A helium atom is the second lightest and second most abundant element in the universe, with the atomic number 2. It consists of two protons in its nucleus, which gives it its atomic number, along with two neutrons (in its most common isotope, helium-4) and two electrons surrounding the nucleus. **Key characteristics of helium:** 1. **Noble Gas:** Helium belongs to the group of noble gases on the periodic table.
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A neutral helium atom contains two electrons bound to a nucleus of charge . Even with a fixed nucleus and nonrelativistic kinetics, the repulsion prevents exact separation into two hydrogenic one-electron problems. Its ground configuration is with total spin zero, while distinct occupied orbitals allow both singlet helium states and triplet helium states.