The Group VII elements - Halogens

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The Group VII elements are called HALOGENS. The name Halogen means “salt – former” and is based on the fact that the elements combine with most metals to form salts. (halides)

 ELECTRON  AFFINITY

Electron affinity is the enthalpy change when gaseous atoms of an element gain electrons to become negative ions.

The FIRST ELECTRON AFFINITY  of an element is the ENTHALPY CHANGE WHEN ONE MOLE OF GASEOUS ATOMS GAINS ELECTRONS TO FORM ONE MOLE OF GASEOUS IONS. 

Electron affinity DECREASES as you DESCEND the group VII:

F         333kJ          

Cl         346kJ        of energy is released for every one mole of F/Cl/Br/I

Br         324kJ        atoms changing to a mole of F/Cl/Br/I ions.

I         295kJ          

This trend can be explained by the fact that as you descend the group, the X- ion becomes less stable and is therefore less likely to form. H is less stable because electrons are less strongly attracted due to the fact that the valence electrons are further from the attractive positive nucleus, and because there is an increase in the number of inner shells which, in turn, increases the shielding effect.

                HALOGENS AS OXIDISING AGENTS

Since their atoms accept electrons, the halogens are oxidising agents (OIL), and in a reaction they are reduced (RIG). Of the halogens, fluorine is the most powerful oxidising agent, and astatine, the least. This can be explained by the relative size of their atoms. The fluorine atom is the smallest (i.e., the outer electrons are closer to the nucleus), with fewer inner – shell shielding electrons, so it’s nucleus can have a greater attraction for an extra electron.

                   

EXPLAINING THE CHANGE IN STATE FROM F2 TO I2

 

           F2      Cl2                   Br2               I2 

       Pale Yellow                  Pale Green             Red-Brown                 Black

            Gas                              Gas                                Liquid                   Solid

  This can be explained by instantaneous dipole-induced dipole forces.

The likelihood is that, at any instant, the charge distribution of an atom will not be symmetrical. If electrons are thought of as “ charge clouds”, wherein the electrons are in constant motion, then it is entirely plausible that, at any instant, there will be more electrons in one area than in another. Therefore, at that instant, the atom is polarized.

 

This INSTANTANEOUS DIPOLE can affect the electron distribution in nearby atoms, so that they too are distorted. The result of this is to INDUCE DIPOLES in nearby atoms ( the area of instantaneous high electron density in the electron cloud will repel the electrons in nearby atoms and so polarize these atoms too ). These atoms will then be attracted to the original dipoles.

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The instantaneous dipole – induced dipole forces are the forces that hold halogen atoms together whether as gases, liquids or solids. The forces increase as you go down the group.

As the atomic number of an element increases, the element becomes more POLARIZABLE and the instantaneous dipole – induced dipole forces become stronger. This is because, as the number of electrons increases, so too does the likelihood that the charge distribution of an atom will not be symmetrical. It also follows that, with more electrons, the forces will be stronger. Therefore, as you descend Group VII the elements ...

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