Q.Ionization enthalpy of Li is 520 kJ mol⁻¹ while that of F is 1681 kJ mol⁻¹. Explain.

Helium's ionisation energy is genuinely the highest of any element, but its electron affinity is NOT the highest — noble gases resist gaining an electron because their shell is already stably filled, so the Reason statement is false even though the Assertion is true.

Assertion — "Helium has the highest value of ionisation energy among all elements": TRUE. Helium's first ionisation energy is about 2372 kJ/mol, the highest of any element, because it has only two electrons held very close to a nucleus with charge +2, with no other electrons to shield them, and it has a completely filled, exceptionally stable 1s2 configuration that strongly resists losing an electron.

Reason — "Helium has the highest value of electron affinity among all elements": FALSE. Electron affinity is the energy change when an atom gains an electron. Noble gases like helium already have a completely filled, highly stable electron shell (1s2). Adding one more electron would force it into a new, higher-energy shell (the n=2 shell) which is poorly shielded and far from the nucleus, so this process is energetically unfavourable — helium's electron affinity is close to zero or even positive (endothermic), not large or negative. The element with the actual highest (most negative/most exothermic) electron affinity among all elements is chlorine (about -349 kJ/mol), not helium.

Since Assertion is true but Reason is false, and the reason does not correctly explain the assertion (indeed it isn't even a true statement), the correct choice is that Assertion is true and Reason is false.

The assertion (helium has the highest ionisation energy of all elements) is TRUE, but the reason given (helium has the highest electron affinity) is FALSE, so the two statements are unrelated and option (d) is correct.

Ionisation energy is the energy needed to remove the most loosely bound electron from an isolated gaseous atom. Helium has the smallest atomic radius of any element and a completely filled, very stable 1s2 configuration, so its nucleus holds its two electrons unusually tightly. This makes helium's first ionisation energy (2372 kJ/mol) the highest among all known elements — the assertion is correct.

Electron affinity is the energy released when an atom gains an extra electron. Because helium already has a completely filled and highly stable 1s2 shell, adding a third electron would have to go into the much higher-energy 2s orbital, which is energetically unfavourable. So helium's electron affinity is not the highest — it is essentially zero (in fact noble gases as a group have the lowest, near-zero or even negative, electron affinities). The reason statement is therefore false, and it is not a valid explanation for the assertion either way.

Ionization enthalpy falls down group 1 as atoms get bigger, so the smallest alkali metal, lithium, has the highest ionization enthalpy of the four choices.

Ionization enthalpy is the minimum energy required to remove the outermost (most loosely held) electron from a gaseous atom. Within a group, ionization enthalpy generally decreases as we move down, because:

• Atomic size increases (additional electron shells are added).
• The outermost electron is farther from the nucleus and is shielded more effectively by the inner electron shells.
• As a result, the outer electron is held less tightly by the nucleus and is easier to remove.

Among the given options — Sodium, Lithium, Potassium, Caesium — lithium is the smallest and topmost alkali metal, so it has the highest ionization enthalpy of the group (roughly: Li 520 kJ/mol > Na 496 kJ/mol > K 419 kJ/mol > Cs 376 kJ/mol).