A chlorine atom can form a single covalent bond, according to the University of Wisconsin chemistry department website. Chlorine atoms have seven electrons in their outer shells and can only share a single electron with another atom to fill that outer electron shell.
The periodic table indicates how many bonds are necessary to fill electron shells by which columns the elements are placed in. Chlorine is in the column for elements with seven electrons in their outer shells. Chlorine is also a highly electronegative element, which means it holds onto its electrons very tightly. As a result, although it shares its single electron with another atom in a covalent bond, the other electrons are not available for additional bonds.