Animation of the formation of a covalent bond between two chlorine (Cl) atoms. Each chlorine atom has seven electrons (black spheres) in its outer shell. Atoms are most stable when they have eight electrons (four pairs) in the outer shell. Two chlorine atoms can achieve this state by sharing their unpaired electrons (purple), which lowers the energy of the system (grey outward flash). This forms the stable chlorine molecule Cl2.
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