Bihar Board Class 10 · Science · Matric exam 2027
Chapter 5: Periodic Classification of Elements — notes
The chapter’s 6 parts in the book’s order — the first idea of each and the points Medha Ma’am writes on the board — and the summary at the end. The full lesson and a check question for each part are in the app.
1. Early attempts – Döbereiner’s Triads
Okay, so you know there are 118 elements known to us.
- • Earliest attempt: metals vs non-metals
- • Döbereiner (1817): triads of 3 elements
- • Rule: atomic mass of middle = average of other two
- • Example: Li 6.9, Na 23.0, K 39.0
- • Average of Li & K = (6.9+39.0)/2 ≈ 23 = Na's mass
- • Limitation: only 3 triads could be found
2. Newlands' Law of Octaves
Okay, so after Döbereiner, there was John Newlands.
- • Newlands (1866): elements arranged by increasing atomic mass
- • Every 8th element resembles the 1st – like musical octaves
- • Example: Li and Na (Na is 8th after Li)
- • Example: Be and Mg resemble each other
- • Limitation: worked only up to calcium
- • Limitation: assumed only 56 elements exist
3. Mendeléev's Periodic Table
Okay, so let's talk about Mendeléev.
- • Mendeléev: 63 elements, arranged by atomic mass + properties
- • Based on hydride and oxide formulas (R used for any element)
- • Periodic Law: properties are periodic function of atomic mass
- • Groups = vertical columns, Periods = horizontal rows
- • Example: Co (58.9) placed before Ni (58.7)
- • Predicted Eka-aluminium → later found as Gallium
4. Limitations of Mendeléev's Table
Okay, so, Mendeléev's table was a great success.
- • Limitation 1: Hydrogen's position not fixed
- • H resembles alkali metals: HCl~NaCl, H2O~Na2O
- • H also resembles halogens: diatomic, forms covalent compounds
- • Limitation 2: Isotopes - same properties, different atomic mass
- • Limitation 3: Atomic mass does not increase regularly
5. The Modern Periodic Table
Okay, so let's talk about Moseley.
- • Moseley (1913): atomic number more fundamental than atomic mass
- • Modern Periodic Law: properties are periodic function of atomic number
- • 18 groups (vertical), 7 periods (horizontal)
- • Group = same number of valence electrons
- • Period = same number of shells
- • Max electrons in a shell = 2n² (K = 2, L = 8)
6. Trends: Valency, Atomic Size and Metallic Character
Okay, so now let's talk about how some properties change across the table.
- • Valency = number of valence electrons
- • Atomic radius of hydrogen = 37 pm
- • Atomic size: increases down a group (more shells)
- • Atomic size: decreases across a period (more nuclear charge)
- • Metals = left side, Non-metals = right side, Metalloids = border (zig-zag line)
- • Metallic character: decreases across period, increases down group
Summary
Okay, so let's look back at everything we covered in this chapter. Scientists wanted order among the many known elements. First, they tried something simple — just metals and non-metals. Then came Döbereiner's triads. There, the middle element's atomic mass was the average of the other two. But only three such triads could be found, so it didn't go far. Next, Newlands gave his Law of Octaves. He arranged elements by atomic mass, and every eighth element turned out similar. Nice idea, but it broke down after calcium. Then Mendeléev built a full periodic table, using atomic mass and the formulae of hydrides and oxides. He even left gaps — and used them to predict undiscovered elements, like gallium. That's a big achievement, isn't it? But hydrogen's position, and isotopes, stayed as problems he couldn't solve. Finally, Moseley showed that atomic number matters more than atomic mass. That gave us our Modern Periodic Table — 18 groups, 7 periods. It fixed the old problems, and showed clear trends — in valency, in atomic size, in metallic character — across periods and groups. So, tell me — between atomic mass and atomic number, which one do you think really decides an element's properties?
AskMedha · Medha Ma’am — Bihar Board Class 10 Science, Chapter 5. Written from the textbook, checked twice.
Questions parents and students ask
- How do I get the "Periodic Classification of Elements" notes as a PDF?
- Press "Save as PDF" — the phone’s print sheet opens; choose "Save as PDF" there. The PDF holds the notes only.
- Where are these notes written from?
- From the chapter’s own textbook text, part by part, in the book’s own terms; a second, independent model then checked every part against the book. The full lesson, a check question for each part and the practice are in the AskMedha app.
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