Lewis Shell Model

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The diagrams above illustrate the first two shells in Lewis's shell model for hydrogen, helium, and lithium, which are the first three elements in the modern version of Mendeleev's periodic table. The atomic nuclei, composed of neutrons (blue) and protons (red), are shown at the center of the shells occupied by the electrons. These shell diagrams are not meant to be an accurate picture of what an atom looks like, but they are useful for understanding chemical properties of the atoms. Lewis determined empirically that the first shell holds two electrons and the second shell holds eight electrons, and judged that full shells correspond to systems of special stability. Hydrogen and lithium, with their unfilled outer shells, are thus more chemically active than helium, with its first shell full and no electrons in its second shell. (Unit: 6)