Periodic Properties and Variations
Introduction
The Periodic Table arranges elements in order of increasing atomic number, revealing periodic trends in properties. In ICSE Class 10 Chemistry, you study how atomic size, ionisation potential, electron affinity, and electronegativity vary across periods and down groups.
Periodic Trends
Atomic Size (Atomic Radius)
- Across a period (L to R): Decreases — nuclear charge increases, electrons are pulled closer.
- Down a group: Increases — new shells are added.
Metallic Character
- Across a period (L to R): Decreases — elements become less metallic.
- Down a group: Increases — easier to lose electrons.
Non-Metallic Character
- Across a period (L to R): Increases — elements become more non-metallic.
- Down a group: Decreases.
Ionisation Potential (IP)
Energy required to remove the outermost electron from an isolated gaseous atom.
- Across a period (L to R): Increases — nuclear charge increases, electrons held more tightly.
- Down a group: Decreases — atomic size increases, outer electrons are further from the nucleus.
Electron Affinity (EA)
Energy released when an electron is added to an isolated gaseous atom.
- Across a period (L to R): Increases (becomes more negative).
- Down a group: Decreases.
Electronegativity
The tendency of an atom to attract shared electrons towards itself in a covalent bond.
- Across a period (L to R): Increases.
- Down a group: Decreases.
- Most electronegative element: Fluorine.
Summary of Trends
| Property | Across a Period (→) | Down a Group (↓) |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Non-metallic character | Increases | Decreases |
| Ionisation potential | Increases | Decreases |
| Electron affinity | Increases | Decreases |
| Electronegativity | Increases | Decreases |
Group 1 — Alkali Metals (Li, Na, K, Rb, Cs, Fr)
Characteristics
- valency: +1
- Soft, low melting points
- Highly reactive (reactivity increases down the group)
- Form alkaline hydroxides: MOH
- Stored under kerosene (to prevent reaction with air/moisture)
Trends
| Property | Trend down the group |
|---|---|
| Atomic radius | Increases |
| Melting point | Decreases |
| Reactivity with water | Increases (Na reacts vigorously, K catches fire) |
Group 17 — Halogens (F, Cl, Br, I, At)
Characteristics
- valency: −1
- Non-metals
- Exist as diatomic molecules (F₂, Cl₂, Br₂, I₂)
- React with metals to form salts (halides)
Trends
| Property | Trend down the group |
|---|---|
| Atomic radius | Increases |
| Melting/boiling point | Increases (F₂, Cl₂ are gases; Br₂ is liquid; I₂ is solid) |
| Reactivity | Decreases (F₂ is most reactive) |
| Colour | Darkens (F₂: pale yellow, Cl₂: green, Br₂: red-brown, I₂: violet) |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Thinking atomic radius increases across a period | It DECREASES — nuclear charge outweighs electron addition |
| Confusing ionisation potential and electron affinity | IP = energy to REMOVE e⁻; EA = energy released when ADDING e⁻ |
| Mixing group and period trends | Memorise: across decreases (atomic radius, metallic); down increases (atomic radius, metallic) |
| Forgetting F is the most electronegative element | Not oxygen or chlorine — fluorine |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: trends across periods and groups, comparison of Group 1 and Group 17 elements, explanation of trends.
Marks Blueprint: Period trends — 3 marks, Group trends — 2 marks, Group 1/Group 17 — 3 marks.
Self-Test Questions
-
Explain why atomic size decreases across a period.
-
Define ionisation potential. How does it vary down a group and why?
-
Compare the reactivity trends of Group 1 and Group 17 elements.
-
Which is the most electronegative element? How does electronegativity vary across a period?
-
Why are alkali metals stored under kerosene?
-
Arrange F, Cl, Br, I in order of increasing (a) atomic radius, (b) reactivity, (c) melting point.
