Properties of elements are periodic functions of their atomic numbers. Modern periodic table arranges elements by increasing atomic number Z.
-- NCERT Class 11 Chemistry, Ch. 3, p. 2Modern Periodic Law
8 MCQs9-step worked example
Source: NCERT Classification of Elements and Periodicity in PropertiesPYQ coverage: NEET 2021, 2024Official key: NTA-verifiedLast reviewed: May 2026
Lesson
Modern Periodic Law — The Organising Principle of Chemistry
Mendeleev arranged elements by increasing atomic mass and predicted missing elements successfully — but his table had awkward patches. Cobalt (58.93) sits before nickel (58.69); tellurium (127.6) before iodine (126.9). Mass ordering breaks.
Henry Moseley's 1913 X-ray experiments on elements resolved this. He found that the square root of the frequency of characteristic X-rays varies linearly with a whole number unique to each element — the atomic number (Z). This gave chemistry a cleaner organising variable than atomic mass.
Modern Periodic Law (NCERT Class 11 Chemistry Chapter 3, page 2): Physical and chemical properties of elements are periodic functions of their atomic numbers.
Why does Z work where mass fails? Atomic number counts protons — an integer that increases by exactly one from element to element. Atomic mass is an average over isotopes, and isotope mixing can invert mass order between neighbours. Z never inverts.
What the law does and does not say. It says properties recur periodically as Z increases. It does NOT say properties change monotonically. Periodicity means a pattern that repeats — rises, falls, and discontinuities at shell boundaries are the pattern, not exceptions to it.
Watch-out for NEET: Questions on this topic test whether you can distinguish the modern law from Mendeleev's original formulation. The key discriminator is "atomic number" versus "atomic mass." A second common test point is identifying Moseley as the scientist who established atomic number as the fundamental property, and recognising that his evidence came from X-ray spectra — not chemical reactivity, not electronegativity measurements, and not radioactivity studies.
Practice MCQs
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The modern periodic law states that the physical and chemical properties of elements are periodic functions of their:
MCQ 2Easy RecallPractice
Who established that atomic number, rather than atomic mass, is the fundamental property of an element?
MCQ 3Easy RecallPractice
Moseley's conclusion about atomic number was based on his study of:
MCQ 4Direct ApplicationPractice
Mendeleev's periodic table had anomalies in the placement of certain element pairs. Which of the following pairs illustrates an atomic mass inversion that the modern periodic law resolves?
MCQ 5Direct ApplicationPractice
The modern periodic law replaced Mendeleev's periodic law primarily because atomic number:
MCQ 6Direct ApplicationPractice
Which of the following statements is correct about the modern periodic law?
MCQ 7Direct ApplicationPractice
Tellurium (Z = 52, mass ≈ 127.6) is placed before iodine (Z = 53, mass ≈ 126.9) in the modern periodic table. This placement is justified because the modern periodic table is arranged by:
MCQ 8Concept TrapPractice
Consider the following statements: (I) Moseley studied characteristic X-ray frequencies of elements. (II) Moseley found that √ν (square root of X-ray frequency) is linearly related to atomic number. (III) Moseley's work proved that atomic mass is more fundamental than atomic number. Which statements are correct?
Worked Example
- 1
Given
- Element X: Z = 19, mass = 39.1 - Element Y: Z = 18, mass = 39.9 - Element Z: Z = 20, mass = 40.1
- 2
Required
Arrangement in the modern periodic table order.
- 3
Concept
The modern periodic law arranges elements by increasing atomic number, not atomic mass (NCERT Class 11 Chemistry Chapter 3, page 2).
- 4
Formula
No mathematical formula needed. The principle is: arrange by increasing Z.
- 5
Substitution
Order by Z: Y (Z = 18) → X (Z = 19) → Z (Z = 20).
- 6
Calculation
Sorting: 18 < 19 < 20, so the order is Y, X, Z. Note that if we had used atomic mass, the order would be X (39.1), Y (39.9), Z (40.1) — a different sequence. This illustrates why mass-based ordering fails: Y (argon, Z = 18) has a higher mass than X (potassium, Z = 19), creating a mass inversion analogous to the Co–Ni and Te–I cases.
- 7
Final answer
**Y → X → Z** (i.e., Ar → K → Ca, in order of increasing atomic number: 18, 19, 20).
- 8
Common trap
Arranging by atomic mass instead of atomic number. Mass ordering gives X → Y → Z (39.1, 39.9, 40.1), which places potassium before argon — violating their known chemical group assignments (argon is a noble gas in Group 18; potassium is an alkali metal in Group 1).
- 9
Similar NEET-style question
"Elements P, Q, and R have atomic numbers 26, 27, and 28 and atomic masses 55.8, 58.9, and 58.7 respectively. Which element appears last in the periodic table, and why does this sequence differ from mass ordering?" ---
Before solving, remember these
Formulas
Ionization energy of hydrogen-like atom
Energy required to ionize an electron from the n-th shell of hydrogen-like atom.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Z | nuclear charge | - |
| n | quantum number | - |
Valid when
- One-electron atom
- Non-relativistic
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student includes inert-gas radius in atomic-radius trends. But inert gases use van der Waals radius (much larger than covalent), making 'monotonic decrease across period' look broken.
When it triggers
Atomic radius comparison includes a noble gas or trends across period 2/3.
How to avoid
Compare like with like: covalent radii for non-noble gases. Noble gas radii are van der Waals (no covalent bond). Don't compare noble-gas radius directly to halogen.
Category: Inorganic Exception
Student expects monotonic increase in IE across period. Anomalies: Be(s²) > B(s²p¹); N(p³ half-filled) > O(p⁴).
When it triggers
Compare IE values across period 2 (Li, Be, B, C, N, O, F).
How to avoid
Be > B (s² stable; B's p¹ easier to remove). N > O (N has p³ half-filled stability; O loses one to attain p³). Memorise these two anomalies.
Root cause: concept gap
Correction
Be>B (s² stability); N>O (N's p³ half-filled stability). Memorise these two anomalies in period 2.
Root cause: concept gap
Correction
Don't compare different radius types. Noble gases use vdW radius (much larger); halogens use covalent radius. Compare like-with-like.
Past Year Questions
3 questions from NEET 2021, 2024. Answers verified against NTA official keys.
NEET 2024Revised key
1Li < Be < B < C < N
2Li < B < Be < C < N
3Li < Be < C < B < N
4Li < Be < N < B < C
NTA Answer: Option 2(revised_final)
NEET 2024Revised key
1Si < C < N < O < F
2Si < C < O < N < F
3O < F < N < C < Si
4F < O < N < C < Si
NTA Answer: Option 1(revised_final)
NEET 2021
1CO < SiO : Increasing 2 2 < SnO < PbO oxidizing power 2 2
2HF < HCl : Increasing acidic < HBr < HI strength
3H O < H S : Increasing pK 2 2 a < H Se < H Te values 2 2
4NH < PH : Increasing 3 3 < AsH < SbH acidic character 3 3
NTA Answer: Option 3(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Periodic trends comparison — atomic/ionic radii, ionization energy, electron gain enthalpy, electronegativity.
Direct ApplicationMedium
Common distractors
swapped classes
Tempts surface-level recall.
Sources
NCERT refs: Class 11 Chemistry Chapter 3, p.2
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