down to absolute zero at normal pressure, and can only be solidified by applying high pressure. It also has a very high specific heat and thermal conductivity, making it an excellent coolant for superconducting magnets, nuclear reactors, and other devices. ^{2 4}
• Abundance and sources: It is the second most abundant element in the universe, after hydrogen. It is produced by the nuclear fusion of hydrogen in stars, and by the radioactive decay of heavy elements on Earth. It is found in trace amounts in the atmosphere, and in larger quantities in natural gas fields, where it is extracted commercially. ^{2 4}