Fluorine, chlorine, bromine, iodine and astatine are members of Group 17. They are also known as Halogens. Halogens are highly reactive non-metals.

Fluorine is present in form of insoluble fluorides and also in teeth and bones of animals. Sea water consists of chlorides, bromides, iodides of Sodium, Magnesium, Potassium and Calcium but as we know sea water majorly contains sodium chloride. Marine life contain Iodine in their systems; various seaweeds contain iodine too. Astatine is a radioactive element.

Outermost shell Electronic Configuration of group 17 elements if ns2 np5

Halogens have the least atomic size in their respective periods because they have maximum effective nuclear charge.

Order of Atomic radius: I>Br>Cl>F

They have little tendency to lose electron hence they have high ionization enthalpies.

Order of Ionization Enthalpies: F>Cl>Br>I

Halogens have maximum negative electron gain enthalpy in their respective periods, this is because they need only one electron to achieve the noble gas configuration.

Order of Electron Gain Enthalpy: Cl>F>Br>I this is because Fluorine has small size and has more interelectronic repulsions so the incoming electron doesn’t face much attraction.

They have high electronegativity values and Fluorine is the most electronegative element in the periodic table.

Order of electronegativity: F>Cl>Br>I>At

Fluorine and Chlorine are gases, Bromine is a liquid and Iodine is a solid.

Their melting and boiling points increase with atomic number.

F2 has yellow, Cl2 has greenish yellow, Br2 has red and I2 has violet color. It can also be observed that Fluorine and chlorine react with water, Bromine and iodine are only sparingly soluble in water but soluble in organic solvents.

Bond Dissociation enthalpies Order: Cl-Cl>Br-Br>I-I. Bond dissociation enthalpy of F-F is less than that of Cl-Cl because of larger electron-electron repulsion among the lone pairs which are much closer in F-F than Cl-Cl.

All halogens exhibit -1 oxidation state apart from that chlorine, bromine and iodine exhibit + 1, + 3, + 5 and + 7 oxidation states too when they are combined with small and highly electronegative fluorine and oxygen atoms. e.g., in interhalogens, oxides and oxoacids.

Fluorine atom doesn’t have d-orbitals and hence can’t expand it’s octet. Being the most electronegative it only exhibits -1 oxidation state.

Oxidizing ability of the halogens decreases down the group with F2 being a powerful oxidizing agent. We can hence see that Fluorine oxidizes water to oxygen whereas chlorine and bromine react with water to form corresponding hydrohalic and hypohalous acids.

Fluorine behaves anomalously due to it’s small size, highest electronegativity, low F-F bond dissociation enthalpy, and non availability of d orbitals in it’s valence shell.

Hydrides of Group 17 elements:

Affinity for hydrogen decreases from fluorine to iodine.

Order of Acidic strength: HF < HCl < HBr < HI

Order of Bond dissociation enthalpy: H–F > H–Cl > H–Br > H–I

Order of stability: H–F > H–Cl > H–Br > H–I

Order of Bond length: HF < HCl < HBr < HI

Oxides of Group 17 elements:

Fluorine forms only two oxides OF2 and O2F2 among which only OF2 is thermally stable. Both of these are strong fluorinating agents.

• FACT 1: O2F2 oxidizes plutonium to PuF6 and the reaction is used in removing plutonium as PuF6 from spent nuclear fuel.

Order of stability of oxides: I>Cl>Br

Chlorine oxides, Cl2O, ClO2, Cl2O6 and Cl2O7 are highly reactive oxidizing agents and tend to

explode. ClO2 is used as a bleaching agent.

Bromine oxides, Br2O, BrO2 , BrO3 are least stable and exist only at low temperatures but they are powerful oxidizing agents.

Iodine oxides, I2O4 , I2O5 , I2O7 are insoluble solids and decompose on heating. I2O5 is a very good oxidizing agent used in the estimation of carbon monoxide.

Group 17 elements reacting with metals:

They react with metals to give metal halides. If a metal exhibits more than one oxidation state, the halides in higher oxidation state will be more covalent than the one in lower oxidation state.

Order of Ionic character of Metal halides: MF > MCl > MBr > MI

Interhalogen compounds:

They form compounds of the form XX ′, XX3 ′, XX5 ′ and XX7 ′ where X is a larger size halogen and X′ is smaller size halogen.

Few compounds of Group 17 elements:

Chlorine (Cl2):

Chlorine can be prepared by heating manganese dioxide with concentrated HCl or by the action of HCl on potassium permanganate.

(i)Deacon’s process: By oxidation of hydrogen chloride gas by atmospheric oxygen in the presence of CuCl2 (catalyst) at 723 K.

(ii) Electrolytic process: Chlorine is obtained by the electrolysis of brine. Chlorine is liberated at anode. It is also obtained as a by–product in many chemical industries.

It is a greenish yellow gas with pungent and suffocating odor. It reacts with a number of metals and non-metals to form chlorides. It has great affinity for hydrogen. It is a powerful bleaching agent because chlorine water on standing loses its yellow color due to the formation of HCl and HOCl. HOCl formed here, gives nascent oxygen which is responsible for oxidizing and bleaching properties of chlorine.

• Formula of Bleaching powder: Ca(OCl)2.CaCl2.Ca(OH)2.2H2O

Chlorine is used in textile industries and in the manufacture of dyes, drugs, organic compounds, poisonous gases. It is also used in sterilizing water.

• Fact 2: The word Tear gas can be heard in most of the military themed movies but do you know it’s chemical formula? Formula of Tear gas CCl3NO2 and it Chlorine is used in it’s manufacture.

Hydrogen Chloride (HCl):

In laboratory, it is prepared by heating sodium chloride with concentrated sulphuric acid.

It is a colorless and pungent smelling gas.

It’s aqueous solution is called hydrochloric acid which is a strong acid.

Hydrochloric acid decomposes salts of weaker acids, e.g., carbonates, hydrogen carbonates, etc.

It is used in the manufacture of chlorine and also as a laboratory agent. When three parts of concentrated HCl and one part of concentrated HNO3 are mixed, aqua regia is formed which is used for dissolving noble metals, e.g., gold, platinum.

Oxoacids of Halogens:

Due to high electronegativity and small size, fluorine forms only one oxoacid, HOF known as fluoric (I) acid or hypofluorous acid. The other halogens form several oxoacids.

Interhalogen compounds:

They form compounds of the form XX ′ hypohalite, XX3 ′ halite, XX5 ′ halate and XX7 ′ perhalate where X is a larger size halogen and X′ is smaller size halogen. As the ratio between X and X ′ increases the number of atoms per molecule also increases. That’s why IF7 is has the most number of atoms.

These compounds can be formed either by direct combination of halogens or by the reaction of halogen on lower interhalogen compounds.

These are all covalent molecules and are diamagnetic in nature. They are all volatile solids or liquids at 298 K except ClF which is a gas. Their physical properties are intermediate between those of the halogens used except that the m.p and b.p are a little higher. In general, Interhalogen compounds are more reactive than halogens (except fluorine). XX3 ′ compounds have the bent ‘T’ shape, XX5 ′ compounds have square pyramidal structure and IF7 has pentagonal bipyramidal structure.

Interhalogen compounds are very useful fluorinating agents.

Group 17 elements study is as important as the study of any other group elements.