Revision Notes: Class 10 Science Periodic Classification of Elements (Supplementary)

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

Note: Not part of the current CBSE board-exam syllabus (excluded since the 2023-24 rationalisation), but retained in the NCERT textbook. These Class 10 Science Chapter 5 notes are ideal for quick revision just before exams.

• Döbereiner’s Triads (1817): groups of 3 elements with similar properties where the middle element’s atomic mass ≈ average of the other two. Worked only for a few triads.
• Newlands’ Law of Octaves (1866): arranged elements by increasing atomic mass; every 8th element had similar properties (like musical octaves). Broke down after calcium and didn’t leave room for new elements.
• Mendeleev’s Periodic Table: arranged ~63 known elements by increasing atomic mass into 8 groups (with sub-groups A/B) and periods. Left gaps for undiscovered elements (predicted properties accurately, e.g. eka-aluminium = gallium). Limitation: could not accommodate isotopes and a few elements didn’t fit strict mass order (e.g. cobalt/nickel).
• Modern Periodic Law: properties of elements are a periodic function of their atomic number, not atomic mass.
• Modern Periodic Table: 18 groups (vertical columns), 7 periods (horizontal rows). Elements in the same group have the same number of valence electrons and similar properties. Elements in the same period have the same number of occupied shells.
• Periodic trends across a period (left to right): valence electrons increase, atomic size decreases, metallic character decreases, non-metallic/acidic character of oxides increases.
• Periodic trends down a group: number of shells increases, atomic size increases, metallic character increases (for metals), reactivity of non-metals decreases, electronegativity decreases.
• Valency = number of valence electrons (if 4 or fewer) or (8 − number of valence electrons) if more than 4.

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Written by Satish

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