Discuss and compare the trend in ionization enthalpy of the elements of group 1 with those of group 17 elements.

The ionization enthalpies decrease regularly as we move down a group from one element to the other. This is evident from the values of the first ionisation enthalpies of the elements of group 1 (alkali metals) and group 17 elements as given in table and figure.

(i) On moving down the group, the atomic size increases gradually due to the addition of one new principal energy shell at each succeeding element. Hence, the distance of the valence electrons from the nucleus increases. Consequently, the force of attraction by the nucleus for the valence electrons decreases and hence the ionisation enthalpy should decrease.

(ii) With the addition of new shells, the shielding or the screening effect increases. As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence the ionisation enthalpy should decrease.

(iii) Nuclear charge increases with increase in atomic number. As a result, the force of attraction by the nucleus for the valence electrons should increase and accordingly the ionisation enthalpy should increase.

The combined effect of the increase in the atomic size and the screening effect more than compensates the effect of the increased nuclear charges. Consequently, the valence electrons become less and less firmly held by the nuclear and hence the ionisation enthalpies gradually decrease as move down the group.

Ionization enthalpy is the energy given to a electron to remove from its outer shell 

If we are removing first electron then it is .              i1 AND if removing second one after first then   i2.  AND so on 

 In case of first group elements  only one electron is there in there outer shell   and in  group 17 there are 7 electron  and every atom wants to be stable by achieving noble gas configuration so ,

Removing a electron from group 1 element is easy  because after losing 1 electron it becomes more stable  so it requires lesser amount of energy to remove its outer electron infact very less or room temperature is enough 

But in tgecase of group 7 Elements removing of electron didn't make that atom stable so it requires more amount of energy to remove electron from it's outer shell 

i7>>>>>>>i1