DR MARK J. WINTER / SCIENCE PHOTO LIBRARY DR MARK J. WINTER / SCIENCE PHOTO LIBRARY
An electron orbital is a region around an atomic nucleus (not seen) in which one or a pair of electrons is most likely to exist. The three 3p orbitals are made up of four lobes, two large outer ones and two smaller inner ones, centred on the nucleus. They are part of the 3 shell, which also contains a spherical, lower energy 3s orbital and five 3d orbitals at a higher energy (not seen). In atoms, electrons fill the lower energy orbitals first. The 3p orbitals are only filled when the lower energy 1s, 2s, 2p and 3s orbitals are full. Hence, the 3p orbitals are full in element 18, Argon (1s2 2s2 2p6 3s2 3p6).
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