What is the definition of first ionisation energy?

The first ionisation energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms. This process involves converting neutral atoms into positively charged ions by overcoming the attractive forces between the nucleus and the electron being removed.

When measuring ionisation energy, it is important that the atoms are in the gaseous state, as this allows for an accurate assessment of the energy changes involved without intermolecular forces that are present in liquids or solids. Therefore, the definition provided in the correct answer aligns perfectly with the standard description of first ionisation energy in chemistry.

The definition reflects the fundamental concept that ionisation involves a gaseous atom, highlighting both the specific phase (gaseous) and the systematic approach (one mole of electrons) required for ionisation. Other options address incorrect states of matter or processes that do not relate to the removal of electrons from atoms.