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The concept involved here is known as covalent bonding of atoms. A Covalent bond is a chemical bond that involves sharing of electron pairs between atoms.

These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding.

Two chlorine atoms are joined by a covalent bond. The reason that the two chlorine atoms stick together is that the shared pair of electrons are attracted to the nucleus of both chlorine atoms.

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