📘 Chapter 5: Periodic Classification of Elements – Class 10 Science Notes

🔷 I. Historical Background

1. Döbereiner’s Triads (1817)

• Grouped elements into triads (groups of 3 elements with similar properties).
• The atomic mass of the middle element ≈ mean of the other two.
  Example:
  Li (7), Na (23), K (39) → (7 + 39)/2 = 23

✅ Limitation: Could not be applied to all known elements.

2. Newlands’ Law of Octaves (1866)

• Every eighth element had properties similar to the first (like musical octaves).
  Example: Li, Be, B, C, N, O, F, Na

✅ Limitation: Valid only up to Calcium.

3. Mendeleev’s Periodic Table (1869)

• Based on increasing atomic mass and similar properties.
• Left gaps for undiscovered elements and predicted their properties.
• Grouped elements into rows (periods) and columns (groups).

✅ Limitations:

• Position of isotopes
• Wrong order of atomic masses in some cases (e.g., Co & Ni)

🔷 II. Modern Periodic Table

Proposed by: Henry Moseley (1913)
Basis: Increasing Atomic Number (Z)

• Atomic number = number of protons
• 18 Groups (vertical) & 7 Periods (horizontal)

🔷 III. Structure of the Modern Periodic Table

🔷 IV. Periodic Trends

1. Valency

• Number of electrons lost/gained/shared to achieve octet.
• In a period: Increases from 1 to 4, then decreases.
• In a group: Remains the same.

2. Atomic Size (Atomic Radius)

• Distance between the nucleus and outermost electron shell.

3. Metallic & Non-metallic Character

• Metallic: Tendency to lose electrons.
• Non-metallic: Tendency to gain electrons.

4. Electronegativity

• Ability of atom to attract shared electrons.
• Increases across a period, decreases down a group.

5. Ionization Energy

• Energy needed to remove an electron.
• Increases across a period, decreases down a group.

🔷 V. Position of Elements

✅ Multiple Choice Questions (MCQs) with Answers

🔹 Conceptual MCQs

1. The basis of the modern periodic table is:
   a) Atomic mass b) Atomic number
   c) Number of neutrons d) Mass number
   ✅ Answer: b
2. In which group are noble gases placed?
   a) Group 16 b) Group 1 c) Group 18 d) Group 17
   ✅ Answer: c
3. Which of the following is the lightest alkali metal?
   a) Na b) Li c) K d) Rb
   ✅ Answer: b
4. How many periods are there in the modern periodic table?
   a) 8 b) 9 c) 7 d) 6
   ✅ Answer: c

🔹 Trend-Based MCQs

5. Which element has the smallest atomic radius in Period 3?
   a) Na b) Cl c) Mg d) S
   ✅ Answer: b
6. Which element shows maximum metallic character in Period 2?
   a) Li b) F c) C d) O
   ✅ Answer: a
7. As we move down Group 1, the metallic character:
   a) Increases b) Decreases c) Remains same d) First increases then decreases
   ✅ Answer: a
8. Which of these is a metalloid?
   a) Boron b) Sodium c) Oxygen d) Neon
   ✅ Answer: a

🔹 Assertion-Reason MCQs

9. Assertion (A): Atomic size increases down a group.
    Reason (R): Number of shells increases down the group.
   ✅ Answer: a (Both A and R are true; R explains A)
10. Assertion (A): Metals are placed on the right side of the periodic table.
     Reason (R): Metals are highly electronegative.
    ✅ Answer: c (A is false, R is false)

🔚 Summary

• Mendeleev classified elements by atomic mass, while Modern Periodic Law is based on atomic number.
• Trends across periods and groups help us understand the chemical behavior.
• Knowledge of valency, atomic size, and metallic character is critical to understanding reactivity.