Ionisation energy is the energy needed to remove 1 mole of electrons from 1 mole of atoms in a gaseous state.

• A. The energy released when a bond forms
• B. The energy required to remove a proton from the nucleus
• C. The energy required to add an electron
• D. The energy needed to remove 1 mole of electrons from 1 mole of atoms in a gaseous state

Answer: (D)

Ionisation energy is defined as the energy required to remove electrons from atoms in the gaseous state, so the atoms are isolated and not bonded to others. This energy is positive because you must supply energy to overcome the attraction of the positively charged nucleus on the electrons.

The statement that matches this definition exactly is the energy needed to remove 1 mole of electrons from 1 mole of atoms in a gaseous state. It captures both the process (removing electrons) and the conditions (gaseous state, per mole of atoms and electrons).

Other ideas describe different quantities: energy released when a bond forms is bond enthalpy, not ionisation energy; removing a proton from the nucleus is nuclear binding energy, not ionisation energy; and adding an electron relates to electron affinity, not removing electrons.