The Halogens


NameFormulaMolar Mass (A) for single atomsAppearance Melting PointBoiling PointElectronic Configuration
FluorineF2 (g)18.9984 yellow gas53 K, -220°C85 K, -188°C1s2 2s2 2p5
ChlorineC2 (g) 35.453 ± 0.001green gas172 K, -101°C238 K, -35°C1s2 2s2 2p6 3s2 3p5
BromineBr2 (g)79.909 ± 0.002orange/brown liquid 266 K, -7°C332 K, 59°C 1s2 2s2 2p6 3s2 3p6 4s2 4p5
Iodine I2 (g)126.9045 bluish-black, lustrous solid (volatilises into purple gas with irritating odour)387 K, 114°C 457 K, 184°C1s2 2s2 2p6 3s2 3p6 4s2 4d10 4p6 5s2 5p5
Astatine At2 (g) 209.9870solid, colour: metallic575 K, 302°C ‡610 K, 337°C ‡ 1s2 2s2 2p6 3s2 3p6 4s2 4d10 4p6 5s2 5d10 5p6 6s2 6p5

‡: Highly uncertain
Hazards are discussed below.


Fluorine gas is extremely corrosive and toxic. The free element has a characteristic pungent odour, detectable in concentrations as low as 20 ppb, which is below the safe working level. Exposure to low concentrations causes eye and lung irritation. Metal fluorides are very toxic. Organic fluorides are generally much less toxic and are often quite harmless. Fluoride ion is a very useful component of drinking water (in very low concentrations) since it renders tooth enamel (replacing the OH group of hydroxyapatite) relatively immune to bacteriological attack.
Chlorine gas is very toxic because of its strong oxidising nature. Even low concentrations of chlorine gas attack eyes, throats, and lungs with painful effects. Chloride is relatively harmless.
Bromine is a very corrosive oxidising liquid whose vapour attacks the eyes and lungs with painful effects. Bromides salts should be treated as toxic. Excessive bromide intake leads to depression and of weight loss.
Elemental iodine, I2, is toxic, and its vapour irritates the eyes and lungs. The maximum allowable concentration in air when working with iodine is just 1 mg m-3. All iodides are toxic if taken in excess.
Astatine does not occur does not occur to any significant extent in the biosphere and so normally never presents a risk. Astatine is studied in a few nuclear research laboratories where its high radioactivity requires special handling techniques and precautions. From a chemical point of view one, can speculate that its toxicity would mimic that of iodine.



Compounds in various oxidation states containing F
Oxidation state of FElectronic configurationExamples
-11s2 2s2 2p6 [Ne]F- (aq), HF, KHF2, CaF2, many salts and derivatives of other elements
01s2 2s2 2p5F2


Compounds in various oxidation states containing Cl
Oxidation state of ClElectronic configurationExamples
-11s2 2s2 2p6 3s2 3p6 [Ar]Cl- (aq), HCl, NaCl etc.
01s2 2s2 2p6 3s2 3p5Cl2
11s2 2s2 2p6 3s2 3p4Cl2O, HOCl, salts, ClO- (aq), ClF
31s2 2s2 2p6 3s2 3p2NaClO2, ClF3
41s2 2s2 2p6 3s2 3p1ClO2
51s2 2s2 2p6 3s2HClO3, salts, ClO3- (aq), ClF5, ClF3O
61s2 2s2 2p6 3s1Cl2O6
71s2 2s2 2p6 [Ne]Cl2O7, HClO4, salts, ClO4- (aq), ClFO3


Compounds in various oxidation states containing Br
Oxidation state of BrElectronic configurationExamples
-11s2 ... 4s2 4p6 [Kr]Br- (aq), HBr, KBr etc.
01s2 ... 4s2 4p5Br2
11s2 ... 4s2 4p4Br2O, BrCl2-
31s2 ... 4s2 4p2BrF3, BrF4-
41s2 ... 4s2 4p1BrO2
51s2 ... 4s2BrO3- (aq), BrF5, BrF6-
71s2 ... (d10?)KBrO4, BrF6+


Compounds in various oxidation states containing I
Oxidation state of IElectronic configurationExamples
-11s2 ... 5s2 5p6 [Xe]I- (aq), HI, KI, etc.
01s2 ... 5s2 5p5I2, I3-, I5-, etc.
11s2 ... 5s2 5p4ICl2-, etc.
31s2 ... 5s2 5p2I4O9 (= I3+(IO3-)3), ICl3
51s2 ... 5s2I2O5, HIO3, IO3+ (aq), IF5, IF6-
71s2 ... (d10?)H5IO6, H4IO6- (aq) etc., HIO4, IO4- (aq), IF7


Compounds in various oxidation states containing At
Oxidation state of AtElectronic configurationExamples
-11s2 ... 6s2 6p6 [Rn]At- (aq)
11s2 ... 6s2 6p4AtBr2-
51s2 ... 6s2AtO3- (aq)


Halogen and Oxidation StateCompoundThermal DecompositionRedox Reagents
Cl (-1)KClReduced by other halogens lower in the group (Br, I) - strong oxidising agent
Br (-1)KBrOxidised by other halogens higher in the group (F, Cl)
I (-1)KIOxidised by other halogens higher in the group (F, Cl, Br) - strong reducing agent
Cl (+5)KClO3 Thermally stable
Br (+5)KBrO3 Releases much Oxygen
I (+5)KIO3 Releases lots and lots and lots of Oxygen
Cl (-1)HClThermally stable
Br (-1)HBrThermally stable
I (-1)HIHydrogen Iodide released purple gas (I2)


HalogenSourceIndustrial UseNatural Use
FluorineCaF2, e.g. from Derbyshirehydrofluoric acid is extensively used for etching the glass of light bulbs, etc. Taken by the tea plant
ChlorineRock Salt (NaCl), sea waterextensively used in the production of paper products, dye stuffs, textiles, petroleum products, medicines, antiseptics, insecticides, foodstuffs, solvents, paints, plastics, and many other consumer products.The chloride ion is the principle anion found in the fluid which bathes our body cells.
BromineSea waterMuch bromine output is used in the production of 1,2-dibromoethane (ethylene dibromide), a lead scavenger used in making petrol antiknock compounds. Lead in petrol, however, is presently being phased out in many countries. This will clearly affect future production of bromine Affects Central Nervous System, e.g. for sedation. Certain species of marine snails, called Murex brandaris, take in bromide ions to produce compounds which are fine dyes, such as Tyrian purple (dibromoindigo).
IodineCaliche (NaNo3), containing NaIO3 Essential in influencing the rate of metabolism in body tissues.
AstatineNuclear Research Labshah! That'd be a laugh... (half life is around 8 hours)...none