QCC Notes
CLASS 11 · CHEMISTRYJEE MAIN × NEETहिंदी
§ 3.2NCERT Class 11 · Chemistry · Chapter 3

Modern Periodic Law and the Present Form of the Periodic Table

One 26-year-old physicist fired electrons at metals, found a straight line where chemistry had seen wiggles — and the periodic table changed its foundation from atomic weight to atomic number. The anomalies Mendeleev apologised for simply stopped existing.

QCC Notes · Class 11 Chemistry NCERT-aligned Updated 26 Sep 2026 13 min read
01

From Weight to Z — Complete Theory

Mendeleev’s table worked brilliantly but carried three open wounds (§ 3.2): hydrogen without a home, isotopes without slots, and three pairs where he had to break his own weight rule. In 1913 Henry Moseley — aged 26 — measured the characteristic X-ray spectra of dozens of elements and found the cleanest relationship in chemistry: the square root of the X-ray frequency varies linearly with the atomic number, √ν ∝ Z — not with atomic weight. Since ν reveals the nuclear charge, Moseley concluded: atomic number Z, the number of protons, is the element’s fundamental identity; atomic weight is merely an average over isotope baggage.

Moseley’s law√ν = a(Z − b) Modern periodic lawproperties = f(Z) Periodicity causerepeating valence configs

So the law was reborn: the physical and chemical properties of the elements are a periodic function of their atomic numbers. Every defect of the weight-based table dissolved on contact. Isotopes of one element share one Z — they belong in one slot, correctly. Hydrogen stays odd, but now for an electronic reason, not a bookkeeping one. And the anomalous pairs heal themselves — Ar (Z = 18) simply precedes K (Z = 19), Te (Z = 52) precedes I (Z = 53), Co (27) precedes Ni (28), with no rule broken because weight no longer arbitrates. Run the flipper below to watch the chemistry break under weights and heal under Z.

Why periodicity at all? The deep answer comes from § 2.6: as Z increases, electronic configurations fill in the fixed aufbau order, and the outermost (valence) shell configuration recurs at regular intervals. Same valence config → same chemistry → same column. Periodicity of properties is periodicity of configurations — the modern table is configuration science (§ 3.5) drawn as furniture.

The long form (18-column) table. The IUPAC-adopted present form has 18 vertical groups and 7 horizontal periods, built so that elements of one group share valence configuration. Period lengths — 2, 8, 8, 18, 18, 32, 32 — follow the subshells filled per period; the f-block (32-element stretches of periods 6–7) is lifted out and printed below to keep the table manageable. Its virtues over Mendeleev’s: no anomalous pairs, a clean metal–non-metal separation along the staircase, and every position justified by configuration rather than weight.

Table 1 — The seven periods: length and the reason for it
PeriodSubshells filledElementsWhy this length
11s2s holds 2
22s, 2p82 + 6
33s, 3p82 + 6 (3d waits)
44s, 3d, 4p182 + 10 + 6 — d joins
55s, 4d, 5p182 + 10 + 6
66s, 4f, 5d, 6p322 + 14 + 10 + 6 — f joins
77s, 5f, 6d, 7p322 + 14 + 10 + 6 (now complete to Og)
Table 2 — Special names of groups (direct one-mark questions)
GroupNameMembersSignature property
1Alkali metalsLi, Na, K, Rb, Cs, FrViolently reactive; hydroxides strongly alkaline
2Alkaline earth metalsBe, Mg, Ca, Sr, Ba, RaEarth-oxides were alkaline; Mg/Ca in chlorophyll/bone
3–12Transition elementsSc…Zn, and all d-blockVariable oxidation states, coloured ions, catalysis
16ChalcogensO, S, Se, Te, Po“Ore-forming” — many metals occur as their oxides/sulfides
17HalogensF, Cl, Br, I, At“Salt-formers” — NaCl is the archetype
18Noble gasesHe, Ne, Ar, Kr, Xe, RnClosed shells — near-zero reactivity
02

Visualising Moseley’s Line & Flipping the Sorting Key

Do graph, ek flipper — the straight line that beat the wiggle first, then three anomalous pairs re-sorted live by weight and by Z.

The same data, two sorting keys — only Z draws a straight line FIG. 1 — MOSELEY’S PLOTS: THE LINE THAT CHANGED THE LAW atomic weight → √ν vs atomic weight — wiggles K and Ar refuse to sit in frequency order by weight atomic number Z → √ν vs Z — a straight line √ν = a(Z − b): Z is the element’s identity same 38 elements, two stories — only one line obeys physics
FIG. 1 — Moseley’s decisive comparison: plotted against atomic weight, the X-ray frequencies wander; plotted against Z, they fall on a perfect line. A property that tracks Z alone is counting protons — and protons, not isotope-averaged weights, define an element.
Try it live

Weight vs Number — the Anomaly Flipper

Mendeleev ke teen anomalous pairs — dono sorting keys me order badalta dekho. Chemistry ki demand neeche likhi hai; kaunsa order usse match karta hai, khud padho.

Weights shown to 2 dp; Z is the true tie-breaker nature uses. Toggle as many times as you like — the chemistry demand line never changes, only which key respects it.

03

Solved Examples (Step-by-Step)

Line → law → order. Jo chain yahan chalti hai, wahi flipper me live chalti hai.

EXAMPLE 01Foundation · Moseley ratio

Frequency ratios from Z alone

Using Moseley’s relation √ν = a(Z − b) with b negligible, estimate the ratio of characteristic X-ray frequencies of two elements with Z = 26 and Z = 13.

  1. Relation√ν ∝ Z (b ≈ 0 approximation)
  2. Ratio√(ν₁/ν₂) = Z₁/Z₂ = 26/13 = 2
  3. Resultν₁/ν₂ = 2² = 4 — the Z = 26 element radiates 4× the frequency.
  4. MeaningFrequency reads off the nuclear charge — no atomic weight appears anywhere in the story.

ν ratio = 4 — Z² scaling of the X-ray line

EXAMPLE 02JEE Main · Anomaly healed

Argon and potassium, two keys compared

Argon (weight 39.95, Z = 18) and potassium (weight 39.10, Z = 19). Show that the weight basis fails and the Z basis succeeds for their placement.

  1. Chemistry demandAr — closed-shell noble gas (IE₁ = 1521 kJ/mol) — must precede K — violent alkali (IE₁ = 419 kJ/mol).
  2. Weight key39.10 < 39.95 → K first → an alkali metal heading the noble-gas column: chemistry inverted.
  3. Z key18 < 19 → Ar then K — noble gas closes period 3, alkali opens period 4. No exception, no apology.
  4. VerifyFlip it live in the lab — the demand line never moves, only the key obeys or breaks it.

Z fixes what weight breaks — the anomaly dissolves

EXAMPLE 03JEE Main · Period arithmetic

Why period 6 is the longest

Account for the 32 elements of period 6 by subshell filling, and name its first and last members.

  1. Fill orderPeriod 6 fills 6s, 4f, 5d, 6p.
  2. Capacities2 + 14 + 10 + 6 = 32 — the f-subshell’s 14 seats stretch the period.
  3. MembersCs (6s¹) opens; Rn (6s² 4f¹⁴ 5d¹⁰ 6p⁶) closes — with 14 lanthanoids (Ce–Lu) riding inside.
  4. ComparePeriods 4–5: 18 (no f); periods 1–3: 2/8/8. Length = Σ subshell capacities, every time.

2 + 14 + 10 + 6 = 32 · Cs → Rn

04

Practice Questions (With Solutions)

Attempt first — options lock after one shot, exactly like the real exam. Then read the working, chahe galti ho ya na ho.

Attempted 0/4 · Correct 0

Q1NEET · The law

The modern periodic law states that properties of elements are a periodic function of their:

Solution

  1. Z — fixed by Moseley’s √ν ∝ Z line; weight was Mendeleev’s key, retired in 1913.
  2. Any option quoting weight (A) is the auto-fail bait of this section.

(B) Atomic numbers

Q2JEE Main · Isotopes

Why does the modern periodic table have no “isotope problem”?

Solution

  1. All isotopes of an element carry the same nuclear charge — one Z, one slot, by the law’s own logic.
  2. B is backwards (isotopes differ in mass number, never in Z); A is historically false; D is nonsense.

(C) One Z, one slot

Q3NEET · Group names

Group 16 elements are collectively called:

Solution

  1. Chalcogens = “ore-formers” — O, S, Se, Te, Po; most metal ores are their oxides/sulfides.
  2. Halogens = group 17; alkaline earths = group 2; noble gases = group 18. Name-column mismatch is the bait.

(B) Chalcogens

Q4JEE Main · Table shape

The lengths of the seven periods of the long form periodic table are, in order:

  1. 2, 8, 8, 18, 18, 32, 32 — the d-subshell (+10) stretches periods 4–5; the f-subshell (+14) stretches 6–7.
  2. Verify by the subshell arithmetic: 2 | 2+6 | 2+6 | 2+10+6 | 2+10+6 | 2+14+10+6 | 2+14+10+6.

(B) 2, 8, 8, 18, 18, 32, 32

05

Key Rules & Takeaways

Law card

Eight lines that solve this topic

Moseley: √ν = a(Z − b)X-ray frequency linear in Z — 1913, the law’s new foundation.
Modern law: properties = f(Z)Never weight — any weight-flavoured statement is false.
Z = proton count = identityIsotopes share Z → share one slot, correctly.
Anomalies auto-heal under ZAr(18)<K(19) · Co(27)<Ni(28) · Te(52)<I(53).
Long form: 18 groups × 7 periodsIUPAC present form; groups = valence-configuration families.
Period lengths: 2, 8, 8, 18, 18, 32, 32= Σ subshell capacities per period; d adds 10, f adds 14.
Names: 1 alkali · 2 alkaline earth3–12 transition · 16 chalcogens · 17 halogens · 18 noble gases.
Periodicity = recurring valence configsSame outer shell pattern → same column → same chemistry.

Anchor pairs: Ar 39.95/18 · K 39.10/19  ·  Co 58.93/27 · Ni 58.69/28  ·  Te 127.60/52 · I 126.90/53  ·  Period 6: Cs → Rn (32)

  1. The line beat the wiggle — Moseley’s √ν vs Z straight line dethroned atomic weight; every modern-law answer must carry Z, never weight.
  2. The anomalies were never chemistry’s fault — they were the sorting key’s fault; flip to Z and Ar–K, Co–Ni, Te–I order themselves.
  3. The table’s shape is configuration geometry — 7 periods of lengths 2-8-8-18-18-32-32, 18 groups of shared valence patterns; § 3.5 fills in every address.
  4. Six group names pay one mark each — alkali, alkaline earth, transition, chalcogens, halogens, noble gases; match name to number cold.
06

FAQs

What is the modern periodic law?

The modern periodic law states that the physical and chemical properties of the elements are a periodic function of their atomic numbers. Moseley showed in 1913 that atomic number Z, not atomic weight, is the fundamental property of an element, so properties repeat as Z increases — each repetition beginning a new period.

Why did Moseley replace atomic weight with atomic number?

Moseley studied the characteristic X-ray spectra of elements and found that the square root of the frequency varied linearly with the nuclear charge — not with atomic weight. Atomic number Z is an element's true identity, while weight depends on the isotope mix. The switch dissolved Mendeleev's anomalous pairs automatically: argon (Z = 18) correctly precedes potassium (Z = 19).

How many periods and groups does the long form periodic table have?

The long-form (18-column) periodic table has 7 horizontal periods and 18 vertical groups. Periods 1–3 are short (2, 8, 8 elements), periods 4–5 are long (18 each), and periods 6–7 are longest (32 each), because the d- and f-subshells join the filling sequence.

What are the special names of the groups in the periodic table?

Group 1 elements are the alkali metals, group 2 the alkaline earth metals, groups 3–12 the transition elements, group 16 the chalcogens (ore-forming), group 17 the halogens (salt-formers) and group 18 the noble gases. Group 15 is sometimes called the pnictogens, though NCERT highlights groups 1, 2, 16, 17 and 18 by name.

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