What did Mendeleev’s Periodic Law state versus the Modern Periodic Law?

- **Mendeleev's Periodic Law:** Properties of elements are a periodic function of their **Atomic Masses**. - **Modern Periodic Law (Henry Moseley):** Properties of elements are a periodic function of their **Atomic Numbers** (Z).

What were the key achievements of Mendeleev's Periodic Table?

1. Left systematic gaps for undiscovered elements (Eka-Boron \to Scandium, Eka-Aluminium \to Gallium, Eka-Silicon \to Germanium) and accurately predicted their atomic masses and properties. 2. Noble gases could be accommodated in a separate zero group without disturbing the existing table.

How does Atomic Radius vary across a Period (left to right) and down a Group (top to bottom)?

- **Across a Period (Left to Right):** Atomic radius **decreases** due to increasing nuclear charge pulling electrons closer to the nucleus. - **Down a Group (Top to Bottom):** Atomic radius **increases** due to the addition of new electron shells shielding the valence electrons.

How does Metallic Character (electropositivity) vary across the periodic table?

- **Across a Period:** Metallic character **decreases** as effective nuclear charge increases, making it harder to lose electrons. - **Down a Group:** Metallic character **increases** as atomic size increases, making it easier for outer valence electrons to be lost.

How does Electronegativity vary across the periodic table?

Electronegativity (tendency to attract shared pair of electrons) **increases across a period** from left to right and **decreases down a group** from top to bottom. Fluorine is the most electronegative element.

What were the limitations of Newlands' Law of Octaves?

1. Applicable only up to Calcium (Z = 20). 2. Assumed only 56 elements existed in nature. 3. Placed dissimilar elements in the same slot (e.g. Cobalt and Nickel under Halogens).

All Revision Notes

ScienceCh-14 15 min comprehensive revision

NCERT Class 10 Science — Chapter 5

Periodic Classification of Elements

Historical developments (Dobereiner's Triads, Newlands' Octaves, Mendeleev's Periodic Law & gaps), Modern Periodic Law (Moseley, atomic number), and periodic trends (valency, atomic radius, metallic/non-metallic character, electronegativity).

Quick Key Takeaways:

Mendeleev's Periodic Law: Physical and chemical properties of elements are periodic functions of their atomic masses. Achievements: Predicted undiscovered elements (Eka-Boron = Scandium, Eka-Aluminium = Gallium, Eka-Silicon = Germanium). Anomalies: Position of isotopes, cobalt before nickel.

Modern Periodic Law (Henry Moseley, 1913): Properties of elements are periodic functions of their atomic numbers ( $Z$ ). Solved Mendeleev's anomalies.

Structure of Modern Periodic Table: 18 vertical columns (Groups) and 7 horizontal rows (Periods).

Periodic Trends across Period (Left to Right): Atomic size decreases; Valency increases (

1 \rightarrow 4

) then decreases (

4 \rightarrow 0

); Metallic character decreases; Non-metallic character & electronegativity increase.

Periodic Trends down Group (Top to Bottom): Atomic size increases (new electronic shells added); Valency remains constant; Metallic character increases; Effective nuclear charge on valence electrons decreases.

Jump to Concept:1. 1. Historical Evolution, Modern Periodic Law & Table Architecture 2. 2. Mastering Periodic Trends in Periods & Groups 3. 3. High-Yield Solved Board Examination Questions (3-Mark & 5-Mark)4. 4. Practical Laboratory & Competency-Based Case Drill: Predicting Properties of Unknown Elements in Periodic Grid 5. 5. CBSE Examiner Marking Scheme, Scientific Notation & Deduction Traps

1. Historical Evolution, Modern Periodic Law & Table Architecture

Fundamental Principles

Core scientific laws, chemical equations, anatomical structures, and visual model for Periodic Classification of Elements.

• Evolution of Periodic Classification

•

1. Döbereiner's Triads (1817): Grouped 3 elements of similar properties such that the atomic mass of the middle element was roughly the arithmetic mean of the other two (e.g.

\text{Li}=6.9, \text{Na}=23, \text{K}=39.1

). Limitation: Could identify only three triads.

•

2. Newlands' Law of Octaves (1866): Arranged elements by increasing atomic mass; every eighth element had properties similar to the first (like musical notes). Limitation: Applicable only up to Calcium (

\text{Ca}

); assumed only 56 elements existed.

•

3. Mendeleev's Periodic Table (1869): Arranged 63 known elements based on atomic mass and formulas of hydrides and oxides (

R_2\text{O}, RH_4

). Left gaps for undiscovered elements.

•

4. Modern Periodic Table (Moseley, 1913): Based on atomic number (

Z

). Group number indicates number of valence electrons (for

p

-block:

10 + \text{valence } e^-

); Period number indicates total number of electron shells (

n

📊 Periodic Trends: Nuclear Charge, Radii & ElectronegativityVisual Model

Visual schematic mapping periodic variations in atomic radius, metallic electropositivity, ionization energy, and effective nuclear charge across groups and periods.

2. Mastering Periodic Trends in Periods & Groups

Mechanisms & Experiments

Step-by-step chemical reaction mechanisms, experimental activities, and physiological pathways for Periodic Classification of Elements.

• Comprehensive Summary of Periodic Trends

•

1. Atomic Radius (Size):
- Across a Period (L $\rightarrow$ R): Decreases because nuclear charge increases (more protons pull the same shell electrons closer to nucleus).
- Down a Group (Top $\rightarrow$ Bottom): Increases because a completely new electronic shell is added at each step, increasing distance between nucleus and outermost electron.

•

2. Metallic Character (Electropositivity):
- Across a Period: Decreases (tendency to lose electrons decreases as nuclear pull strengthens).
- Down a Group: Increases (valence electrons are farther away from nucleus, easily lost).

•

3. Non-Metallic Character (Electronegativity):
- Across a Period: Increases (atoms attract electrons more strongly).
- Down a Group: Decreases (nuclear attraction for incoming electrons weakens).

•

4. Valency:
- Across a Period: Increases from 1 to 4, then decreases to 0.
- Down a Group: Remains constant (all elements in a group have identical number of valence electrons).

3. High-Yield Solved Board Examination Questions (3-Mark & 5-Mark)

Solved Board Questions

Standard CBSE board exam questions with complete scientific justifications and marking scheme step protocols.

• 3-Mark Standard Board Question: An element 'X' has atomic number 17.
(a) Write its electronic configuration.
(b) Find its group number and period number in the Modern Periodic Table.
(c) State whether 'X' is a metal or a non-metal, and name the type of ion it forms.

•

(a) Electronic Configuration: Atomic number

17 \implies \mathbf{2, 8, 7}

•

(b) Group & Period Numbers:
- Period Number: It has 3 electron shells (

K, L, M

)

\implies \mathbf{\text{Period } 3}

.
- Group Number: It has 7 valence electrons (

> 2

)

\implies \text{Group} = 10 + 7 = \mathbf{\text{Group } 17}

(Halogens).

•

(c) Nature & Ion:
- It needs 1 electron to complete octet

\implies

Non-Metal (Chlorine, $\text{Cl}$ ).
- It gains 1 electron to form an anion with

-1

charge: Chloride Ion ( $\text{Cl}^-$ ).

• 5-Mark Comprehensive Question / Numerical: (a) State Mendeleev's Periodic Law and Modern Periodic Law.
(b) How did the Modern Periodic Table resolve the anomalies of: (i) Position of isotopes, (ii) Anomalous pair of Cobalt and Nickel?

•

Part (a) Periodic Laws:
- Mendeleev's Periodic Law: 'The physical and chemical properties of elements are periodic functions of their atomic masses.'
- Modern Periodic Law: 'The physical and chemical properties of elements are periodic functions of their atomic numbers.'

•

Part (b) Resolution of Mendeleev's Anomalies:
1. Position of Isotopes: Isotopes of an element (e.g.

^{35}\text{Cl}

and

^{37}\text{Cl}

) have different atomic masses but the same atomic number ( $Z = 17$ ). In the Modern Periodic Table based on atomic number, all isotopes of an element naturally occupy the same single position.
2. Anomalous Pair of Cobalt and Nickel: In atomic mass, Cobalt (

58.9

u) appeared before Nickel (

58.7

u), violating increasing mass order. But Cobalt has atomic number $Z = 27$ and Nickel has atomic number $Z = 28$ . When arranged by atomic number, Cobalt correctly precedes Nickel without any anomaly.

4. Practical Laboratory & Competency-Based Case Drill: Predicting Properties of Unknown Elements in Periodic Grid

Practical & Case Drill

Experimental observation analysis, chemical gas tests, and assertion-reason drills.

• Laboratory Activity Context: Predicting Properties of Unknown Elements in Periodic Grid

Consider three elements A, B, and C belonging to Period 3 with atomic numbers 11, 14, and 17 respectively.

•

Q1: Write the chemical formulas of their oxides.

\rightarrow

A (

Na: 2,8,1

) forms basic oxide

\mathbf{A_2O}

(

\text{Na}_2\text{O}

); B (

Si: 2,8,4

) forms amphoteric/acidic oxide

\mathbf{BO_2}

(

\text{SiO}_2

); C (

Cl: 2,8,7

) forms acidic oxide

\mathbf{C_2O_7}

(

\text{Cl}_2\text{O}_7

•

Q2: Arrange A, B, C in increasing order of atomic radius.

\rightarrow

Atomic radius decreases across Period 3:

\mathbf{C < B < A}

(Chlorine < Silicon < Sodium).

•

Q3: Which element will exhibit the highest metallic character?

\rightarrow

Element A (Sodium), because metallic character is highest on the far left of the period.

5. CBSE Examiner Marking Scheme, Scientific Notation & Deduction Traps

Important Solved Board Questions

Examiner step-marking allocations, mandatory scientific terminology, and common error avoidance.

• Step-by-Step Marking Rubric & Key Terminology

•

1 Mark: Stating exact definitions of Mendeleev's vs Modern Periodic Laws.

•

1 Mark: Deducing Period number from shell count and Group from valence electrons.

•

2 Marks: Providing scientific rationale for atomic radius decrease (effective nuclear charge increase).

•

1 Mark: Correct oxide formula and acidic/basic nature determination.

• Common Error Deduction Traps

•

Trap 1: For elements with

> 2

valence electrons, forgetting to add 10 to find Group number (e.g. 7 valence electrons is Group 17, not Group 7).

•

Trap 2: Confusing atomic mass (Mendeleev) with atomic number (Modern / Moseley).

•

Trap 3: Stating that noble gases have valency 8 (noble gases have valency 0 because their octet is already complete).

Authentic Board Question (3 Marks)Topic: Periodic Classification of Elements Chemical Equations & Reaction Mechanisms

Explain the fundamental chemical principles, write the balanced chemical equations with state symbols, and describe the observable phenomena for Periodic Classification of Elements.

Official CBSE Step-by-Step Marking Breakdown:

Step 1: Balanced Chemical Equation with State Symbols: Write the stoichiometrically balanced chemical equation including state symbols

(s, l, g, aq)

for all reactants and products.

1 Mark

Step 2: Observable Physical Changes & Test Confirmation: State key observable indicators: color changes, gas evolution with confirmation test (pop sound/lime water), precipitate formation, or temperature changes.

1 Mark

Step 3: Chemical Reasoning & Mechanism: Explain the underlying chemical mechanism (neutralisation, displacement, redox electron transfer, or functional group properties).

1 Mark

Model Student Answer (Target: Full 3/3 Marks):

To score full 3 marks on Periodic Classification of Elements in CBSE Science (Chemistry):

1. Balanced Chemical Equation: State the exact equation with mandatory physical states:

\text{Reactant}_1(s) + \text{Reactant}_2(aq) \longrightarrow \text{Product}_1(aq) + \text{Product}_2(g)

2. Observable Phenomena: State the exact visual observation (e.g., brisk effervescence, blue to green color shift, white precipitate).
3. Chemical Logic: Explain the underlying ionic dissociation, reactivity series order, or pH shift that drives the reaction.

Examiner Mark Deduction Traps:

•Always include physical state symbols

(s), (l), (g), (aq)

in chemical equations to prevent deduction of ½ mark.

•Name specific indicator color changes (e.g. blue litmus to red, pink phenolphthalein turning colorless).

High-Frequency Conceptual Doubts & FAQs

Curated answers to the most common questions asked by Class 10 students.

The standalone chapter on Periodic Classification of Elements (Mendeleev and Modern Periodic Table) has been removed from the Class 10 CBSE examination syllabus as part of NCERT rationalization. Periodic trends are introduced later in Senior Secondary Chemistry.

Periodic Classification of Elements

1. Historical Evolution, Modern Periodic Law & Table Architecture

2. Mastering Periodic Trends in Periods & Groups

3. High-Yield Solved Board Examination Questions (3-Mark & 5-Mark)

4. Practical Laboratory & Competency-Based Case Drill: Predicting Properties of Unknown Elements in Periodic Grid

5. CBSE Examiner Marking Scheme, Scientific Notation & Deduction Traps

Official CBSE Step-by-Step Marking Breakdown:

High-Frequency Conceptual Doubts & FAQs

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