First ionization energy is the amount of energy needed to take first electron from an atom. The trend for ionization energy is that it increases from the bottom to the top and from the left to the right.
Why? The bigger an atom's radius is, the further away outer electrons from the pulling of the nucleus are and the less energy is needed to take them away. Thus, ionization energy trend is opposite of that of radius.
Therefore, as the elements in Period 3 are considered from left to right, the first ionization energy will increase since the radius decreases in this direction.
The ionization energy for the second electron is much greater than first, since it's harder to put a charge on an already charged ion.
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