Final answer:
The amount of energy needed to remove successive electrons from an atom increases, especially for the innermost electrons. Within a group, the ionization energy generally decreases as the size of the atom increases. However, across a period, the ionization energy generally increases as the electrons are more strongly attracted to the nucleus.
Step-by-step explanation:
The amount of energy needed to remove successive electrons from an atom increases. The innermost electrons are particularly difficult to remove due to their close proximity to the nucleus and the strong electrostatic attraction. As we move down a group in the periodic table, the ionization energy generally decreases because the atomic size increases. However, as we move across a period, the ionization energy generally increases because the electrons are more strongly attracted to the nucleus.