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Ionization energy decreases as you go down a group because as you go down a group, you add another shell of electrons. This makes the valence electrons further away from the nucleus' positive charge, which makes the electron easier to remove (thus lower ionization energy, or energy required to move an electron from an atom). Ionization energy increases as you go down a period because as you go down a period, the nuclear charge (number of protons) increases and the electrons are pulled closer to the nucleus. This makes the electrons harder to remove (thus higher ionization energy).
As you go down a group of atoms their radii increases. Thus, there are more levels/ shells shielding the outermost electrons from the pull of the nucleus making it easier for valence electrons to be removed. As you go across a period more electrons are filling the valence shell as that happens the radii decreases and the attractive forces between the nucleus and valence electrons becomes stronger. Because of the stronger attractive force, their is a higher ionization energy since there is more resistance to the change of removing an electron.
Kevin Morden 1E wrote:Is this the same answer to the question of why the second ionization energy of an atom is always higher than the first?
Ionization energy is a measure of how difficult it is to remove an electron, so the second ionization energy is usually considerably larger, especially in the case of a Group 2 elements because the second electron comes from a noble-gas core. These electrons have a lower principal quantum number and are closer to the nucleus, so the protons have a stronger attraction over the electrons as the number of electrons increase.
As you go across a period the ionization energy is going to increase the the nuclear charge is increasing also the z effective charge increases making it harder for the electrons to be pulled from certain elements. However, the ionization energy decreases going down a period because there are more shells therefore the z effective becomes lower since there is more shielding, also the atomic radius increases, reducing the pull of the nuclear charge therefore reducing the ionization energy as you move down a group.
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