Question

Which element is predicted to have the highest ionization energy? Justify the answer.

Answer 1

Since Ionization energy is the amount of energy required to take out one electron from an isolated gaseous atom....and we know that ionization energy is dependent on atomic radius..."The less will be atomic radius , more tightly electrons will be bounded with nucleous...more difficulty will be in taking out electrons..more will be ionization energy"

But along with atomic radius there some more terms which are responsible for the value of ionization energy...these are shielding effect and effective nuclear charge.

If we see the trends in a group we can see that...as we move from top to bottom in a group...the no of shells increases which will increase the atomic radius... Increased atomic radius means the outermost shell electrons will be less tightly bounded with nucleous ...so can be easily taken out which means less ionization energy will be....but if we move in a period from left to right...the no of shells remains...and the valence shell electrons keep on increasing which increases the effective nuclear charge that makes nucleus to attract outermost electrons more tightly, will result in high value of ionization energy...with the above mentioned facts FLUORINE (Atomic no -9 - 1s²2s²2p⁵) will be the element with highest effective nuclear charge and highest ionization energy.

Since electrons in He and Ne are well protected due to full filled 2s and 2p orbitals respectively so these two elements will be having some exceptional values of ionization energy. And are taken as exception.