NCERT Solutions for Class 11 Chemistry Chapter 3 Classification of Elements & Periodicity in Properties
NCERT Solutions for Class 11 Chemistry Chapter 3 Classification of Elements and Periodicity in Properties covers historical development of periodic tables, Modern Periodic Law, division of elements into s, p, d, and f blocks, IUPAC nomenclature for elements with Z > 100, and comprehensive periodic trends in physical and chemical properties including atomic radius, ionic radius, ionization enthalpy, electron gain enthalpy, electronegativity, and chemical reactivity.
Overview of Topics & Subtopics (Click to Jump)
1. Historical Development & Modern Periodic Law
Short Note:
• Dobereiner's Triads & Newlands' Law of Octaves: Early attempts to classify elements based on atomic masses.
• Mendeleev's Periodic Law: Properties of elements are a periodic function of their atomic masses. Arranged elements in periods and groups based on similarities in formula of hydrides and oxides.
• Modern Periodic Law (Henry Moseley): Physical and chemical properties of the elements are periodic functions of their atomic numbers (Z).
• Long Form of Periodic Table: Consists of 7 horizontal rows called Periods and 18 vertical columns called Groups.
2. IUPAC Nomenclature (Z > 100) & Electronic Configuration in Blocks
Short Note:
• IUPAC Nomenclature for Z > 100: Roots used based on digits: 0=nil, 1=un, 2=bi, 3=tri, 4=quad, 5=pent, 6=hex, 7=sept, 8=oct, 9=enn. Suffix -ium is added (e.g., Z = 101 → Unnilunium, Symbol: Unu; Z = 120 → Unbinilium, Symbol: Ubn).
• Classification into Blocks:
- s-Block (Groups 1 & 2): Outer configuration ns1-2. Reactive metals, low ionization energy.
- p-Block (Groups 13 to 18): Outer configuration ns2 np1-6. Includes metals, non-metals, and metalloids.
- d-Block (Groups 3 to 12): Transition elements, configuration (n-1)d1-10 ns1-2. Form colored ions and exhibit variable oxidation states.
- f-Block (Lanthanoids & Actinoids): Inner transition elements, configuration (n-2)f1-14 (n-1)d0-1 ns2.
3. Periodic Trends in Physical Properties
Short Note:
• Atomic & Ionic Radius:
- Across a Period: Decreases left to right due to increase in effective nuclear charge (Zeff).
- Down a Group: Increases top to bottom due to addition of new energy shells.
- Isoelectronic Species: Cations are smaller than neutral atoms; anions are larger. Higher positive charge = smaller size; higher negative charge = larger size.
• Ionization Enthalpy (ΔiH): Energy required to remove the most loosely bound electron from an isolated gaseous atom.
- Increases across a period, decreases down a group.
- Anomalies: N > O (due to half-filled stable 2p3 shell) and Be > B (due to fully-filled stable 2s2 shell).
• Electron Gain Enthalpy (ΔegH): Enthalpy change when an electron is added to a neutral gaseous atom.
- Becomes more negative across a period, less negative down a group.
- Anomaly: Chlorine has a more negative ΔegH than Fluorine due to less inter-electronic repulsion in Chlorine's larger 3p shell.
• Electronegativity: Ability of an atom in a compound to attract shared pair of electrons towards itself (Pauling scale: F = 4.0).
4. Periodic Trends in Chemical Properties & Anomalous Behaviors
Short Note:
• Valence / Oxidation States: Valence increases from 1 to 4 and then decreases to 0 across a period for main group elements.
• Anomalous Properties of Second Period Elements: Li, Be, B, C, N, O, F differ significantly from rest of their group members due to small size, high electronegativity, high charge/radius ratio, and absence of d-orbitals.
• Diagonal Relationship: Similarity in properties between certain elements of period 2 and period 3 placed diagonally (e.g., Li ~ Mg, Be ~ Al, B ~ Si).
Intext & Conceptual Questions
Explanation:
The electronic configurations of Nitrogen (Z = 7) and Oxygen (Z = 8) are:
• Nitrogen (N): 1s2 2s2 2px1 2py1 2pz1 (Half-filled 2p subshell)
• Oxygen (O): 1s2 2s2 2px2 2py1 2pz1
Nitrogen has an extra stable half-filled 2p subshell configuration, which requires more energy to remove an electron compared to Oxygen, where removing one electron yields a stable half-filled configuration. Therefore, Nitrogen has a higher first ionization enthalpy than Oxygen.
Textbook Chapter End Exercise Questions
(a) What is common in them?
(b) Arrange them in order of increasing ionic radii.
(a) Common Feature:
All these species contain 10 electrons each. Therefore, they are isoelectronic species.
(b) Order of Increasing Ionic Radii:
For isoelectronic species, as the nuclear charge (atomic number) increases, the attractive force on the valence electrons increases, causing a decrease in ionic radius.
Nuclear charges (Z): Al3+ (13), Mg2+ (12), Na+ (11), F- (9), O2- (8), N3- (7).
Increasing order of ionic radii:
Al3+ < Mg2+ < Na+ < F- < O2- < N3-
For atomic number 120:
• Digit 1 = un
• Digit 2 = bi
• Digit 0 = nil
Combining the roots with the suffix '-ium':
IUPAC Name = Unbinilium
IUPAC Symbol = Ubn
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.