This MCQ module is based on: History Modern Periodic Law
TOPIC 9 OF 28
History Modern Periodic Law
🎓 Class 11
Chemistry
CBSE
Theory
Ch 3 – Classification of Elements and Periodicity in Properties
⏱ ~14 min
🌐 Language: [gtranslate]
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History of Classification and the Modern Periodic Law
3.1 Why Do We Need to Classify Elements?
We know by now that the elements are the basic units of all types of matter. In 1800, only 31 elements were known. By 1865, the number of identified elements had more than doubled to 63. Today, we have ~118 elements known to us. Of these, the recently discovered elements are man-made. With such a large number of elements, it is very difficult to study individually the chemistry of all these elements and their innumerable compounds. To ease our effort and to ensure that this study can be carried out systematically, we find it useful to classify the elements into groups with similar properties. This procedure has resulted into grouping known as classification of elements.
3.2 Genesis of Periodic Classification
Classification of elements into groups and development of Periodic Law and Periodic Table are the consequences of systematizing the knowledge gained by a number of scientists through their observations and experiments.
Döbereiner's Triads (1817)
Döbereiner, in 1817, observed certain similarity between the properties of several groups of three elements (called triads). The atomic mass of the middle element was found to be approximately the arithmetic mean of the other two:
| Triad | Element 1 (mass) | Element 2 (mass) | Mean of 1 & 3 | Element 3 (mass) |
|---|---|---|---|---|
| Alkali metals | Li (7) | Na (23) | (7+39)/2 = 23 ✓ | K (39) |
| Alkaline earth metals | Ca (40) | Sr (88) | (40+137)/2 = 88.5 ≈ 88 ✓ | Ba (137) |
| Halogens | Cl (35.5) | Br (80) | (35.5+127)/2 = 81.25 ≈ 80 ✓ | I (127) |
Newlands' Law of Octaves (1865)
In 1865, English chemist John Newlands proposed the Law of Octaves: when elements are arranged in increasing order of atomic masses, every eighth element has properties similar to the first, like the eighth note in a musical octave (Do-Re-Mi-Fa-Sol-La-Ti-Do).
Newlands' Law worked well for lighter elements (up to Ca, atomic mass 40), but failed for heavier elements. The discovery of inert gases (Ar, Ne) and lanthanides made his law inadequate. Newlands' work was initially ridiculed but later recognized.
Mendeleev's Periodic Law (1869)
The Russian chemist Dmitri Mendeleev and the German chemist Lothar Meyer independently published their periodic classification in 1869.
Mendeleev's Periodic Law: The properties of the elements are a periodic function of their atomic masses.
Mendeleev's most striking achievements:
- Left gaps for elements not yet discovered (Eka-Boron, Eka-Aluminium, Eka-Silicon)
- Predicted properties of these elements with remarkable accuracy
- Switched some atomic masses (Te before I) when chemistry demanded it
Eka-Aluminium (Gallium): Mendeleev's Prediction vs. Reality
| Property | Eka-Aluminium (predicted, 1871) | Gallium (discovered 1875) |
|---|---|---|
| Atomic mass | ~68 | 69.72 |
| Density (g/cm³) | 5.9 | 5.94 |
| Melting point | Low | 30.2°C (low!) |
| Formula of oxide | E₂O₃ | Ga₂O₃ |
| Formula of chloride | ECl₃ | GaCl₃ |
The remarkable agreement was a triumph for Mendeleev's vision — the periodic table not only ordered known elements but predicted unknown ones!
3.3 Modern Periodic Law and the Modern Periodic Table
Mendeleev's table had some anomalies: e.g., Te (atomic mass 127.6) appeared before I (126.9) — opposite of strict mass ordering. The reason became clear in the early 20th century.
In 1913, the English physicist Henry Moseley showed via X-ray studies that the basic property determining element identity is the atomic number (Z) — the number of protons in the nucleus — not atomic mass. This led to:
Modern Periodic Law (Moseley): The physical and chemical properties of the elements are periodic functions of their atomic numbers.
The atomic number is also equal to the number of electrons in a neutral atom, and electrons govern chemical behaviour. Hence the periodicity of properties is a consequence of the periodicity of electron configurations.
Structure of the Modern Periodic Table
- 118 elements arranged in 7 horizontal rows (periods) and 18 vertical columns (groups)
- Period number = principal quantum number (n) of outermost shell
- Group number indicates valence electron count (with new IUPAC notation 1–18)
Blocks of the Periodic Table
| Block | Last electron enters | Groups | Examples |
|---|---|---|---|
| s-block | s-orbital | 1, 2 (+ He) | Li, Na, Be, Mg |
| p-block | p-orbital | 13–18 | B, C, N, O, F |
| d-block | d-orbital | 3–12 (transition metals) | Fe, Cu, Zn, Sc |
| f-block | f-orbital | Lanthanides, Actinides (inner transition) | Ce, U, Pu |
🎯 Interactive: Element Block Identifier
Enter an atomic number and the simulation will identify its block, period, group and electronic configuration.
Element: Na (Sodium)
Block: s-block | Period: 3 | Group: 1
Electronic config: [Ne] 3s¹
📐 Activity 3.1 — Predict Properties of an Unknown Element
Setup: Imagine that element 119 has just been synthesized (the next undiscovered element after Oganesson, Z = 118).
Predict: Using your knowledge of the periodic table, predict: (a) which group and period it belongs to, (b) its likely properties, (c) the formula of its oxide.
Element 119 would be the next alkali metal after Francium (Fr, Z=87). Filling order: 119 = 87 (Fr core) + 32 (8s²8p⁶ skipped, then 8s¹ available).
Expected configuration: [Og] 8s¹.
(a) Group 1 (alkali metals), Period 8.
(b) Likely properties: silvery, soft, very reactive metal. Would react vigorously with water → MOH + H₂. Highly electropositive (extremely low ionisation energy).
(c) Oxide formula: M₂O (like Na₂O, K₂O).
Real life: This is exactly how chemists today predict properties of super-heavy elements (113–118 already discovered)! Mendeleev's method endures.
Worked Example 1: Identifying Period and Group
An element X has electronic configuration [Ar] 3d¹⁰ 4s² 4p³. Identify its period, group, and block.
The outermost (highest n) shell is n = 4 → Period = 4.
The last electron enters a p-orbital → Block = p-block.
For p-block: Group = 10 + (number of valence electrons) = 10 + 5 = 15. (Or: 4s² + 4p³ = 5 valence electrons, group 13 + 2 = 15.)
Element X: Period 4, Group 15, p-block — this is Arsenic (As, Z=33).
The last electron enters a p-orbital → Block = p-block.
For p-block: Group = 10 + (number of valence electrons) = 10 + 5 = 15. (Or: 4s² + 4p³ = 5 valence electrons, group 13 + 2 = 15.)
Element X: Period 4, Group 15, p-block — this is Arsenic (As, Z=33).
Worked Example 2: Newlands' Octaves
Test Newlands' law of octaves with the early elements: Li (7), Na (23), K (39). What's the relationship?
Counting from Li (in order of atomic mass): Li (1st), Be (2nd), B (3rd), C (4th), N (5th), O (6th), F (7th), Na (8th).
Indeed, Na is the 8th element after Li — and they share similar chemistry (both alkali metals!).
Continue: K is the 8th element after Na (Na, Mg, Al, Si, P, S, Cl, K). Again similar properties.
Conclusion: Among lighter elements, every 8th element (octave) shares similar chemical properties. This is what Newlands captured. The law breaks down beyond Ca because longer periods (with d- and f-blocks) appear.
Indeed, Na is the 8th element after Li — and they share similar chemistry (both alkali metals!).
Continue: K is the 8th element after Na (Na, Mg, Al, Si, P, S, Cl, K). Again similar properties.
Conclusion: Among lighter elements, every 8th element (octave) shares similar chemical properties. This is what Newlands captured. The law breaks down beyond Ca because longer periods (with d- and f-blocks) appear.
🎯 Competency-Based Questions
Q1. What is the basis of the Modern Periodic Law?L1 Remember
Answer: (b) Atomic number. Established by Henry Moseley's X-ray studies (1913).
Q2. Why did Mendeleev's table place Te (atomic mass 127.6) before I (atomic mass 126.9), against strict atomic mass order?L4 Analyse
Answer: Mendeleev prioritized chemical similarity over strict mass ordering. Te is chemically similar to S, Se (group 16), while I is chemically similar to F, Cl, Br (group 17). Strict mass ordering would have placed Te in group 17 and I in group 16 — chemically wrong. Today, we know this is justified by atomic numbers: Z(Te)=52 < Z(I)=53.
Q3. An element has electronic configuration [Kr] 4d¹⁰ 5s² 5p⁵. Identify period, group and block. L3 Apply
Answer: Period 5 (highest n=5). Block: p-block (last electron in p-orbital). Group: 10 + 7 = 17 (or count 5s²5p⁵ = 7 valence; group 13 + 4 = 17). Element: Iodine (I, Z=53).
Q4. Critique Newlands' Law of Octaves: why did it fail to gain widespread acceptance? L5 Evaluate
Answer: Newlands' Law had three major weaknesses: (1) It worked only for lighter elements (up to Ca), failing beyond. (2) It could not accommodate the noble gases (discovered after 1894), which would have changed the "octave" interval. (3) It forced unrelated elements into the same group (e.g., Co, Ni next to halogens). The Royal Society initially mocked Newlands, comparing him to a chemist arranging elements alphabetically. However, his core insight — periodic recurrence of properties — was vindicated by Mendeleev and Moseley.
Q5. HOT (Create): Imagine you are designing a new periodic table for a planet where atoms have only 3 quantum numbers (n, l, m_l) — no spin. Design the structure of this hypothetical periodic table. L6 Create
Sample Answer: Without spin (no Pauli exclusion via spin), each orbital holds only ONE electron (not 2). Capacities of orbitals: s = 1, p = 3, d = 5, f = 7. Period 1 (n=1, only s) = 1 element. Period 2 (n=2, s+p) = 1+3 = 4 elements. Period 3 (n=3, s+p+d) = 1+3+5 = 9 elements. Period 4 = 9. Period 5 = 16 (adding f). The shape of the table would be drastically different — longer rows, narrower groups. This shows how fundamentally the periodic table reflects the underlying quantum-mechanical structure of atoms.
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Mendeleev predicted the existence and properties of Gallium before its discovery.
R: The gaps in Mendeleev's table corresponded to elements not yet discovered, whose properties could be inferred from neighboring elements.
Answer: (A). Both true; R explains A. Mendeleev called the unknown element "Eka-Aluminium." When Gallium was discovered in 1875, its properties matched Mendeleev's predictions almost exactly.
A: The Modern Periodic Law uses atomic number, not atomic mass.
R: Atomic number determines the number of electrons, which determines chemical behaviour.
Answer: (A). Both true; R correctly explains A. Atomic mass varies with isotopes, but Z (number of protons) is the unique element identifier and directly governs the electron count.
A: Newlands' Law of Octaves works for all elements.
R: Like musical notes, every 8th element has similar properties.
Answer: (D). A is FALSE — works only up to Ca (the first ~16 elements). R is TRUE for those light elements (Li-Na-K share group 1). The "8" pattern fails when d-block (10 elements) and f-block (14) are included.
Frequently Asked Questions — History of Classification and the Modern Periodic Law
Who first proposed the periodic table and how has it evolved?
The first attempts at element classification came from Lavoisier and Dobereiner's law of triads (1817). John Newlands proposed the law of octaves (1865). Dmitri Mendeleev gave the first true periodic table in 1869, arranging elements by atomic mass with gaps for undiscovered elements. Lothar Meyer published a similar table independently. Henry Moseley (1913) showed that atomic number, not atomic mass, is the fundamental property. The modern long-form periodic table arranges elements by atomic number into 7 periods and 18 groups, which is the version studied in NCERT Class 11 Chemistry.
What was Mendeleev's contribution to the periodic table?
Dmitri Mendeleev (1869) created the first widely accepted periodic table by arranging 63 known elements in order of increasing atomic mass into rows and columns based on similar properties. His key contributions: (1) he left gaps for undiscovered elements and predicted their properties accurately (eka-aluminium = gallium, eka-silicon = germanium), (2) he reordered some pairs (Te before I) to keep similar properties together. Mendeleev's table established the periodic law and is celebrated in NCERT Class 11 Chemistry as the foundation of modern element classification, though atomic number later replaced atomic mass as the ordering basis.
Why is atomic number a better basis than atomic mass for periodicity?
Atomic number (Z), the number of protons, is a better basis than atomic mass for periodic classification because: (1) Z is a fundamental property that uniquely identifies each element while atomic mass varies due to isotopes; (2) Z explains anomalies in Mendeleev's table (e.g., Ar is before K in atomic number but heavier in mass); (3) Z determines chemical behaviour because chemical properties depend on electron configuration which depends on Z. Moseley demonstrated this experimentally in 1913 using X-ray spectroscopy. NCERT Class 11 Chemistry emphasises this shift as a major advance in chemistry.
How is the modern periodic table organised?
The modern long-form periodic table organises 118 elements into 7 horizontal periods and 18 vertical groups. Each period corresponds to a principal quantum number n. Groups 1, 2 are s-block (active metals); 13–18 are p-block; 3–12 are d-block (transition metals); lanthanides and actinides are f-block. Electron configuration determines an element's position. The metals are on the left, non-metals on the right, with metalloids forming a staircase between them. In NCERT Class 11 Chemistry, this structure is the basis for predicting properties and trends across all chemistry topics.
What is the law of octaves and triads?
The law of triads (Dobereiner, 1817) noted that in groups of three similar elements, the atomic mass of the middle element approximately equals the average of the other two. Example: Cl (35.5), Br (80), I (127) — average of Cl and I is 81 ≈ Br mass. The law of octaves (Newlands, 1865) observed that when elements are arranged by atomic mass, every eighth element has properties similar to the first, like notes in a music octave. Both laws were limited but historically important, paving the way for Mendeleev's full periodic table — covered in NCERT Class 11 Chemistry.
Why is the periodic table important in chemistry?
The periodic table is the single most important organising principle in chemistry because it: (1) classifies all known elements by atomic number, (2) groups elements with similar chemical behaviour, (3) predicts physical and chemical properties such as ionisation energy, electronegativity and oxidation states, (4) helps explain bonding patterns and reactivity, (5) enables prediction of new elements and compounds. NCERT Class 11 Chemistry uses the periodic table throughout — from understanding bonding to redox to organic chemistry — making mastery of its structure and trends essential for the entire curriculum and beyond.
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