The Halogens

GENERAL

Name	Formula	Molar Mass (A) for single atoms	Appearance	Melting Point	Boiling Point	Electronic Configuration
Fluorine	F₂ (g)	18.9984	yellow gas	53 K, -220°C	85 K, -188°C	1s² 2s² 2p⁵
Chlorine	C₂ (g)	35.453 ± 0.001	green gas	172 K, -101°C	238 K, -35°C	1s² 2s² 2p⁶ 3s² 3p⁵
Bromine	Br₂ (g)	79.909 ± 0.002	orange/brown liquid	266 K, -7°C	332 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°C	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4d¹⁰ 4p⁶ 5s² 5p⁵
Astatine	At₂ (g)	209.9870	solid, colour: metallic	575 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.

HAZARDS AND RISKS

Fluorine: 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: 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: 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.
Iodine: 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.
Astantine: 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

Fluorine

Compounds in various oxidation states containing F
Oxidation state of F	Electronic configuration	Examples
-1	1s² 2s² 2p⁶ [Ne]	F^- (aq), HF, KHF₂, CaF₂, many salts and derivatives of other elements
0	1s² 2s² 2p⁵	F₂

Chlorine

Compounds in various oxidation states containing Cl
Oxidation state of Cl	Electronic configuration	Examples
-1	1s² 2s² 2p⁶ 3s² 3p⁶ [Ar]	Cl^- (aq), HCl, NaCl etc.
0	1s² 2s² 2p⁶ 3s² 3p⁵	Cl₂
1	1s² 2s² 2p⁶ 3s² 3p⁴	Cl₂O, HOCl, salts, ClO^- (aq), ClF
3	1s² 2s² 2p⁶ 3s² 3p²	NaClO₂, ClF₃
4	1s² 2s² 2p⁶ 3s² 3p¹	ClO₂
5	1s² 2s² 2p⁶ 3s²	HClO₃, salts, ClO₃^- (aq), ClF₅, ClF₃O
6	1s² 2s² 2p⁶ 3s¹	Cl₂O₆
7	1s² 2s² 2p⁶ [Ne]	Cl₂O₇, HClO₄, salts, ClO₄^- (aq), ClFO₃

Bromine

Compounds in various oxidation states containing Br
Oxidation state of Br	Electronic configuration	Examples
-1	1s² ... 4s² 4p⁶ [Kr]	Br^- (aq), HBr, KBr etc.
0	1s² ... 4s² 4p⁵	Br₂
1	1s² ... 4s² 4p⁴	Br₂O, BrCl₂^-
3	1s² ... 4s² 4p²	BrF₃, BrF₄^-
4	1s² ... 4s² 4p¹	BrO₂
5	1s² ... 4s²	BrO₃- (aq), BrF₅, BrF₆-
7	1s² ... (d¹⁰?)	KBrO₄, BrF₆+

Iodine

Compounds in various oxidation states containing I
Oxidation state of I	Electronic configuration	Examples
-1	1s² ... 5s² 5p⁶ [Xe]	I^- (aq), HI, KI, etc.
0	1s² ... 5s² 5p⁵	I₂, I₃^-, I₅^-, etc.
1	1s² ... 5s² 5p⁴	ICl₂^-, etc.
3	1s² ... 5s² 5p²	I₄O₉ (= I³⁺(IO₃^-)₃), ICl₃
5	1s² ... 5s²	I₂O₅, HIO₃, IO₃⁺ (aq), IF₅, IF₆^-
7	1s² ... (d¹⁰?)	H₅IO₆, H₄IO₆^- (aq) etc., HIO₄, IO₄^- (aq), IF₇

Astatine

Compounds in various oxidation states containing At
Oxidation state of At	Electronic configuration	Examples
-1	1s² ... 6s² 6p⁶ [Rn]	At^- (aq)
1	1s² ... 6s² 6p⁴	AtBr₂^-
5	1s² ... 6s²	AtO₃^- (aq)

REACTIVITY

Halogen and Oxidation State	Compound	Thermal Decomposition
Cl (-1)	KCl	Reduced by other halogens lower in the group (Br, I) - strong oxidising agent
Br (-1)	KBr	Oxidised by other halogens higher in the group (F, Cl)
I (-1)	KI	Oxidised 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)	HCl	Thermally stable
Br (-1)	HBr	Thermally stable
I (-1)	HI	Hydrogen Iodide released purple gas (I₂)

SOURCES AND USES

Halogen	Source	Industrial Use	Natural Use
Fluorine	CaF₂, e.g. from Derbyshire	hydrofluoric acid is extensively used for etching the glass of light bulbs, etc.	Taken by the tea plant
Chlorine	Rock Salt (NaCl), sea water	extensively 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.
Bromine	Sea water	Much 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).
Iodine	Caliche (NaNo₃), containing NaIO₃		Essential in influencing the rate of metabolism in body tissues.
Astatine	Nuclear Research Labs	hah! That'd be a laugh... (half life is around 8 hours)...	none