Answer:
The correct answer is C. they have fewer electron shells to shield the protons from the electrons.
Ionization energy is the energy required to remove an electron from an atom or ion in the gaseous state. Elements in the same group of the periodic table have the same number of valence electrons, which are the outermost electrons involved in chemical bonding. However, as we move up the group, the ionization energy increases. This is because the elements at the top of the group have fewer electron shells, which means that the valence electrons are closer to the nucleus and experience a stronger attraction to the protons in the nucleus.
The valence electrons are shielded from the attractive force of the protons by the inner electrons in the atom. Therefore, as we move down the group, the increasing number of electron shells provides more shielding and reduces the effective nuclear charge experienced by the valence electrons. This makes it easier to remove an electron from the outer shell, resulting in a lower ionization energy.
Therefore, elements at the top of the periodic table have higher ionization energies because they have fewer electron shells to shield the protons from the electrons. This results in a stronger attraction between the valence electrons and the nucleus, making it more difficult to remove an electron from the outer shell.
Option A is incorrect because fewer layers of electron shells actually mean less shielding, which leads to a higher ionization energy. Option B is incorrect because electronegativity is a measure of an atom's ability to attract electrons, not its ionization energy. Option D is incorrect because atomic radius also affects ionization energy, but it is not the primary reason why ionization energy increases up a group.