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I get that as you go down a group, the electrons get further away from the nucleus and are shielded by the inner electrons so the ionization energy decreases. Why does ionization energy increase across a period?
As we go across a period, the atomic radius decreases so there is a greater attraction between the negatively charged electrons and positively-charged nucleus thus it will take more energy to remove a valence electron (greater ionization energy).
Going across a period on the periodic table the number of protons an element has increases. Since the number of protons increase as you move down the period the elements will have a stronger nuclear charge. Due to the greater nuclear charge the electrons are held much more tightly and the electrostatic attraction is greater. The ionization energy increases as it is much more harder to remove an electron when they are strongly attracted to the nucleus. Much more energy has to be put it to remove an electron when the nuclear charge becomes stronger.
it's easier to think about whether an elememt wants to gain or lose an electron to become stable. elements in group 1 and 2 want to lose their electrons to become stable so the ionization energy is low. elements in groups further along the period are closer to becoming stable (filling their electron shell) by adding electrons, and thus are going to hold more tightly to their electrons, giving a higher cost to remove an electron (a higher ionization energy)
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