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.
Common Mistakes Students Make in Periodic Classification of Elements
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- Confusing Mendeleev’s and the Modern Periodic Law: Mendeleev arranged elements by increasing atomic mass; the Modern Periodic Law arranges them by increasing atomic number. Students often quote Mendeleev’s basis when asked about the Modern Periodic Table, or vice versa.
- Mixing up groups and periods: vertical columns are groups (elements with the same number of valence electrons and similar properties); horizontal rows are periods (elements with the same number of shells). This left-right/up-down mix-up is one of the most frequent slips in this topic.
- Valency vs group number: for main-group elements, valency generally equals the group number (for groups 1–2) or 18 minus the group number (for groups 13–17) — students often try to apply a single flat rule across all groups without this adjustment.
- Getting periodic trend directions backwards: atomic size decreases across a period (left to right) but increases down a group; metallic character decreases across a period but increases down a group. These two trends are commonly swapped or stated in the wrong direction.
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- 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.
NCERT Solutions | Extra Questions (HOTS) | Formulas Handbook
Class 10 Science Chapter 5 – Solutions and Important Questions
Need full answers or more practice? See the Class 10 Science Chapter 5 Solutions and Class 10 Science Chapter 5 Extra Questions.
What to Revise First (and Last) in Periodic Classification of Elements
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This chapter is a supplementary/legacy topic on this site’s current NCERT syllabus listing, so revise it strictly for conceptual clarity rather than exam-weightage — focus first on the group-vs-period distinction and the modern periodic law, since these are the ideas every other trend in the chapter builds on. Revise the periodic trends (atomic size, metallic character, valency) as a single comparison table rather than as separate facts — it is the fastest way to refresh the whole chapter in one pass if you are revisiting it for background or foundational understanding.
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