Element 17 · Halogen
Chlorine (Cl) — Element 17
Greek chloros — pale green
Chlorine (Cl) is a halogen in Period 3, Group 17 of the periodic table with atomic number 17, standard atomic weight 35.45 u, and ground-state electron configuration [Ne] 3s² 3p⁵. At standard temperature and pressure it is classified as a gas with 7 valence electrons and common oxidation states -1, +1, +5, +7.
Disinfectants, PVC, bleach
What are the atomic properties and physical constants of Chlorine?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 17
- Standard atomic mass
- 35.45 u
- Chemical family
- Halogen
- Ground-state configuration
- [Ne] 3s² 3p⁵
- Electrons per shell
- 2, 8, 7
- Valence electrons
- 7
- Common ion charges / oxidation states
- -1, +1, +5, +7
- Phase at STP
- Gas
- Pauling electronegativity
- 3.16 Pauling
- First ionization energy
- 1,251 kJ/mol
- Atomic radius
- 79 pm
- Melting point
- -102 °C
- Boiling point
- -34 °C
- Discovery year
- 1774
- Table coordinate
- Period 3, Group 17
- Radioactivity
- Has stable isotope(s)
How are electrons arranged across Chlorine’s shells?
A neutral Chlorine atom contains 17 electrons distributed across 3 occupied principal energy shells ([Ne] 3s² 3p⁵). The table below compares Chlorine’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Cl | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 7 e⁻ | 18 e⁻ |
Frequently Asked Questions about Chlorine (Cl)
What is the atomic mass and atomic number of Chlorine (Cl)?
Chlorine (Cl) has atomic number Z = 17 (17 protons in its nucleus) and an IUPAC standard atomic weight of 35.45 u. It sits in Period 3, Group 17 and is classified as a halogen.
What is the ground-state electron configuration of Chlorine?
The neutral ground-state electron configuration of Chlorine (Z = 17) is [Ne] 3s² 3p⁵. Across its 3 occupied principal shells (n = 1 to n = 3), the electrons per shell are 2, 8, 7, with 7 valence electrons participating in bonding.
What are the common ion charges and electronegativity of Chlorine?
Chlorine commonly forms -1, +1, +5, +7 oxidation states or ion charges in compounds. Its Pauling electronegativity is 3.16 and its first ionization energy is 1,251 kJ/mol.
When was Chlorine discovered and what is it used for?
Chlorine was discovered in 1774 (Greek chloros — pale green). Disinfectants, PVC, bleach
How does Chlorine compare to neighboring elements?
Other halogen elements: