What does the term '1st ionisation energy' refer to?

The term '1st ionisation energy' specifically refers to the energy change associated with the removal of one mole of electrons from one mole of gaseous atoms to form positively charged ions. This process unfolds in the gas phase, emphasizing the significance of overcoming the electrostatic attraction between the negatively charged electron and the positively charged nucleus of an atom.

Choosing the correct definition helps underscore the significance of ionisation energy in understanding an element's reactivity and its ability to form ions, which is fundamental in various chemical processes. The first ionisation energy tends to increase across a period in the periodic table due to increasing nuclear charge, affecting the attraction between the nucleus and the electrons.

The other options do not accurately define first ionisation energy. For example, the idea of forming an ion from a solid misconstrues the phase involved in the ionisation process, while the total energy for all electrons in a molecule doesn’t directly relate to the concept of ionisation energy. Finally, the concept of energy released when an electron is added pertains to electron affinity rather than ionisation. Thus, the definition that refers to the removal of electrons from gaseous atoms captures the essence of what first ionisation energy represents.