Why is it possible for an oxygen atom to form a double covalent bond, but it is not possible for a chlorine atom to form a double covalent bond?

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An oxygen atom has six valence electrons, like all elements in group 6A of the periodic table. A chlorine atom has seven valence electrons, like all elements in group 7A. The octet rule states that atoms will gain, lose or share enough electrons to have eight valence electrons. This results...

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An oxygen atom has six valence electrons, like all elements in group 6A of the periodic table. A chlorine atom has seven valence electrons, like all elements in group 7A. The octet rule states that atoms will gain, lose or share enough electrons to have eight valence electrons. This results in the stable electron configuration of a noble gas.

Oxygen therefore needs to gain two electrons to be stable. When two oxygen atoms come together they share two pairs of electrons between them, resulting in a double bond and a complete octet for each atom. The shared or bonding pairs of electrons complete the octet for both atoms. Oxygen can form a double covalent bond with any atom that tends to share two valance electrons. In carbon dioxde, for examaple, there's a double covalent bond between the central carbon and each oxgen. 

Each chlorine atom needs to gain just one electron to achieve a noble gas electron configuration. When two chlorine atoms come together they share one pair of electrons between them, resulting in a single bond. Since the single bond completes the octet for both chlorine atoms they don't have any other valence electrons available to share in additional bonds.  This is also true of bonds between chorine and other atoms that need an additional valence electron. For example, carbon tetrachloride has a central carbon atom that's single bonded to each of four different chlorine atoms.

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