Q.Justify that the fifth period of the periodic table should have 18 elements on the basis of quantum numbers.

Every element's position in the periodic table follows from which orbital receives its last ("differentiating") electron, and this single idea sorts all 18 groups into four families or blocks.

s-block (Groups 1-2, general configuration $ns^{1-2}$): the alkali metals and alkaline earth metals -- soft, low-melting, low-ionisation-enthalpy, highly reactive, electropositive metals that form ionic compounds and often colour a flame.

p-block (Groups 13-18, general configuration $ns^2\,np^{1-6}$): the representative elements, including the chalcogens (Group 16), halogens (Group 17) and noble gases (Group 18, fully filled $ns^2\,np^6$). High electron gain enthalpy magnitude, higher ionisation energy than the s-block, mostly covalent compounds, frequently multiple oxidation states.

d-block (Groups 3-12, general configuration $ns^{1-2}\,(n-1)d^{1-10}$): the transition elements -- multiple oxidation states, ionic/covalent/coordination compounds, interstitial compounds and alloys, common catalytic activity, high melting points, good conductors. …