Chlorine at a glance
- Atomic number
- 17
- Symbol
- Cl
- Atomic mass
- 35.45 u
- Category
- Halogen
- Group / period
- 17 / 3
- Electron configuration
- [Ne] 3s 2 3p 5
- State at 25 °C
- Gas
- Melting point
- -101.5 °C
- Boiling point
- -34.04 °C
- Density
- 3.21 g/L
- Electronegativity
- 3.16
- 1st ionization energy
- 1251 kJ/mol
- Oxidation states
- −1, +1, +5, +7
- Discovered
- 1774
Atomic structure
A neutral chlorine atom has 17 protons and 17 electrons, and its most common isotope carries about 18 neutrons. One atom weighs about 5.89 × 10⁻²⁶ kg, so a mole of chlorine (6.022 × 10²³ atoms) weighs 35.45 g. That nucleus holds 1.06 neutrons for every proton, close to the one-to-one balance that light nuclei prefer. The electrons fill 3 shells (K 2, L 8, M 7), which gives the configuration [Ne] 3s 2 3p 5. With 7 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has 2 stable isotopes: Cl-35 and Cl-37. No other element has a stable isotope with mass number 35, so Cl-35 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) chlorine is a gas. It melts at -101.5 °C and boils at -34.04 °C, so it stays liquid across a 67.51-degree range. To turn liquid it has to be cooled below -34.04 °C, colder than the freezing point of water. Only 9 elements have a lower melting point. As a gas it weighs 3.21 g per litre, 2.7 times heavier than air. Its atoms have a radius of about 100 pm (Sulfur next door is 100 pm). In the solid, its atoms pack into an orthorhombic lattice.
Chemical behaviour
Its electronegativity is 3.16 on the Pauling scale, the 3rd highest of all elements, so it pulls hard on shared electrons. Pulling off its outermost electron takes 1251 kJ/mol, less than Fluorine above it (1681 kJ/mol). Bonded to sodium, the electronegativity difference of 2.23 makes the bond in table salt mostly ionic. It releases 349 kJ/mol when it gains an extra electron, one of the largest electron affinities known. Its common oxidation states are −1, +1, +5, +7, so in compounds you will usually meet it as Cl⁻, Cl⁺ and Cl⁵⁺. Group 17 also holds Fluorine, Bromine, Iodine, Astatine and Tennessine, which share its outer-shell pattern. Its Lewis symbol is Cl surrounded by 7 dots. Handle with care: as a pure element it is toxic, oxidiser and corrosive.
Where it comes from
It makes up 145 mg/kg of Earth's crust, the 19th most abundant element there. In the human body it accounts for about 0.2 % of body mass. It was first identified in 1774, making it the 22nd element to be discovered (counting the 11 known since antiquity together).