Chlorine (Cl): electron configuration, atomic mass and facts
Chlorine (Cl) is element 17 of the periodic table, a halogen in period 3, group 17. Its electron configuration is [Ne] 3s2 3p5, with electrons arranged in shells 2, 8, 7, and its standard atomic weight is 35.453 u.
💡 A toxic yellow-green gas, used in small amounts to make drinking water safe.
▶ Watch Chlorine’s electrons orbit in the interactive periodic tableChlorine at a glance
| Symbol | Cl |
|---|---|
| Atomic number | 17 |
| Atomic mass | 35.453 u |
| Category | Halogen |
| Group | 17 |
| Period | 3 |
| Block | p-block |
| Electron configuration | 1s2 2s2 2p6 3s2 3p5 |
| Noble-gas shorthand | [Ne] 3s2 3p5 |
| Electrons per shell | 2, 8, 7 |
| Outer-shell electrons | 7 |
| Electronegativity (Pauling) | 3.16 |
| State at 20 °C | gas |
Experiments with chlorine
🧪 Acid–base titration
Find the end-point of HCl vs NaOH with phenolphthalein and plot the titration curve.
🫧 CO₂ and limewater
Marble + acid makes CO₂. Bubble it through limewater: milky, then clear again in excess.
🧂 Electrolysis of brine
Chlorine at the anode, hydrogen at the cathode, sodium hydroxide left behind.
🟤 Halogen displacement
Chlorine water releases iodine from iodide; starch shows it as blue-black.
Where is Chlorine on the periodic table?
Chlorine is in period 3, group 17, in the p-block. The period number matches its 3 electron shells, and the block names the subshell its last electrons fill. In the same group, Fluorine sits directly above it and Bromine sits directly below it. Elements in one group have the same number of outer-shell electrons (7 here), which is why they react in similar ways.
How to work out the electron configuration of Chlorine
Put 17 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁵ (17). In shorthand, [Ne] stands for the 10 electrons of neon, giving [Ne] 3s² 3p⁵.
How electronegative is Chlorine?
Chlorine has a Pauling electronegativity of 3.16, ranking 3 of the 100 elements with a known value (fluorine is highest at 3.98). That is high: it pulls shared electrons strongly towards itself in a bond.
Chlorine in the virtual lab
Faradium’s chemistry workbench includes these chemicals that contain chlorine: Sodium chloride, Potassium chloride, Ammonium chloride, Calcium chloride, Potassium chlorate, Dilute hydrochloric acid, Hydrochloric acid 0.1 M, Conc. hydrochloric acid, Brine (saturated), Lithium chloride, Strontium chloride, Caesium chloride, Barium chloride, Copper(II) chloride, Iron(III) chloride, Chlorine water, Chlorine cylinder, Hydrogen chloride. Open the Chemistry Lab to experiment with them.
Frequently asked questions
What is the electron configuration of Chlorine?
The electron configuration of Chlorine (Cl) is 1s2 2s2 2p6 3s2 3p5. In noble-gas shorthand it is written [Ne] 3s2 3p5.
How many electrons does Chlorine have in each shell?
Chlorine has 17 electrons arranged in shells as 2, 8, 7. It has 7 electrons in its outer shell.
What is the atomic mass of Chlorine?
The standard atomic weight of Chlorine is 35.453 u, so one mole of Chlorine atoms has a mass of about 35.453 g.
Is Chlorine a metal?
No. Chlorine is a halogen.
Explore nearby elements
More halogens: Fluorine, Bromine, Iodine, Astatine, Tennessine. See all elements on the periodic table.