Why does ionization energy vary along a period???
Due to increase in the Nuclear charge.
In a period as we move from left to right , the number of electrons in the outermost shell increase along with the increase in Nuclear charge ( due to increase in number of protons) .
But , the increase in nuclear charge is much dominant factor which results in the size of the atom to decrease along a period ; and this decrease in the size results in the increase in Ionization energy.