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Periodic classification
Chemistry
Useful for: OSSC RHT — Regular High School Teacher, OSEPA Upper Primary — Science, OSSC Science, OSSSC Govt TGT Science — CBZ, OSSSC Govt TGT Science — PCM, SSB Lecturer (Degree Colleges), SSB TGT CBZ, SSB TGT PCM
Overview
The periodic table arranges elements by increasing atomic number so that elements with similar properties fall in the same group.
Key points
- Döbereiner grouped elements in triads; Newlands proposed the law of octaves.
- Mendeleev (1869) arranged elements by atomic mass and left gaps, predicting elements such as eka-aluminium (gallium).
- Moseley showed atomic number is the basis; the modern periodic law follows atomic number.
- The modern table has 18 groups and 7 periods.
- Across a period, atomic radius decreases and electronegativity increases.
- Down a group, atomic radius increases and metallic character increases.
Explanation
- Elements of a group have the same number of valence electrons.
- Noble gases are in group 18.
- Metals are on the left, non-metals on the right, metalloids along the zig-zag line.
- Isotopes posed a problem for Mendeleev’s mass-based table.
- Ionisation energy generally increases across a period.
Step by step
- Identify the concept, method or fact asked.
- Recall the definition, thinker, year or rule.
- Apply it and eliminate extreme or mismatched options.
Examples
- Mendeleev’s eka-aluminium was later discovered as → gallium.
- Sodium and potassium are in the same group → similar properties.
- Across period 3 from Na to Cl, atomic radius → decreases.
Traps and advanced points
- Trap: Saying the modern table uses atomic mass.
- Trap: Wrong trend direction.
- Trap: Mixing groups and periods.
Quick revision
- Mendeleev 1869; Moseley atomic number.
- 18 groups, 7 periods.
- Radius ↓ across, ↑ down.
Practice MCQs
The law of octaves was proposed by —
- Mendeleev
- Newlands
- Döbereiner
- Moseley
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Answer: (b) Newlands. Newlands.
Triads were proposed by —
- Newlands
- Mendeleev
- Bohr
- Döbereiner
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Answer: (d) Döbereiner. Döbereiner.
Mendeleev arranged elements by —
- atomic number
- atomic mass
- valency only
- density
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Answer: (b) atomic mass. Atomic mass (1869).
The modern periodic law is based on —
- atomic mass
- atomic number
- density
- colour
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Answer: (b) atomic number. Moseley.
The modern periodic table has —
- 16 groups and 8 periods
- 18 groups and 9 periods
- 8 groups and 7 periods
- 18 groups and 7 periods
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Answer: (d) 18 groups and 7 periods. 18 × 7.
Eka-aluminium predicted by Mendeleev is —
- silicon
- germanium
- gallium
- scandium
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Answer: (c) gallium. Gallium.
Across a period from left to right, atomic radius —
- doubles
- increases
- decreases
- stays same
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Answer: (c) decreases. Nuclear charge increases.
Down a group, metallic character —
- stays same
- disappears
- decreases
- increases
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Answer: (d) increases. Atoms get larger.
Noble gases are in group —
- 18
- 2
- 1
- 17
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Answer: (a) 18. Group 18.
Elements in the same group have the same —
- atomic mass
- number of shells
- density
- number of valence electrons
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Answer: (d) number of valence electrons. Similar properties.
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