Na → Na⁺ + e⁻ ;ΔH = 5.1eV

Na⁺ + e⁻ → Na; ΔH = −5.1eV

The atomic radii decrease on moving from left to right in a period, thus order of sizes for Cl, P and Mg is Cl < P < Mg. Down the group size increases.

Thus overall order is : Cl < P < Mg < Ca

Cl-atom has the highest electron affinity in the periodic table. F being a member of group 17 has higher electron gain enthalpy than S which belongs to group 16 . This in turn is higher than the electron affinity of O -atom. Thus, Cl > F > S > O

It is worth noting that the electron gain enthalpy of oxygen and fluorine, the members of the second period, have less negative values than the elements sulphur and chlorine of the third period.

This is due to small size of the atoms of oxygen and fluorine. As a result, there is a strong interelectronic repulsion when extra electron is added to these atoms, i.e., electron density is high and the addition of an extra electron is not easy.

The larger the atomic size, smaller is the value of the ionisation enthalpy. Again higher the screening effect, lesser is the value of ionisation potential. Hence option (b) has lowest ionisation enthalpy.

Amongst isoelectronic ions, ionic radii of anions is more than that of cations. Further size of the anion increases with increase in -ve charge and size of cation decreases with increase in +ve charge. Hence, correct order is

O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺

State functions or state variables are those which depend only on the state of the system and not on how the state was reached.

Path function depends on the path followed during a process as well as the end states. Work and heat are the path functions.

Chromium is more electropositive metal than iron. In stainless steel, chromium forms an oxide layer and thus it protects steel from corrosion.

In the process of electro decomposition for purification of metal, impure metal acts as anode.