Vidaara.orgClass 11 · Chemistry
CodeVID-C11-03-T2-01
Periodic Trends: Size & Energy — Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show full working for Sections B, C and D; only final answers are provided.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Across a period, the atomic radius generally:
- A.increases
- B.decreases
- C.stays constant
- D.doubles
2.
A cation is always _____ than its parent atom.
- A.larger
- B.smaller
- C.equal
- D.twice
3.
Which has the most negative electron gain enthalpy?
- A.F
- B.Cl
- C.Br
- D.I
4.
Effective nuclear charge $Z_{eff}$ is given by:
- A.$Z + S$
- B.$Z - S$
- C.$Z \times S$
- D.$Z / S$
5.
Successive ionisation enthalpies of an element:
- A.decrease
- B.stay equal
- C.increase
- D.alternate
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Define first ionisation enthalpy and write its defining equation.
7.
Arrange $\text{Na}^+, \text{Mg}^{2+}, \text{F}^-, \text{O}^{2-}$ in increasing size.
8.
Why is electron gain enthalpy of noble gases positive (large)?
Section C — Long Answer (3 marks)
2 × 3 = 6 marks
9.
Explain, with $Z_{eff}$ and shielding, why ionisation enthalpy increases across a period.
10.
Account for the two dips in first ionisation enthalpy across period 2.
Section D — Detailed Answer (5 marks)
1 × 5 = 5 marks
11.
Discuss the variation of atomic radius, ionisation enthalpy and electronegativity across a period and down a group, giving the underlying reasons.
Answer Key
Section A — Multiple Choice Questions
- (B) decreases
- (B) smaller
- (B) Cl
- (B) $Z - S$
- (C) increase
Section B — Short Answer (2 marks)
- Energy to remove the most loosely bound electron from one mole of gaseous atoms: $\text{X}(g) \rightarrow \text{X}^+(g) + e^-$.
- $\text{Mg}^{2+} < \text{Na}^+ < \text{F}^- < \text{O}^{2-}$ (isoelectronic; size falls as $Z$ rises).
- Their $ns^2np^6$ shells are full; an added electron must enter a higher shell, requiring energy, so $\Delta_{eg} H$ is positive.
Section C — Long Answer (3 marks)
- Across a period electrons enter the same shell (poor shielding) while $Z$ rises, so $Z_{eff}$ increases, atoms get smaller, and the outer electron is held more tightly, raising ionisation enthalpy.
- B
Section D — Detailed Answer (5 marks)
- Across a period: radius decreases, ionisation enthalpy and electronegativity increase (rising $Z_{eff}$, poor shielding). Down a group: radius increases, ionisation enthalpy and electronegativity decrease (new shells added, more shielding, outer electron farther out). All three trends are driven by the balance between nuclear charge and shielding/distance.
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