Anand Classes provides detailed NCERT Solutions for Class 11 Chemistry Chapter 3 – Classification of Elements and Periodicity in Properties (Questions 3.31 to 3.35). These step-by-step solutions explain important concepts such as ionization enthalpy, electron gain enthalpy, reactivity trends, electronic configurations, periodic table structure, quantum numbers, and factors affecting valence electrons. These solutions are highly useful for CBSE board exams as well as competitive exams like NEET and JEE. Click the print button to download study material and notes.
NCERT Solutions of Problems 3.31 – 3.35 of Chapter Classification of Elements and Periodicity in Properties Chemistry Class 11 pdf Download
NCERT 3.31 : The first (ΔiH₁) and the second (ΔiH₂) ionization enthalpies (in kJ mol⁻¹) and the (ΔegH) electron gain enthalpy (in kJ mol⁻¹) of a few elements are given below:
| Element | ΔiH₁ (kJ mol⁻¹) | ΔiH₂ (kJ mol⁻¹) | ΔegH (kJ mol⁻¹) |
|---|---|---|---|
| I | 520 | 7300 | −60 |
| II | 419 | 3051 | −48 |
| III | 1681 | 3374 | −328 |
| IV | 1008 | 1846 | −295 |
| V | 2372 | 5251 | +48 |
| VI | 738 | 1451 | −40 |
Which of the above elements is likely to be:
(a) the least reactive element
(b) the most reactive metal
(c) the most reactive non–metal
(d) the least reactive non–metal
(e) the metal which can form a stable binary halide of the formula MX₂ (X = halogen)
(f) the metal which can form a predominantly stable covalent halide of the formula MX (X = halogen) ?
Answer
(a) Least reactive element: V.
- The element V has very high ionization enthalpies ($\Delta iH_1 = 2372,\; \Delta iH_2 = 5251$) and a positive electron gain enthalpy ($\Delta egH = +48$) — behaviour of a noble gas; chemically unreactive.
(b) Most reactive metal: II.
- The element II which has the Lowest first ionization enthalpy ($\Delta iH_1 = 419$) and a low negative electron gain enthalpy ($\Delta egH = -48$) is the most reactive metal→ easiest to lose an electron → most metallic/reactive metal among the list.
(c) Most reactive non–metal: III.
- Most negative electron gain enthalpy ($\Delta egH = -328$) → strongest tendency to gain an electron → most reactive non-metal (halogen-like).
(d) Least reactive non–metal: V.
- Positive $\Delta egH$ and very large $\Delta iH$ values indicate extremely low reactivity (noble-gas behaviour), hence least reactive non-metal.
(e) Metal that can form a stable binary halide MX₂: VI.
- Relatively low $\Delta iH_1$ and moderate $\Delta iH_2$ ($738$ and $1451$) — formation of a +2 oxidation state is energetically feasible → stable MX₂ (alkaline-earth-like behaviour).
(f) Metal that can form a predominantly covalent halide MX: I.
- Low $\Delta iH_1$ (520) but very large $\Delta iH_2$ (7300) → stable +1 state; small highly polarising cation leads to covalent character in MX (Li-like behaviour).
NCERT 3.32 : Predict the formulas of the stable binary compounds that would be formed by the combination of the following pairs of elements.
(a) Lithium and oxygen
(b) Magnesium and nitrogen
(c) Aluminium and iodine
(d) Silicon and oxygen
(e) Phosphorus and fluorine
(f) Element 71 and fluorine
Answer :
(a) Lithium and oxygen
Lithium is an alkali metal (Group 1). It has only one electron in the valence shell, therefore, its valency is 1. Oxygen is a group 16 element with a valence of 2. Therefore, formula of the compound formed would be $Li_2O$ (Lithium oxide).
(b) Magnesium and nitrogen
Magnesium is an alkaline earth metal (Group 2) and hence has a valence of 2. Nitrogen is a group 15 element with a valence of $8-5=3$. Thus, the formula of the compound formed would be $Mg_3N_2$ (Magnesium nitride).
(c) Aluminium and iodine
Aluminium is a group 13 element with a valence of 3 while iodine is a halogen (group 17) with a valence of 1. Therefore, the formula of the compound formed would be $AlI_3$ (Aluminium iodide).
(d) Silicon and oxygen
Silicon is a group 14 element with a valence of 4 while oxygen is a group 16 element with a valence of 2. Hence the formula of the compound formed is $SiO_2$ (Silicon dioxide).
(e) Phosphorus and fluorine
Phosphorus is a group 15 element with a valence of 3 or 5 while fluorine is a group 17 element with a valence of 1. Hence the formula of the compound formed would be $PF_3$ or $PF_5$.
(f) Element 71 and fluorine
Element with atomic number 71 is a lanthanoid called lutetium (Lu). Its common valence is 3. Fluorine is a group 17 (halogen) element with a valence of 1. Therefore, the formula of the compound formed would be $LuF_3$ (Lutetium fluoride).
NCERT 3.33 : In the modern periodic table, the period indicates the value of :
(a) atomic number
(b) atomic mass
(c) principal quantum number
(d) azimuthal quantum number
Answer :
In the modern periodic table, each period begins with the filling of a new shell. Therefore, the period indicates the value of the principal quantum number.
Thus, the correct option is:
(c) principal quantum number
NCERT 3.34 : Which of the following statements related to the modern periodic table is incorrect?
(a) The p–block has 6 columns, because a maximum of 6 electrons can occupy all the orbitals in a p–subshell.
(b) The d–block has 8 columns, because a maximum of 8 electrons can occupy all the orbitals in a d–subshell.
(c) Each block contains a number of columns equal to the number of electrons that can occupy that subshell.
(d) The block indicates value of azimuthal quantum number (l) for the last subshell that received electrons in building up the electronic configuration.
Answer
Statement (b) is incorrect while the other statements are correct.
✅ Corrected statement: The d–block has 10 columns, because a maximum of 10 electrons can occupy all the orbitals in a d–subshell.
NCERT 3.35 : Anything that influences the valence electrons will affect the chemistry of the element. Which one of the following factors does not affect the valence shell?
(a) Valence principal quantum number (n)
(b) Nuclear charge (Z)
(c) Nuclear mass
(d) Number of core electrons
Answer
Nuclear mass does not affect the valence shell because the nucleus consists of protons and neutrons.
- Protons (i.e., nuclear charge) affect the valence shell.
- Neutrons do not influence valence electrons.
Thus, the correct option is:
(c) Nuclear mass
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