Periodic Chemistry

Halogens (Group 17)

Q: Why is HF a weak acid while HCl, HBr, HI are strong?

Aqueous HF only partially dissociates (Ka ~6.3 x 10^-4, pKa 3.2), while HCl, HBr, and HI dissociate essentially fully. The reason is the H-F bond strength of 565 kJ/mol — energetically expensive to break — combined with strong solvent stabilization of the molecular HF by hydrogen bonding. Going down the group, H-X bonds weaken (H-Cl 431, H-Br 366, H-I 299) and the conjugate base X- becomes more diffuse and stable. Bond strength dominates over electronegativity in determining acid strength in this series. HI is the strongest hydrohalic acid despite I being the least electronegative halogen.

Q: What are interhalogen compounds?

Compounds of two different halogens — XY, XY3, XY5, XY7 — where the lighter halogen is more electronegative and acts as the negative end. Examples include ClF, BrF3, IF5, IF7. They are powerful oxidizers and fluorinating agents. ClF3 ignites concrete and asbestos and was investigated as a rocket oxidizer (and rejected — it is too aggressive to handle safely). IF7 is the only known stable XY7 because iodine alone is large enough to coordinate seven fluorines around its central atom in pentagonal-bipyramidal geometry. Interhalogens follow the same trend: Y is always F or Cl when X is heavier.

Q: Why does iodine sublime at room temperature?

Solid I2 sublimes (becomes vapor without melting first) above 184 °C at 1 atm but produces noticeable purple vapor even at room temperature because the I2-I2 London dispersion forces in the molecular crystal are weak relative to thermal energy. Fluorine is gaseous (BP -188 °C), chlorine is gaseous (BP -34 °C), bromine is liquid (BP 59 °C), and iodine is solid (BP 184 °C; sublimes readily). The trend reflects increasing polarizability: heavier halogens have larger electron clouds, stronger dispersion forces, and higher boiling points despite being chemically less reactive.

F, Cl, Br, I, At — most electronegative reactive nonmetals; F most reactive (BDE F-F only 159 kJ/mol)

The halogens are Group 17 of the periodic table — fluorine, chlorine, bromine, iodine, astatine (and synthetic tennessine). They are the most electronegative reactive nonmetals; F is the most reactive element on Earth despite an unusually weak F-F bond (BDE only 159 kJ/mol versus Cl-Cl 243). Halogens form -1 anions, hydrohalic acids HX, interhalogen compounds (ClF₃, IF₇), and the entire chlor-alkali, fluoropolymer, and pharmaceutical industries depend on them. Mendeleev's 1869 periodic table grouped them by reactivity, and Henry Moseley's 1913 X-ray work fixed their atomic numbers.

Group17 (formerly VIIA)
F-F BDE159 kJ/mol (anomalously low)
F electronegativity3.98 Pauling (highest of all)
F₂/F^- potential+2.87 V (strongest oxidant)
States at 25 °CF₂ g, Cl₂ g, Br₂ l, I₂ s
Industry~80 Mt/yr Cl₂ via chlor-alkali

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Why halogens matter

F is the most reactive element on Earth. Standard reduction potential E°(F₂/F^-) = +2.87 V. The next strongest common oxidant is ozone at +2.07 V. F₂ oxidizes water (E° = +1.23 V), reacts with noble gases (Bartlett's 1962 XePtF₆ kicked off noble-gas chemistry), and even etches glass by attacking SiO₂: 4HF + SiO₂ → SiF₄ + 2H₂O.
Chlor-alkali is one of the largest chemical industries. ~80 Mt Cl₂/yr globally, paired with ~80 Mt NaOH/yr from the same brine electrolysis. The output supplies 40% of global PVC, water treatment for ~3 billion people, and the chlorine atom in roughly 25% of pharmaceutical molecules.
Fluoropolymers transformed materials. Teflon (PTFE, polytetrafluoroethylene) was discovered by Roy Plunkett at DuPont in 1938 and patented 1941. The C-F bond's strength (485 kJ/mol) makes PTFE chemically inert from -200 to +260 °C — a property no other polymer matches.
Halogens dominate pharmaceuticals. Roughly 30% of FDA-approved drugs contain at least one F, Cl, or Br atom. Atorvastatin (Lipitor), fluoxetine (Prozac), ciprofloxacin, and the entire fluoroquinolone antibiotic class rely on aromatic C-F bonds for metabolic stability.
The Frasch process and brine wells supply Br, I. Bromine comes from Dead Sea brine and Arkansas wells (~600 kt/yr). Iodine comes from Chilean caliche and Japanese natural-gas brines (~30 kt/yr). Both are produced by oxidation of X^- with Cl₂: Cl₂ + 2Br^- → 2Cl^- + Br₂, exploiting the standard-potential trend.
Halogenation in organic synthesis. Free-radical halogenation, electrophilic addition (Markovnikov for HBr/alkene), aromatic substitution with Cl₂/AlCl₃, and SN1/SN2 displacement of X^- are foundational reactions; the C-X bond's polarization makes alkyl halides versatile electrophiles in C-C bond forming chemistry (Grignard, Suzuki, etc.).
Photographic and X-ray imaging once relied on AgX. Silver halides AgCl, AgBr, AgI absorb visible/UV light and reduce Ag⁺ to Ag⁰; this photographic chemistry, dominant from 1839 (Daguerre) until the 2000s digital transition, depended entirely on Group 17.

Common misconceptions

Reactivity decreases monotonically down the group. True for oxidative reactivity (F₂ > Cl₂ > Br₂ > I₂), but not for everything. F has the smallest atomic radius and the smallest electron affinity of F vs Cl (counter-intuitively), and HF is a weak acid while HCl/HBr/HI are strong. The trend has subtle reversals everywhere F is involved.
Higher electronegativity always means stronger acid. F is more electronegative than I, but HI is a much stronger acid than HF. Acid strength here is dominated by H-X bond strength (565 vs 299 kJ/mol) and X^- solvation, not electronegativity.
Halogens always have oxidation state -1. F is always -1 in compounds, but Cl, Br, I commonly show +1, +3, +5, +7 in oxoanions: HClO (+1), HClO₂ (+3), HClO₃ (+5), HClO₄ (+7). F has no expanded octet because n = 2 lacks d-orbitals.
I₂ dissolves well in water. Solubility of I₂ in pure water is only ~0.3 g/L. Iodine tincture works because I₂ dissolves much more in KI(aq) by forming triiodide: I₂ + I^- → I₃^-, which is brown and water-soluble.
The lightest halogen has the strongest X-X bond. Wrong — F-F is unusually weak at 159 kJ/mol. The trend Cl-Cl 243 > Br-Br 193 > F-F 159 > I-I 151 violates the naive size argument because lone-pair-lone-pair repulsion in F₂ destabilizes the small molecule disproportionately.
Astatine is well-characterized. Astatine (Z = 85) is so radioactive that no macroscopic sample has ever been isolated. The longest-lived isotope ²¹⁰At has a half-life of 8.1 hours. All At chemistry is inferred from tracer studies; many properties are extrapolations down the group.

Periodic trends in Group 17

Each halogen has the valence configuration ns²np⁵, one electron short of a noble-gas octet. That single electron deficiency is the thread that ties the entire group together. Adding one electron to make X^- closes the shell and releases substantial energy (electron affinity). Forming a single bond to one other atom achieves the octet through covalent sharing. Both routes are favorable, and which dominates depends on the bonding partner — electropositive metals lose electrons to give halide ions, while electronegative non-metals share electrons in covalent halides.

Down the group, atomic radius grows (F 64 pm, Cl 99 pm, Br 114 pm, I 133 pm) so the pull on the bonding electrons weakens, electronegativity drops (3.98, 3.16, 2.96, 2.66 on the Pauling scale), and the X^- ion becomes larger and more polarizable. Standard reduction potentials for X₂/X^- follow this monotonically: F₂/F^- +2.87 V > Cl₂/Cl^- +1.36 V > Br₂/Br^- +1.07 V > I₂/I^- +0.54 V. So the lighter halogen always displaces the heavier from a halide solution, which is exactly the chemistry behind Br₂ and I₂ production from natural brines.

Bond strengths and acid strengths show the most counter-intuitive deviations. The F-F single bond (159 kJ/mol) is anomalously weak because the small F atoms force their lone pairs into close repulsion. The H-X bond, however, weakens monotonically down the group (565, 431, 366, 299 kJ/mol), and aqueous acid strength reverses the electronegativity expectation: HI is the strongest hydrohalic acid because its H-X bond is the easiest to break heterolytically and I^- is the most stabilized large anion.

Halogens trend table

Halogen	Atomic radius (pm)	X-X BDE (kJ/mol)	Electron affinity (kJ/mol)	E°(X₂/X^-) (V)	Oxidizing power	Common compound
F (Z = 9)	64	159	-328	+2.87	Strongest known	HF, NaF, PTFE
Cl (Z = 17)	99	243	-349	+1.36	Strong	NaCl, HCl, PVC
Br (Z = 35)	114	193	-325	+1.07	Moderate	NaBr, AgBr (photographic)
I (Z = 53)	133	151	-295	+0.54	Mild	KI, I₂, thyroid hormone T₄
At (Z = 85)	~150	~110 (est.)	~-270 (est.)	~+0.3 (est.)	Weakest of stable group	HAt, NaAt (tracer scale only)
Ts (Z = 117)	~165 (est.)	~80 (est.)	~-160 (est.)	n/a	Predicted metallic-like	None isolated; relativistic effects expected

Halide-analog comparison: HF vs HCl vs HBr vs HI

Property	HF	HCl	HBr	HI
H-X bond energy (kJ/mol)	565	431	366	299
Boiling point (°C)	+19.5 (H-bonded)	-85	-67	-35
Aqueous pK_a	3.2 (weak)	-7 (strong)	-9 (strong)	-10 (strongest)
Industrial source	CaF₂ + H₂SO₄	Chlor-alkali by-product	Brine + Cl₂	Caliche + reduction
Major use	Etching glass, fluoropolymer feedstock	Pickling steel, organic synthesis	Brominating agent, sedatives (historical)	Iodate, iodide salts, contrast agents
Hazard class	Extreme: bone-seeking; HF skin burns	Severe pulmonary irritant	Severe corrosive	Corrosive; oxidizes easily to I₂

Applications and examples

F₂ etches glass. SiO₂ + 4HF → SiF₄ + 2H₂O. Hydrofluoric acid is the only common acid that attacks silicates, used in semiconductor etching, glass-frosting, and stainless-steel pickling. Stored in polyethylene because glass is unsuitable.
Chlor-alkali process. Brine electrolysis at ~3 V/cell yields ~80 Mt Cl₂/yr and ~80 Mt NaOH/yr globally. Modern membrane cells use Nafion (a perfluorosulfonate cation-exchange membrane) to keep anolyte and catholyte separate while permitting Na⁺ migration.
Pharmaceutical fluorination. Roughly one-third of FDA-approved drugs contain at least one C-F bond. The high C-F bond strength (485 kJ/mol) blocks oxidative metabolism by cytochrome P450 and extends half-life. Fluoxetine, atorvastatin, and 5-fluorouracil are canonical examples.
Iodine biochemistry. Thyroid hormones T₃ and T₄ are tri- and tetraiodinated tyrosine derivatives; iodine deficiency causes goiter and developmental delay (the rationale for iodized salt). Ratio Iodine/body content is only ~15 mg total but its metabolic role is essential.
Bromine flame retardants and silver bromide imaging. Tetrabromobisphenol-A is the dominant flame retardant in printed circuit boards (~120 kt/yr). AgBr was the workhorse of black-and-white photography for over a century before digital sensors took over after ~2005.

Frequently asked questions

Why is fluorine so reactive despite a weak F-F bond?

Two reasons compound. First, the F-F single bond is anomalously weak — BDE only 159 kJ/mol versus 243 for Cl-Cl, 193 for Br-Br, and 151 for I-I — because the small F atoms force the lone pairs into close lone-pair-lone-pair repulsion. Second, the bonds F forms with everything else are extraordinarily strong: H-F is 565 kJ/mol versus H-Cl 431, C-F 485 versus C-Cl 339, Si-F 565. So homolyzing F₂ costs little while the resulting M-F bonds release a great deal — combustion of essentially anything in F₂ is exergonic by hundreds of kilojoules per mole.

Why does F have a smaller electron affinity than Cl?

Counter-intuitively, fluorine's first electron affinity (-328 kJ/mol) is less exothermic than chlorine's (-349 kJ/mol). Adding an electron to F packs it into an already-crowded n=2 shell where lone-pair-lone-pair repulsion is severe; the n=3 shell of Cl has more room. Trends down the group then resume normally: Br -325, I -295. EA decreases predictably from Cl onward because the added electron sits farther from the nucleus. The F anomaly is the same effect that produces N₂'s filled-shell stability and Be's high IE.

Why is HF a weak acid while HCl, HBr, HI are strong?

Aqueous HF only partially dissociates (K_a ~6.3 x 10^-4, pK_a 3.2), while HCl, HBr, and HI dissociate essentially fully. The reason is the H-F bond strength of 565 kJ/mol — energetically expensive to break — combined with strong solvent stabilization of the molecular HF by hydrogen bonding. Going down the group, H-X bonds weaken (H-Cl 431, H-Br 366, H-I 299) and the conjugate base X^- becomes more diffuse and stable. Bond strength dominates over electronegativity in determining acid strength in this series. HI is the strongest hydrohalic acid despite I being the least electronegative halogen.

What are interhalogen compounds?

Compounds of two different halogens — XY, XY₃, XY₅, XY₇ — where the lighter halogen is more electronegative and acts as the negative end. Examples include ClF, BrF₃, IF₅, IF₇. They are powerful oxidizers and fluorinating agents. ClF₃ ignites concrete and asbestos and was investigated as a rocket oxidizer (and rejected — it is too aggressive to handle safely). IF₇ is the only known stable XY₇ because iodine alone is large enough to coordinate seven fluorines around its central atom in pentagonal-bipyramidal geometry. Interhalogens follow the same trend: Y is always F or Cl when X is heavier.

Why does iodine sublime at room temperature?

Solid I₂ sublimes (becomes vapor without melting first) above 184 °C at 1 atm but produces noticeable purple vapor even at room temperature because the I₂-I₂ London dispersion forces in the molecular crystal are weak relative to thermal energy. Fluorine is gaseous (BP -188 °C), chlorine is gaseous (BP -34 °C), bromine is liquid (BP 59 °C), and iodine is solid (BP 184 °C; sublimes readily). The trend reflects increasing polarizability: heavier halogens have larger electron clouds, stronger dispersion forces, and higher boiling points despite being chemically less reactive.

What is the chlor-alkali process?

The industrial electrolysis of brine (NaCl(aq)) to co-produce chlorine, sodium hydroxide, and hydrogen. At the anode 2Cl^- gives Cl₂ + 2e^-; at the cathode 2H₂O + 2e^- gives H₂ + 2OH^-. Modern membrane cells separate the half-cells with a perfluorinated cation-exchange membrane (typically Nafion). Global production runs ~80 Mt Cl₂/yr and ~80 Mt NaOH/yr — roughly stoichiometric since the same reactor makes both. Chlor-alkali is one of the largest electrochemical industries; it consumes ~3% of global electricity and is the dominant source of chlorine for PVC, water treatment, and pharmaceutical synthesis.