a) 2.8.7 (A chlorine atom has 17 protons and therefore 17 electrons. The first shell holds 2 electrons, and those after hold 8 electrons)
b) Chlorine exists as a diatomic molecule at room temperature. This means two chlorine atoms share a pair of electrons to complete their octets and form a covalent bond. Covalent bonds are very strong, but the forces between distinct chlorine molecules are weak. Therefore, chlorine exists as a gas at room temperature.