IONISATION POTENTIAL

          The minimum energy required to remove most loosely bound electron from the isolated gaseous atom is known as ionization potential or ionization energy.     


* The most loosely bound electron means the electron present in the outermost shell.

           

This energy is generally measured in electron volt per atom (e.v./ atom) or kilo calories per mole (k.cal/mole) or kilo joule per mole (k.j./mole).  The electrons are removed in stages one-by-one. Consequently, there may be first, second, third or nth ionization potential, depending upon the removal of first, second, third or nth ionization potential, depending upon the removal of first, second, third or n electron from the atom.   Thus,

First ionization potential is the energy required to convert 

M(g) = M+(g).

Second ionization potential is the energy required to convert 

M+(g) = M2+ (g), and so on.

It has been observed that the value of ionization potential increases in the order: first< second < third<....... so on. This is, because the removal of second, third or nth electron is done against greater and greater nuclear attraction. 

Measurement of ionization potential:

  For the measurement of ionization potential, the vapours of the element are taken in the discharge tube. The tube is then connected with a source current. The voltage applied is then slowly increased. When ionization occurs, a current flows at a certain applied voltage. This gives the value of first ionization potential. The voltage is again increased.  After sometime a stage comes when the sudden rise of current takes place. This gives the value of second ionization potential. In the same way, we can know the value of third ionization potential and so on.


  Factors affecting the value of ionization potential  

In general, the ionization potential depends upon the following factors:

  1.  Size of the atom: The larger the distance of the electron from nucleus weaker will be the forces of attraction and thus ,less will be the amount of energy required to remove the electron. In other words, larger the size of the atom, smaller will be the ionization potential.
  2. The charge on the nucleus: The higher the charge on the nucleus the greater will be the force of attraction and thus, it will be more difficult to remove an electron. Hence, higher will be the value of ionization potential.
  3.  Screening effect of electrons in the inner shells: The electrons in the inner shells produce a screening effect on the nucleus. The electrons of outermost shell are, therefore attracted with less force and hence it is easier to remove the valency electrons. Thus, larger the number of electrons in the inner shells, lesser will be the value of ionization potential.
  4. Completed shells, completed or half completed subshells: The atoms having such type of electronic structures are relatively more stable. Therefore, more energy is required to remove electrons in such cases. Hence, ionization potential will be high.
  5. Shape of orbital: The shape of the orbitals also affects ionization potential. s-orbitals closer to the nucleus than the p-orbital of the same orbit. Thus, it is easier to remove electrons from a p-orbital as compared to s-orbital. In general, for the same orbit, ionization potential decreases in the order. 

s>p>d>f.

Applications in explaining the chemical behavior

  1. The elements having lower value of ionization potential e.g. alkali and alkaline earth metals are more reactive, strong reducing agents and are basic in nature.
  2.  The elements having higher value of ionization potential are less reactive, strong oxidizing agents and are acidic in nature (except inert elements).
  3. The relative value of ionization potential and electron affinity of two elements gives an idea about the nature of bond formed between two elements. If the difference between both the values is greater, the bond will be more covalent.
  4. The value of ionization potential gives evidence about the number of valence electrons in the atom. For example, in case of sodium, the second ionization potential (47.3 e.v.)   is too much higher than the first ionization potential (5.1 e.v.).  This indicates the presence of one electron in the valence shell of sodium.

 

 

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