Question

ionisation potential increases across a period from left to right
why

Answer 1

Because atomic size decreases due to increase in nuclear pull.
I hope it will help you!!!!!

Answer 2

Thus, on moving from left to right in a period, the tendency of atoms to lose electrons decreases. Hence, the ionization energy increases across the period. ... So each electron experiences a greater attraction to the nuclear region due to the increased nuclear positive charge.