(solved)Question 3.14 of NCERT Class XI Chemistry Chapter 3

$\displaystyle \underline{\underline{\text{Isolated Gaseous Atom}}}$

Ionization and electron gain enthalpies, like atomic radii, are documented on single atom basis in the units kJ/mol of single atoms

Even though atomic radius is defined for a single atom, but it is indirectly measured from the covalent, ionic or metallic radii of atoms in bonded state.

But ionization and electron gain enthalpies cannot be measured on an atom while it is in a bonded state because bond energies and dynamics also become a factor.

Hence, for measurement of IE, an atom has to be isolated as a monatomic gas.

Only noble gases exist as monatomic gases. Other gases are first split as single atoms. Some solids can be heated and vaporized into single atoms. Such single atoms are called "isolated" or "separated" single atoms. They are isolated, "segregated" out of their bonded states as monatomic gases.

$\displaystyle \underline{\underline{\text{Ground State}}}$

The isolated monatomic atoms are kept in an evacuated tube and ensured that the electrons radiate any excess energy and the atom is in its ground state before the ionization enthalpy is measured.

$\displaystyle \underline{\underline{\text{Measurement of IE}}}$

The isolated atoms are held in a tube between two electrodes connected to a voltage source.

An external ultraviolet source is applied to ionize the atoms. The ionized atoms and the released electrons cause an electric current to flow through the electrodes. The frequency of the UV source is gradually increased till the current rises steeply. The energy corresponding to this frequency $\displaystyle h\nu$ gives the IE, which is then expressed as kJ/mol.