Chapter 2: Periodic Classification of Elements Notes – Class 10 (MSBSHSE)

In this chapter, we explore how elements are classified based on their properties, from early attempts to modern periodic tables. This detailed guide covers laws, trends, formulas, solved examples, and exam highlights according to Maharashtra Board syllabus.

🔸 1. Classification of Elements – Need & Methods

• Early chemists discovered many elements—organizing them helped predict properties.
• Classification arranged elements by atomic mass or atomic number.

🔸 2. Dobereiner’s Law of Triads (1817)

Grouped elements by similar properties in threes—middle element’s mass was approximate average of the other two.

• Example: Li‑Ca‑Sr: (20+40)/2 = 30 ≈ mass of Ca 1

🔸 3. Newlands’ Law of Octaves (1866)

Elements arranged by atomic mass—every 8th element had similar properties.

🔸 4. Mendeleev’s Periodic Table (1869)

• Elements arranged by increasing atomic mass.
• Similar properties lined up in columns.
• Successfully predicted properties of unknown elements before discovery. 3

🔸 5. Modern Periodic Law & Table

Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers (Henry Moseley). 5

• Arranged by atomic number → 7 periods, 18 groups.
• Elements grouped into s-, p-, d-, and f-blocks.
• Lanthanides and actinides separated at the bottom.

📌 Table Layout Summary

• s-block: Groups 1–2 + H and He
• p-block: Groups 13–18 (including noble gases)
• d-block: Transition metals (Groups 3–12)
• f-block: Lanthanides & actinides

🔸 6. Periodic Trends in the Modern Table

• Valency: Increases across a period, same down groups.
• Atomic radius: Decreases across a period, increases down groups.
• Metallic character: Decreases across, increases down groups.

🔸 7. Important Definitions & Rules

• Period: Horizontal row of elements, indicates number of electron shells.
• Group: Vertical column of elements, based on valence electrons.
• Valency trend: In periods, valency increases up to a point then decreases back toward noble gases.

🔸 8. Solved Examples

1. Atomic Configuration: 2,8,2 → Atomic number = 12 → Mg (Group 2, Period 3)
2. Identify Element: 2,7 → F (Valency = 1)
3. Dobereiner’s Triad: S‑Se‑Te → yes, average atomic mass ≈ middle.

🔸 9. Exam Tips (Last 5 Years)

• State laws: Dobereiner, Newlands, Mendeleev with examples.
• Describe modern periodic table layout & trends.
• Numericals: electronic configuration, identifying valency, periodic trend questions.
• Draw a neat modern periodic table with group/period labels.

🔗 You may also like:

• Chapter 1 Gravitation notes
• Class 10 Kumarbharati Textbook Solutions
• Class 10 All Subject Notes
• Class 10 English Paper Pattern

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