Periodic Classification of Elements — Science Class 10 Notes (CBSE & HBSE)
Free NCERT Science notes for Periodic Classification of Elements (Class 10) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 4 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Periodic Classification of Elements (CBSE & HBSE)
Examines the historical evolution of the periodic table and the recurring physical/chemical trends among elements. CBSE focuses on periodic trends; HBSE focuses on early attempts and definitions.
Early Attempts: Dobereiner and Newlands
Early scientists tried to group elements based on their properties to simplify their study.
1. Dobereiner’s Triads (1817)
Johann Wolfgang Döbereiner arranged elements with similar properties into groups of three (triads).
- Rule: Atomic mass of the middle element was roughly the average of the other two.
- Example: Lithium (6.9), Sodium (23.0), Potassium (39.0).
- Failure: He could only identify three triads from the elements known at that time.
2. Newlands’ Law of Octaves (1866)
John Newlands arranged elements in increasing order of atomic masses.
- Rule: Every eighth element had properties similar to that of the first (like musical notes).
- Failure: Applicable only up to Calcium. It assumed only 56 elements existed.
Mendeleev’s Periodic Table
Dmitri Mendeleev used atomic mass as the fundamental property.
Mendeleev’s Periodic Law
"The properties of elements are the periodic function of their atomic masses."
Achievements:
- Left gaps for undiscovered elements (e.g., Eka-boron, Eka-aluminium).
- Could predict properties of undiscovered elements accurately.
- Accommodated noble gases without disturbing the main table.
Limitations:
- No fixed position for Hydrogen.
- Position of Isotopes could not be explained.
- Wrong order of atomic masses for some elements (e.g., Co before Ni).
The Modern Periodic Table
In 1913, Henry Moseley showed that Atomic Number is a more fundamental property than atomic mass.
Modern Periodic Law
"Properties of elements are a periodic function of their atomic number."
Features:
- Periods: 7 Horizontal rows. Period number indicates the number of shells.
- Groups: 18 Vertical columns. Elements in a group have the same number of valence electrons.
- It successfully explained the position of isotopes and the anomalies of Mendeleev's table.
Periodic Trends in Properties
Properties of elements change predictably across the table.
1. Valency
- Across a Period: Increases from 1 to 4 and then decreases to 0.
- Down a Group: Remains the same.
2. Atomic Size (Radius)
- Across a Period: Decreases (nuclear charge increases, pulling electrons closer).
- Down a Group: Increases (new shells are added).
3. Metallic Character
- Across a Period: Decreases (tendency to lose electrons decreases).
- Down a Group: Increases (effective nuclear charge decreases, making it easier to lose electrons).
Frequently asked questions
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Periodic Classification of Elements.