What is the first ionization energy?

The first ionization energy refers specifically to the energy required to remove one mole of electrons from one mole of gaseous atoms, resulting in the formation of positively charged ions. This process involves overcoming the attraction between the negatively charged electron and the positively charged nucleus of the atom, which requires a significant amount of energy.

This definition is critical in understanding periodic trends, as the first ionization energy typically increases across a period due to the increasing nuclear charge that holds the electrons more tightly, making them harder to remove. Additionally, it tends to decrease down a group because the added electron shells reduce the effective nuclear charge experienced by outer electrons, making them easier to remove.

The other options refer to different concepts in chemistry: breaking bonds (which relates to bond dissociation energy), gaining an electron (which pertains to electron affinity), and overall energy changes in chemical reactions (associated with enthalpy changes), rather than specifically addressing the ionization process. Therefore, the definition of first ionization energy encapsulates the concept accurately and aligns with common principles established in chemistry.