Final answer:
Ionization energy is the energy needed to remove an electron from a neutral atom. It is correct that ionization energy decreases down a group due to larger atom size and increases across a period due to decreasing atomic radius and stronger nuclear hold on electrons.
Step-by-step explanation:
Ionization Energy Explanation
Ionization energy is a key concept in chemistry that refers to the minimum amount of energy required to remove an electron from a neutral atom in its gaseous state. When an atom is ionized, it becomes a cation, that is, it has one less electron than it did when it was neutral. The ionization energy reflects how tightly an electron is bound to an atom.
Statement A) "Ionization energy is the energy required to remove an electron from a neutral atom" is correct. Larger atoms, as mentioned in statement B), typically have lower ionization energy because the outer electrons are farther from the nucleus and are less tightly bound, making them easier to remove. This coincides with statement D), as the ionization energy typically decreases down a group on the periodic table, which corresponds to an increase in the size of the atoms.
Statement C) is also correct; ionization energy increases across a period on the periodic table. This is because the atomic radius generally decreases from left to right across a period, resulting in a tighter hold on the valence electrons by the nucleus and therefore a higher energy requirement to remove an electron.