Helium atom

An atom with two protons in its nucleus and two electrons. Its nucleus usually contains one or two neutrons, producing helium-3 or helium-4.

Atomic nucleus: The dense central region of an atom, containing protons and usually neutrons. The helium nucleus contains two protons and, in common isotopes, one or two neutrons.
Helium electronic configuration: The distribution of helium’s two electrons among atomic orbitals, normally 1s² in its ground state. This configuration is the simplest complete, closed electron shell.
Jules Janssen: A French astronomer who studied the Sun’s spectrum and observed the 1868 solar eclipse. His eclipse observations helped establish the solar spectral line later associated with helium.
Helium-3: A stable helium isotope whose nucleus contains two protons and one neutron. It is the lighter common helium isotope, with different nuclear properties from helium-4.
Electron: A negatively charged elementary particle found in atoms and other systems. Two electrons occupy the bound states around a helium nucleus.
Pauli exclusion principle: The quantum rule that identical fermions cannot occupy the same quantum state. The two 1s electrons must have opposite spins to share that orbital.
Norman Lockyer: An English astronomer who identified a previously unknown yellow line in the solar spectrum in 1868. He proposed that the line came from a new element, naming it helium.
Helium-4: A stable helium isotope whose nucleus contains two protons and two neutrons. It is by far the most abundant helium isotope and the usual helium nucleus.
Proton: A positively charged particle found in atomic nuclei. The two protons define helium’s atomic number and positive nuclear charge.
Electron-electron interaction: The electrostatic repulsion and quantum correlations between electrons in an atom or material. Repulsion between helium’s two electrons makes its energy levels harder to calculate than hydrogen’s.

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Old quantum theoryRelated: Old quantum rules could not reliably predict its spectrum, revealing their limited scope.