Why does the ionization energy increase from group 1 to group 8A of the periodic table?

Question

Natalie Anton · Accepted Answer

Ionization energy increases ACROSS a Period (left to right), but decreases DOWN a Group. Why?

Since the electron is farther from the nuclear core and must overcome LESS electrostatic attraction, it is simple to understand why the ionization energy should decrease down a group. Explaining the increase in ionization energy across the period from left to right is less intuitive. Two factors are at work here: (i) increased nuclear charge, #Z#; and (ii) shielding by other electrons. When electrons add to the same shell, they shield each other very imperfectly from the increased nuclear charge. As #Z# ascends sequentially across the Period, the valence electrons experience increased effective nuclear charge. As a physical scientist, you should check the atomic radii to confirm that this decrease occurs. The result is that atomic radii decrease sequentially and markedly across the Period. For example, the lithium atom is much larger than the neon atom, and helium has an ATOMIC radius much lower than the HYDROGEN atom. Ionization energies for right-handed elements on the same Period should be higher because of this property, which leads us to believe that it would be more difficult to remove a valence electron. Is that right?

Naomi Aronson · Answer

undefined The ionization energy increases from Group 1 to Group 8A of the periodic table due to the increasing effective nuclear charge experienced by the outermost electrons. As you move from left to right across a period, the number of protons in the nucleus increases, leading to a greater attraction between the nucleus and the outermost electrons. This stronger attraction makes it more difficult to remove an electron, resulting in higher ionization energies. Additionally, as you move across a period, the electrons are added to the same principal energy level, which does not provide effective shielding against the increasing nuclear charge, further contributing to the increase in ionization energy.