How is ionization energy defined?

Ionization energy is defined as the amount of energy required to remove the outermost electron from a gaseous atom. This process involves overcoming the attractive forces between the negatively charged electron and the positively charged nucleus.

When an atom is in a gaseous state, the removal of an electron results in the formation of a positively charged ion. The energy needed for this process can vary significantly among different elements, as it is influenced by factors such as atomic size, effective nuclear charge, and electron shielding.

This definition captures the essence of ionization energy by emphasizing the gaseous state of the atom and focusing on the removal of the outermost electron, which is typically the one most easily removed due to its distance from the nucleus and its interactions with other electrons. The other choices present alternative concepts related to atomic interactions, such as electron affinity or energy considerations in different states, which do not directly define ionization energy.