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Chlorine is a chemical element on the periodic table with the atomic symbol Cl, atomic number 17, and standard atomic weight 35.45 u. Classified as a Halogen within Group 17 and Period 3 (p-block), chlorine possesses a ground-state electron configuration of [Ne] 3s² 3p⁵ distributed across 3 concentric Bohr electron shells (2 · 8 · 7) with 7 outer-shell electrons. Primary extracellular anion in biological fluids, maintaining electroneutrality, gastric hydrochloric acid (pH 1.5–2.0), GABAergic inhibitory neurotransmission, and photosynthetic water-splitting complex cofactor.

⚛️ Interactive Periodic Table Lab: Explore Chlorine (Cl, Z = 17) in the CRV Science Dynamic Periodic Table & Bohr Orbital Lab to visualize its animated 2 · 8 · 7 electron shells, simulate live phase transitions from 0 K to 6,000 K, and compare periodic heatmaps across all 118 IUPAC elements.

Electron Shell Architecture and Ground-State Configuration

A neutral atom of chlorine contains 17 protons in its atomic nucleus balanced by 17 electrons occupying quantized atomic orbitals. In the Bohr-Sommerfeld and quantum-mechanical orbital models, these electrons populate 3 principal energy shells (K (n=1): 2e⁻, L (n=2): 8e⁻, M (n=3): 7e⁻), terminating in the directional dumbbell-shaped p-orbital (azimuthal quantum number l = 1) with ground-state configuration [Ne] 3s² 3p⁵. Removing the most loosely bound valence electron from an isolated gaseous chlorine atom requires a first ionization energy of 1,251 kJ/mol, while its tendency to attract shared bonding electrons is quantified by a Pauling electronegativity of 3.16.

Periodic Table Position and Neighboring Elements

Within the periodic table, Chlorine occupies Period 3, Group 17 of the p-block (Halogens). Along Period 3, it is flanked by Sulfur (S, Z = 16) and Argon (Ar, Z = 18). Vertically within its periodic family, Chlorine shares valence orbital symmetry and homologous chemical trends with Fluorine (F), Bromine (Br), Iodine (I), Astatine (At). Learners can interactively compare Chlorine's orbital radii, ionization energy, and Pauling electronegativity against all neighboring elements in the Dynamic Periodic Table Lab.

Standard Phase, Melting Point, and Boiling Point Dynamics

At standard ambient temperature and pressure (298.15 K / 25 °C, 1 atm), elemental chlorine exists as a molecular or monatomic gas. As thermal energy increases, its crystal or molecular lattice undergoes solid-to-liquid fusion at a melting point of 171.6 K (-101.55 °C / -150.8 °F) and transitions to the vapor phase at a normal boiling point of 239.11 K (-34.04 °C / -29.3 °F), spanning a liquid thermodynamic range of 67.51 K. In the CRV Science Dynamic Periodic Table, dragging the live temperature slider between absolute zero (0 K) and solar-photosphere temperatures (6,000 K) dynamically tracks Chlorine's state transitions in real time alongside enthalpy and entropy phase equilibria.

Quantitative Atomic and Thermodynamic Parameter Matrix

Characteristic Oxidation States and Coordination Chemistry

As a Group 17 p-block halogen with an ns² np⁵ valence shell (one electron short of a closed octet), it exhibits high electron affinity and oxidizing power, forming halide (−1) anions, covalent organohalogens, and interhalogen compounds. In chemical compounds, chlorine exhibits characteristic oxidation states of -1, +1, +5, +7. Depending on the electronegativity difference (Δχ) of its bonding partners—such as Oxygen (χ = 3.44), Fluorine (χ = 3.98), Chlorine (χ = 3.16), Nitrogen (χ = 3.04), Carbon (χ = 2.55), or Hydrogen (χ = 2.20)—chlorine forms redox-specific oxides, halides, hydrides, or coordination complexes governed by Gibbs free energy (ΔG = ΔH − TΔS).

Nucleosynthesis, Discovery History, and Elemental Origins

First identified or isolated in 1774, Chlorine carries a distinct cosmological and geochemical signature. As an element with atomic number Z ≤ 26, chlorine is forged exothermically inside stars or via cosmic nucleosynthesis before being dispersed into protoplanetary nebulae.

Biochemical Classification: Essential Macro-Ion & Electrolyte

In comparative biochemistry and systems physiology, Chlorine is classified as Essential Macro-Ion & Electrolyte. Major cellular ion regulating membrane action potentials, osmotic pressure, ATP chelation, and skeletal biomineralization. Across terrestrial and marine ecosystems, its chemical speciation interacts with cell membrane, cellular homeostasis, adenosine triphosphate, cytosol.

Key Mechanistic Functions and Technological Uses

Primary extracellular anion in biological fluids, maintaining electroneutrality, gastric hydrochloric acid (pH 1.5–2.0), GABAergic inhibitory neurotransmission, and photosynthetic water-splitting complex cofactor. Researchers and students can further investigate Chlorine's functional role alongside all 118 elements by launching the Interactive Periodic Table & Element Detective Lab on CRV Science Learn.

What is the electron configuration and valence shell structure of Chlorine (Cl)?

Chlorine (atomic number Z = 17) has the ground-state electron configuration [Ne] 3s² 3p⁵, distributing its 17 electrons across 3 shells as 2 · 8 · 7. Its outermost shell contains 7 electrons, placing Chlorine in Group 17, Period 3 (p-block) as a member of the Halogens. You can watch these 17 electrons orbit the nucleus in real time in the Dynamic Periodic Table Bohr Lab.

What phase of matter is Chlorine at room temperature, and when does it melt or boil?

At standard room temperature (298 K / 25 °C) and 1 atm pressure, Chlorine is a molecular or monatomic gas. It melts at 171.6 K (-101.55 °C / -150.8 °F) and boils at 239.11 K (-34.04 °C / -29.3 °F). Using the 0–6,000 K temperature slider in the Interactive Periodic Table, you can observe Chlorine's exact solid, liquid, and gas transitions relative to all other elements.

How does Chlorine function in biochemistry, materials science, and technology?

Primary extracellular anion in biological fluids, maintaining electroneutrality, gastric hydrochloric acid (pH 1.5–2.0), GABAergic inhibitory neurotransmission, and photosynthetic water-splitting complex cofactor. In biological systems, it is categorized under Essential Macro-Ion & Electrolyte, with principal oxidation states of -1, +1, +5, +7 and a first ionization energy of 1,251 kJ/mol.