What does the term "ionization energy" refer to?

Ionization energy is defined as the energy required to remove an electron from an atom or ion in the gas phase. This process involves overcoming the attractive forces between the negatively charged electron and the positively charged nucleus of the atom. The result of this energy input is the formation of a positively charged ion when an electron is removed.

The concept of ionization energy is crucial in understanding the reactivity and chemical properties of elements since it reflects how easily an atom can lose an electron to form cations. Typically, ionization energy increases across a period in the periodic table due to the increasing nuclear charge, which attracts electrons more strongly, and decreases down a group due to the increased distance of the outer electrons from the nucleus and increased electron shielding.

The other definitions presented do not accurately describe ionization energy. For instance, the energy released when an electron is added to an atom refers to electron affinity, which is related but distinct. The process of forming an ion from a neutral atom may involve multiple steps, including ionization, but does not capture the specific requirement of energy input for the removal of an electron. Likewise, the energy released during electron capture describes a different process involving the absorption of an electron and is not relevant to ionization energy.