Which of the following statements best defines ionization energy?

The definition of ionization energy specifically refers to the amount of energy necessary to remove an electron from a gaseous atom. This process involves overcoming the attraction between the negatively charged electron and the positively charged nucleus, which requires a certain amount of energy input.

As the atomic structure consists of positively charged protons in the nucleus and negatively charged electrons surrounding it, the stability of the atom relies on these opposing charges. When energy is added in the form of ionization energy, it can overcome this attractive force, allowing the electron to be released from the atom. This definition is fundamental in understanding how atoms interact with one another and how they form ions, which play a crucial role in chemical reactions.

The other statements pertain to different concepts in chemistry. One describes the energy change that occurs when gaining an electron, which refers to electron affinity, while another concerns breaking chemical bonds in molecules, which relates to bond dissociation energy, and the last defines the phase change known as fusion. Each of these concepts involves energy changes but in different contexts than that of ionization energy.