1. “Isolated Gaseous Atom”
In the definitions of ionization enthalpy and electron gain enthalpy, the atom is taken as an isolated gaseous atom.
- “Gaseous” means the atoms are far apart, not influenced by neighbouring atoms or ions (no intermolecular or lattice interactions).
- “Isolated” means a single atom, free from external fields or bonding, so the measured energy change depends only on the atom itself.
- This gives a standard, comparable value for different elements because all are measured under the same physical state and conditions.
Symbolically, for ionization enthalpy: X(g) -> X^+(g) + e^- and for electron gain enthalpy: X(g) + e^- -> X^-(g)
2. “Ground State”
The atom is also specified to be in its ground state, i.e., its lowest-energy electronic configuration.
- If excited states were allowed, the energy required to remove or add an electron would vary with each excited level.
- Using the ground state fixes a unique reference energy for each element, making ionization enthalpy and electron gain enthalpy well‑defined.
- This again is essential for meaningful comparison of these quantities across different elements.
3. Link to Comparison (Hint)
For comparison of periodic trends, all values must be obtained under the same conditions: \[ \text{isolated, gaseous, ground‑state atoms} \] Only then can differences be attributed to nuclear charge, size, shielding, etc., and not to physical state or excitation.