CBSE Class 10 Science: Periodic Classification — Complete Notes 2026

Early Classification Attempts

• Dobereiner's Triads (1817): groups of 3 elements where middle element's atomic mass ≈ average of other two; e.g., Li (7), Na (23), K (39)
• Newlands' Law of Octaves (1865): every 8th element has properties similar to first; worked only up to calcium
• Limitations: many elements unknown; no accommodation for isotopes; physical states mixed

Mendeleev's Periodic Table (1869)

• Arranged 63 known elements by atomic mass; elements with similar properties in same column (Group)
• Predicted existence of undiscovered elements: eka-aluminium (Gallium), eka-silicon (Germanium)
• Limitations: no fixed position for hydrogen; isotopes of same element assigned different positions

Modern Periodic Table

• Henry Moseley (1913): periodic law based on atomic number; elements arranged in increasing atomic number
• 18 groups (vertical), 7 periods (horizontal); s, p, d, f block elements
• Period 1: H, He; Period 2: Li to Ne (8 elements); Period 3: Na to Ar

Trends Across a Period

• Atomic size decreases left to right (nuclear charge increases, electrons in same shell)
• Ionisation energy increases left to right; metallic character decreases
• Electronegativity increases across a period; non-metallic character increases

Trends Down a Group

• Atomic size increases down the group (new electron shells added)
• Ionisation energy decreases; metallic character increases down the group
• Reactivity: metals increase down group (alkali metals); non-metals decrease (halogens)

Valency and Electronic Configuration

• Valency = number of electrons in outermost shell (or 8 minus that number for non-metals)
• Group 1 elements (alkali metals): valency 1; Group 17 (halogens): valency 1
• Noble gases (Group 18): valency 0; stable octets; do not form compounds normally

CBSE Exam Tips

• Periodic table trends diagram is frequently asked (draw and label); practice drawing atomic diagrams
• Mendeleev vs Modern periodic table — comparison table is high-value for board exam
• Define Dobereiner's triads with an example; explain one limitation of each classification