Modern Periodic Law

Answer: D. The modern periodic law, as stated in NCERT Class 11 Chemistry Chapter 3 (page 75), explicitly defines properties as periodic functions of atomic number, not atomic mass or any other property.

Why A is wrong: A is the basis of Mendeleev's original periodic law, not the modern periodic law. Mendeleev used atomic mass; Moseley replaced it with atomic number.

Why B is wrong: B (atomic radius) is a periodic property that is explained by the law, not the independent variable on which the law is based.

Why C is wrong: C (atomic volume) was used by Lothar Meyer in his atomic volume vs. atomic mass curve — it is a periodic property itself, not the organising variable of the modern law.

Answer: B. Henry Moseley (1913) used characteristic X-ray spectra of elements to show that atomic number (Z) is the true organising property, as described in NCERT Class 11 Chemistry Chapter 3 (page 75).

Why A is wrong: A — Mendeleev arranged elements by atomic mass and formulated the original periodic law. He did not identify atomic number as the fundamental property.

Why C is wrong: C — Lothar Meyer independently constructed a periodic table based on physical properties (atomic volume) but used atomic mass as the ordering variable.

Why D is wrong: D — Newlands proposed the law of octaves (1866), which was based on atomic mass ordering and applied only to lighter elements.

Answer: A. Moseley bombarded different elements with high-energy electrons and measured the frequency of the characteristic X-rays emitted. He found √ν ∝ Z, establishing atomic number as the fundamental quantity (NCERT Class 11 Chemistry Chapter 3, page 75).

Why B is wrong: B — Radioactive decay helped discover isotopes and transmutation, but Moseley's atomic number work used X-ray spectroscopy, not radioactivity measurements.

Why C is wrong: C — Chemical reactivity was the basis of Mendeleev's groupings, not the evidence Moseley used. Moseley's method was a physical (spectroscopic) technique, not a chemical one.

Why D is wrong: D — Electronegativity is a periodic property that was systematised after Moseley's work. Pauling's electronegativity scale came in the 1930s; Moseley's X-ray work was in 1913.

Answer: D. Cobalt (Z = 27, mass ≈ 58.93) has a higher atomic mass than nickel (Z = 28, mass ≈ 58.69). Mendeleev's mass-based ordering would place Ni before Co, but their chemical properties demand the reverse. Ordering by atomic number resolves this (NCERT Class 11 Chemistry Chapter 3, page 75).

Why A is wrong: A — Na (Z = 11, mass ≈ 23.0) and Mg (Z = 12, mass ≈ 24.3) have both atomic number and atomic mass in the same ascending order. There is no mass inversion between them.

Why B is wrong: B — Mn (Z = 25, mass ≈ 54.9) and Fe (Z = 26, mass ≈ 55.8) follow normal mass order. No inversion exists.

Why C is wrong: C — Ca (Z = 20, mass ≈ 40.1) and Sc (Z = 21, mass ≈ 45.0) follow normal mass order. No inversion exists.

Answer: A. Atomic number (Z) equals the proton count, which increases by exactly one from element to element — it is an unambiguous integer. Atomic mass is an isotope-weighted average that can produce inversions between neighbouring elements. Z's integer uniqueness eliminates the anomalies in Mendeleev's mass-based ordering (NCERT Class 11 Chemistry Chapter 3, page 75).

Why B is wrong: B — Atomic number is actually much smaller than atomic mass for all elements beyond hydrogen (e.g., Na: Z = 11, mass ≈ 23). This statement is factually incorrect.

Why C is wrong: C — The modern periodic law explicitly states that both physical AND chemical properties are periodic functions of atomic number. It does not exclude physical properties.

Why D is wrong: D — Atomic mass was known and used in periodic classifications long before Moseley established atomic number in 1913. The chronological order is the opposite of what this option claims.

Answer: A. The modern periodic law states that properties are periodic functions of atomic number — periodicity here means the pattern of variation recurs at regular intervals (each period), not that the property climbs monotonically. A periodic function can rise, fall, and show discontinuities within a single cycle; what makes it periodic is that the same qualitative shape repeats from one cycle to the next. The described rise–dip–rise–fall shape, recurring every period, is precisely that kind of repeating pattern, so it confirms rather than contradicts the law (NCERT Class 11 Chemistry Chapter 3, page 75: properties are periodic functions of their atomic numbers).

Why B is wrong: B is wrong because it treats 'periodic' as requiring a smooth, uninterrupted climb; the law defines periodicity as the recurrence of a pattern across periods, not an increase without interruption. A dip at a particular atomic number is part of the repeating pattern, not a violation of it.

Why C is wrong: C is wrong because the modern periodic law applies uniformly to every period; nothing in its statement restricts it to only the first two periods — a pattern repeating in later periods is confirming evidence for the law, not an exception to it.

Why D is wrong: D is wrong for the same reason as B: it demands an uninterrupted climb across each period. Periodicity means the pattern — rises, falls, and discontinuities — recurs from period to period; it does not forbid dips within a single period.

Answer: B. Te (Z = 52) comes before I (Z = 53) because the modern periodic table arranges elements by increasing atomic number. Despite Te having a higher atomic mass than I, Z-based ordering places Te first — this is one of the classic anomalies that the modern periodic law resolves (NCERT Class 11 Chemistry Chapter 3, page 75).

Why A is wrong: A — If elements were arranged by decreasing atomic mass, the entire table would be reversed. This is not the basis of any standard periodic table.

Why C is wrong: C — Neutron number is not the organising principle. Neutron count varies between isotopes of the same element and does not increase uniformly across elements.

Why D is wrong: D — If arranged by increasing atomic mass, iodine (126.9) would be placed before tellurium (127.6), which contradicts their known chemical properties and group assignments.

Answer: C. Statements I and II are correct: Moseley measured characteristic X-ray frequencies and discovered the linear relationship √ν = a(Z − b). Statement III is the opposite of his conclusion — Moseley established that atomic number is more fundamental than atomic mass (NCERT Class 11 Chemistry Chapter 3, page 75).

Why A is wrong: A — Statement III is false (as explained above) and Statement II is correct (the √ν vs. Z linear relationship is Moseley's key finding). This option wrongly excludes II and wrongly includes III.

Why B is wrong: B — Statement III is false. Moseley's entire contribution was to prove atomic number, not atomic mass, is the fundamental property. Including III makes this option incorrect.

Why D is wrong: D — Statement III is false. Moseley proved the opposite: atomic number is more fundamental. Since III is wrong, 'all three correct' cannot be right.