Question

all the exceptions of ionisation enthalpy in periodic table

Answer 1

Generally, ionization energy increases from left to right along a period, but there are special cases against this general rule. For example, the ionization energy of of Boron (B) is in fact lower than the one of Be (Beryllium), while the general trend indicates the first ionization energy of B is higher than Be. The reason this exception happens is that Boron (B) has only one electron in the p orbital and p orbital has higher energy than s orbital, so this electron in the p orbital tends to go to a lower energy level or lose in order to make the atom more stable. (The lower the energy, the more stable the atom will be.) Therefore the energy required to remove one electron from the p orbital of Boron (B) is lower than the one required to remove one electron from the s orbital (fully filled) of Beryllium (Be).  The same thing applies to the other periods as well, such as the first ionization energy of Al is lower than that of Mg.
group 5A and elements O, S, and Se in group 6A

the reasons are

1) in case of Be electron should be removed from stable configuration of s2

so it will take higher energy to remove

2) similarly N has half filled P obritals , P3 configuration which is stable than that of Oxygen
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Answer 2

Answer: an example is N (nitrogen) and O (oxygen)

Explanation: p orbital of nitrogen is half full that makes p orbital more stable. But the p orbital of oxygen is not half full which means it is less stable than nitrogen's p orbital .

So nitrogen needs more ionisation energy than oxygen ; )