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Group 17: Chemical Properties

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Group 17: Chemical Properties - Lesson Summary

Group -17 elements are more reactive than all other elements in the periodic table. They react with metals and non-metals to form halides. The reactivity of these elements decreases down the group.

All these elements are ready to accept an electron from other elements. Hence, they are all good oxidising agents. The oxidising capacity of these elements decreases down the group. So fluorine is the strongest oxidising agent among them.

Fluorine oxidises other halide ions. In general, a halogen oxidises higher halides to their respective halogens.

F2      + 2Br -       →       2F-          + Br2
          Bromide                              Bromine

Cl2      + 2Br -       →       2Cl-          + Br2
          Bromide                              Bromine

Br2      + 2I -          →       2Br -          + I2
            Iodide                                Iodine

When these elements react with hydrogen gives hydrogen halides.

  X2(g)           +             H2(g)          →           2HX(g)
 Halogen                     Hydrogen                  Hydrogen halide

   2HX(g)        +             H2O(I)        →           2HX(Aq)
 Hydrogen                    Water                      Hydrogen acid

                            X = F,Cl, Br, I

These hydrogen halides react with water to give hydrohalic acids.

The acidic strength and bond lengths of these halides increases and bond dissociation energy decrease from top to bottom.

The melting and boiling points of these halides gradually increase from top to bottom except for hydrogen fluoride due to the high electro-negativity and small atomic size of fluorine.

Halogens form many oxides with oxygen. The stability of these oxides increases down the group.

Fluorine forms only two oxides - OF2 and O2F2. These two are thermally stable at room temperature, and both are fluorinating agents due to the high electro-negativity of fluorine. Chlorine forms the oxides Cl2O, ClO2, Cl2O6 and Cl2O7. All these are powerful bleaching agents. ClO2 is used as a bleaching agent in the wood pulp, water purification and paper industries.

Bromine forms the oxides Br2O, and BrO2. All these are less stable, exist at very low temperatures and act as powerful oxidising agents.

Iodine forms I2O4, I2O5 and I2O7. All these are solids and are insoluble in water. Among these, I2O5 is a good oxidising agent.

Halogens react with metals to give metal halides.

Ex: Chlorine reacts with Calcium to give calcium chloride. All these metal halides are ionic in nature. The ionic nature of monovalent metal halides decreases from fluorine to iodine.

Due to electro-negative difference and atomic size variations, these halogens combine amongst themselves to form complexes known as inter-halogen compounds of the type AX, AX3, AX5 and AX7.

                                            Interhalogen Compounds         AX         AX3         AX5         AX7        ClF        ClF 3        ClF 5           -     BrF,BrCl        BrF 3        BrF 5           -         ICl         ICl 3        ICl 5          ICl 7

Anomalous behaviour of fluorine:

The anomalous behaviour in properties like ionisation energy, electro-negativity, bond dissociation energy, electrode potentials, electron-gaining enthalpy, ionic and covalent radii, melting point, and boiling point is due to the high electro-negativity, small atomic size, low bond dissociation energy and no availability of d-orbitals in the valence shell of Fluorine.


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