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Ionization energy is the energy required to remove an electron from an atom or ion. It can be classified into two categories: first ionization energy and successive ionization energy. The first ionization energy is the energy required to remove the first electron from the valence shell, while successive ionization energies correspond to the removal of subsequent electrons.

Ionization energy and atomic size are inversely proportional. As the size of an atom increases, the ionization energy decreases. This is because, in larger atoms, the valence electrons are further from the nucleus and experience weaker nuclear attraction. As a result, it becomes easier to remove electrons in larger atoms, reducing their ionization energy.

Ionization energy generally increases across a period (from left to right) and decreases down a group (from top to bottom) in the periodic table. The explanation behind this behavior is that as we move across a period, the nuclear charge (number of protons) increases, so electrons are more strongly attracted to the nucleus. In contrast, electrons in a larger atom experience weaker nuclear forces and can be removed with less energy.

(a) The general relationship between the size of an atom and its first ionization energy is that they are inversely proportional: Larger atoms have lower ionization energies, and smaller atoms have higher ionization energies. (b) To find the element with the largest ionization energy, we can look for the element with the smallest atomic size in the periodic table. Helium (He), being in the top right corner of the periodic table, has the smallest atomic size and the largest first ionization energy. For the element with the smallest ionization energy, we can look for the element with the largest atomic size. Francium (Fr), being in the bottom left corner of the periodic table, has the largest atomic size and the smallest first ionization energy.