Classification of Elements and Periodicity in Properties Notes: Class 11 Chemistry Chapter 3

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.

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 ns^1-2. Reactive metals, low ionization energy.

  - p-Block (Groups 13 to 18): Outer configuration ns² np^1-6. Includes metals, non-metals, and metalloids.

  - d-Block (Groups 3 to 12): Transition elements, configuration (n-1)d^1-10 ns^1-2. Form colored ions and exhibit variable oxidation states.

  - f-Block (Lanthanoids & Actinoids): Inner transition elements, configuration (n-2)f^1-14 (n-1)d^0-1 ns².

Short Note:

• Atomic & Ionic Radius:

  - Across a Period: Decreases left to right due to increase in effective nuclear charge (Z_eff).

  - 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 (Δ_iH): 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 (Δ_egH): 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 Δ_egH 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).

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).