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Animation of the bonding in a chlorine molecule (Cl2). Each chlorine atom has 17 electrons, with twelve in full electron orbitals and not shown for clarity, and five in its three outer 3p orbitals (blue, red and orange). Each orbital can hold two electrons, so having five means that one 3p orbital (blue) is half full, which is unstable. To achieve a more stable, lower energy state, two chlorine atoms can come together and share their unpaired electrons by merging their half-full orbitals, shown here as a merging of the adjacent blue orbitals. This process lowers the energy of the system, releasing energy in a flash.
Model release not required. Property release not required.