SSC CGL Chemistry · Atomic Structure and Periodic Classification
The Periodic Table
Newlands' octaves and Mendeleev's table with their years and predictions, the group names and numbers, the blocks, and how atomic size changes across the table.
Why this matters
Twenty-one questions in the bank: 13 from the 2023 shifts and eight from the 2024 shifts, and none from the 2025 shifts. They ask a year, a group number or a family name, so the two tables here are the whole page.
Concept 1 of 2: Newlands and Mendeleev
Definition
Newlands' Law of Octaves (1865):
- The 56 known elements in order of atomic mass, from hydrogen (the lightest) to thorium (the 56th).
- Every eighth element resembles the first, like notes of music: lithium and sodium match.
- A flaw: cobalt and nickel sat under 'Do' with the halogens.
Mendeleev's table (1869):
- 63 elements by atomic weight, with gaps for undiscovered ones.
- Eka-aluminium became gallium (predicted mass 68); eka-silicon became germanium (72); eka-boron became scandium (44).
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Atomic Structure and Periodic Classification · The Periodic Table
Hydrogen is the lightest, not the heaviest
Concept 2 of 2: Groups, periods, blocks and trends
Definition
Families:
- Group 1: alkali metals, highly electropositive (sodium). Group 2: alkaline earth metals (Mg, Ca, Sr, Ba, Ra).
- Group 16: chalcogens. Group 17: halogens (F, Cl, Br, I). Group 18: noble gases (He, Xe).
- Groups 3 to 12: transition elements, the d-block. Lanthanides: the rare earth elements (f-block).
Rows and trends:
- Every period ends in a noble gas: period 5 ends at xenon. Period 6 holds tungsten, platinum; silicon is period 3.
- Atomic size increases down a group (a shell is added each step). Atomic radius is measured in picometres.
- Fluorine has electronegativity 4.0 on Pauling's scale, the highest.
- Period 2, group 17 means shells 2, 7: atomic number 9.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 2 · Atomic Structure and Periodic Classification · The Periodic Table
| Column-A (Common name) | Column-B (Group number in the Modern Periodic Table) |
|---|---|
| i. Alkali metals | a. 18 |
| ii. Alkaline earth metals | b. 17 |
| iii. Halogens | c. 2 |
| iv. Noble gases | d. 1 |
Lanthanides are f-block
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Watch out for (2)
- Hydrogen is the lightest, not the heaviest→ Newlands and Mendeleev
- Lanthanides are f-block→ Groups, periods, blocks and trends
Test yourself on Atomic Structure and Periodic Classification
15 past SSC CGL questions from this chapter, timed at 9 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.