🔶 Introduction – Periodic Classification of Elements

Elements ko unke properties ke hisaab se systematically arrange karna hi periodic classification kehlaata hai.
Is chapter me aap Mendeleev’s Periodic Table, modern periodic table, periodicity of properties, and trends ke bare me detail me padhenge.

Table of Contents

Agar aapne previous chapter nahi padha, to pehle Class 10 Science Chapter 4: Carbon and Its Compounds padh sakte hain.

🔶 Why Classification of Elements Important?

Sab elements ki properties similar hote hain to unhe group me rakha jata hai.
Prediction of properties easy ho jata hai.
Chemistry ki study systematic ho jati hai.
Unknown elements ki discovery me help milti hai.

🔶 Early Attempts at Classification

1. Dobereiner’s Triads (1817)

3 elements group me rakhe jate hain
Middle element ka atomic mass average of other two
Example: Li (6), Na (23), K (39)

2. Newlands’ Law of Octaves (1864)

Elements ko increasing atomic mass ke according arrange kiya
Har 8th element similar properties ka tha
Problem: Only light elements me kaam kiya, heavy elements me fail

🔶 Mendeleev’s Periodic Table (1869)

Elements ko atomic mass ke order me arrange kiya
Similar properties wale elements same group me rakhe
Prediction of undiscovered elements: Gallium, Scandium, Germanium

Advantages:

Predict new elements
Properties systematic

Disadvantages:

Atomic mass se kuch anomalies (Te, I)
Modern periodic table me replace

🔶 Modern Periodic Table (Based on Atomic Number)

1913: Moseley ne atomic number discover kiya
Elements atomic number ke according arrange karte hain
Modern periodic table me 7 periods aur 18 groups hain

Rules:

Same group → similar chemical properties
Same period → gradually change in properties

🔶 Groups and Periods

Groups (Vertical Columns)

1–18
Similar properties
Number of valence electrons same

Periods (Horizontal Rows)

1–7
Gradual change in properties
Same number of shells

🔶 Classification of Elements (Main Blocks)

1. s-block Elements

Groups 1 & 2 (Alkali & Alkaline earth metals)
Soft metals, reactive
Examples: Li, Na, K, Mg, Ca

2. p-block Elements

Groups 13–18
Non-metals, metals & metalloids
Examples: B, C, N, O, F, Cl

3. d-block Elements (Transition Metals)

Groups 3–12
Hard, high melting points, coloured compounds
Examples: Fe, Cu, Zn

4. f-block Elements (Inner Transition Metals)

Lanthanoids & Actinoids
Rare earth elements
Examples: La, Ce, Th, U

🔶 Periodicity in Properties

Elements show repeating pattern in properties across periods:

1. Atomic Size (Radius)

Increases down the group
Decreases across the period

2. Ionization Energy

Energy required to remove electron
Decreases down the group
Increases across the period

3. Electronegativity

Ability to attract electrons
Decreases down the group
Increases across the period

4. Metallic & Non-metallic Properties

Metals → Left side
Non-metals → Right side
Metallic character ↓ across period
Non-metallic character ↑ across period

🔶 Special Groups (Important for Boards)

1. Group 1 – Alkali Metals

Soft, highly reactive, stored in kerosene
Form hydroxides & hydrogen gas
Examples: Li, Na, K

2. Group 2 – Alkaline Earth Metals

Less reactive than group 1
Form oxides & hydroxides
Examples: Mg, Ca

3. Group 17 – Halogens

Highly reactive non-metals
Form salts with metals
Example: F, Cl, Br, I

4. Group 18 – Noble Gases

Inert gases
Monatomic, non-reactive
Example: He, Ne, Ar

🔶 Important Trends (Board Focus)

Property	Down the Group	Across the Period
Atomic size	↑	↓
Ionization energy	↓	↑
Electronegativity	↓	↑
Metallic character	↑	↓
Non-metallic character	↓	↑

🔶 Transition Elements (d-block)

Hard, high melting points
Coloured compounds
Good catalysts
Example: Fe, Cu, Zn, Ni

🔶 Preparation and Uses of Elements

Sodium, Potassium → lab preparation via electrolysis
Chlorine → disinfect water, bleaching
Iron → construction, machinery
Copper → electrical wiring

🔶 Important Board Questions

Q1. Why modern periodic table is better than Mendeleev’s?

Based on atomic number
Eliminates anomalies

Q2. Define periodicity.

Repetition of properties after regular intervals in periodic table.

Q3. Give example of metalloid.

Boron, Silicon, Arsenic

Q4. Which group elements form salts with Halogens?

Group 1 (Alkali metals)

🔶 Conclusion

Periodic Classification of Elements chapter students ke liye fundamental hai. Elements ke groups, periods, trends aur modern periodic table ke rules samajhne se aap board exams me high scoring aur chemistry concepts me strong base paate hain.