Q.(a) What is the basic difference in Mendeleef's Periodic Law and Modern Periodic Law?

(a) Mendeleev's Periodic Law is based on atomic mass; the Modern Periodic Law replaces this with atomic number, resolving the anomalies Mendeleev's version had (e.g. some elements had to be placed out of mass order). (b) Nitrogen's exactly half-filled 2p³ configuration is extra stable, so it resists losing an electron more than oxygen (2p⁴) does, giving N a higher first ionisation energy than O despite O being to its right in the period.

(a) Mendeleev vs Modern Periodic Law:

• Mendeleev's Periodic Law: "The physical and chemical properties of elements are a periodic function of their atomic masses." Elements were arranged in order of increasing atomic mass.
• Modern Periodic Law: "The physical and chemical properties of elements are a periodic function of their atomic numbers." Elements are arranged in order of increasing atomic number (number of protons), which fixed the few anomalies (like Ar/K, Co/Ni) that atomic-mass ordering couldn't explain.

(b) Why IE₁(N) > IE₁(O):

Across a period, ionisation energy generally increases due to increasing effective nuclear charge — so oxygen, being to the right of nitrogen, would 'normally' be expected to have a higher IE. But the actual trend reverses here because of electron configuration:

• N (Z=7): $1s^2 2s^2 2p^3$ — the 2p subshell is exactly half-filled (one electron in each of pₓ, p_y, p_z). A half-filled subshell has extra stability (symmetrical distribution + maximum exchange energy), so removing an electron from this arrangement requires extra energy. …