17Chlorine

Chlorine is a halogen in group 17, period 3. At 25 °C it is a gas.

Shells: 2, 8, 7. A simple picture: electrons do not move in circles.

Symbol: Cl
Atomic number: 17 protons in the nucleus
Standard atomic weight: 35.45 ± 0.01Commercial samples can have a changed isotope mix, because material has been processed or isotopes removed (for example lithium sold after its lithium-6 was taken out).
Electron configuration: [Ne] 3s² 3p⁵
Electrons in each shell: 2, 8, 7
Group, period, block: 17, 3, p
Family: Halogens
State at 25 °C: gas
Electronegativity: 3.16 (Pauling scale)
First ionisation energy: 1251.2 kJ/mol (12.967633 eV)

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Where these numbers come from

Atomic weight: IUPAC CIAAW, abridged standard atomic weights 2024. Electron configuration and ionisation energy: NIST Atomic Spectra Database. Family, state and electronegativity: PubChem. See Sources. Melting points, boiling points and densities are left out for now: published values differ between sources, and we will add them once each has been checked.