Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Quick Summary
The periodic table is chemistry's master organiser — 118 elements arranged by atomic number, with physical and chemical properties repeating periodically. Understanding the blocks and trends allows you to predict properties of any element without memorising everything individually. This chapter is heavily tested in NEET.
History of the Periodic Table
| Scientist | Contribution |
|---|---|
| Döbereiner | Law of triads (1829) |
| Newlands | Law of octaves (1864) |
| Mendeleev | Periodic law by atomic mass; predicted missing elements (1869) |
| Moseley | Modern periodic law by atomic number (1913) |
Modern Periodic Law: Properties of elements are a periodic function of their atomic number.
Structure of the Periodic Table
- Periods (rows): 7 periods. Period number = number of electron shells.
- Groups (columns): 18 groups. Group number relates to valence electrons.
Blocks
| Block | Location | Outer subshell |
|---|---|---|
| s-block | Groups 1–2 | ns¹ or ns² |
| p-block | Groups 13–18 | np¹ to np⁶ |
| d-block | Groups 3–12 | (n-1)d¹⁻¹⁰ |
| f-block | Bottom rows | (n-2)f¹⁻¹⁴ |
Periodic Trends
Atomic Radius
- Across period: Decreases (↑ nuclear charge, same shell)
- Down group: Increases (↑ number of shells)
- Order: Cs > Rb > K > Na > Li (down Group 1)
Ionic Radius
- Cations (M⁺) are smaller than parent atoms (electrons lost).
- Anions (X⁻) are larger than parent atoms (electrons gained).
- Isoelectronic series: species with same number of electrons — size decreases as nuclear charge increases.
Ionisation Energy (IE)
- Energy required to remove the outermost electron from a gaseous atom.
- Across period: Increases (↑ nuclear charge)
- Down group: Decreases (↑ shielding, farther from nucleus)
- Exception: IE(Be) > IE(B); IE(N) > IE(O) (due to filled/half-filled stability)
- Highest: He > Ne > F; Lowest: Cs (among metals)
Electron Affinity (EA)
- Energy released when an electron is added to a neutral gaseous atom.
- Across period: Increases (generally)
- Exception: Noble gases (stable config) have zero EA; F has lower EA than Cl (small size → electron–electron repulsion)
Electronegativity (EN)
- Across period: Increases
- Down group: Decreases
- Highest: F (4.0) → O (3.5) → N (3.0) → Cl (3.0)
- Lowest: Cs (0.7)
Summary of Trends
| Property | Across period | Down group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionic radius (for same charge) | Decreases | Increases |
| Ionisation energy | Increases | Decreases |
| Electron affinity | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
| Non-metallic character | Increases | Decreases |
Diagonal Relationship
Some elements in Period 2 resemble the next element diagonally in Period 3: Li–Mg, Be–Al, B–Si
This is due to similar charge/radius ratios (ionic potential).
Important Questions
One Mark
- Who proposed the Modern Periodic Law?
- Which element has the highest electronegativity?
- Name the element with the highest first ionisation energy among all elements.
- To which block does iron (Fe) belong?
Short Answer (3 marks)
- Explain why atomic radius decreases across a period but increases down a group.
- Why is the ionisation energy of oxygen less than that of nitrogen?
- Arrange F, Cl, Br, I in order of increasing electronegativity with reasons.
Long Answer (5 marks)
- Define and explain the periodic trends in ionisation energy with exceptions. Why does the second ionisation energy always exceed the first?
- Compare and contrast Mendeleev's periodic table with the Modern periodic table. What were the limitations of Mendeleev's table that the modern table resolved?
Common Mistakes
- Confusing atomic radius and ionic radius trends — cations are SMALLER, anions are LARGER than their parent atoms.
- Forgetting the IE exceptions: Be > B and N > O — ask "is the subshell filled or half-filled?"
- Electron affinity of F < Cl (not the expected trend) — due to small atomic size and high electron density causing repulsion.
- Mixing up electronegativity (bonding property) with electron affinity (isolated atom property).
Board Exam Tips
- Periodic trends (radius, IE, EA, EN) appear as 3-mark questions every year — know the trends AND the exceptions.
- "Arrange in order" questions require a brief reason — don't just write the order.
- NEET: 2–3 questions on periodic trends appear in nearly every paper. Focus on IE exceptions and isoelectronic series.
- The diagonal relationship (Li–Mg, Be–Al) is a favourite 2-mark question in boards.
Frequently Asked Questions
s-block: Groups 1–2 (alkali and alkaline earth metals). p-block: Groups 13–18. d-block: Groups 3–12 (transition metals). f-block: Lanthanides and Actinides (inner transition metals).