Question

Why the ionisation energy increases from left to right across a period?

Answer 1

Moving left to right within a period or upward within a group, the first ionization energy generally increases. As the atomic radius decreases, it becomes harder to remove an electron that is closer to a more positively charged nucleus. ... They experience a weaker attraction to the positive charge of the nucleus.

Answer 2

as we know that ionization energy means the minimum amount of energy required to remove the most loosely bound electron of an isolated neutral gaseous atom.If atom is large we can easily take out electron without spending more energy because in atom of large size the attraction between the electrons and protons is less.so ionization energy increases from left to right across a period