Introduction & Historical Development

The periodic table is a tabular arrangement of chemical elements organized based on their atomic number, electron configuration, and recurring chemical properties. Elements are presented in order of increasing atomic number.

Historical Development

Dobereiner's Law of Triads (1817)

States that the atomic weight of the middle element is the arithmetic mean of the other two elements in a triad of elements with similar properties.

Newland's Law of Octaves (1864)

When elements are arranged in order of increasing atomic weights, the properties of every eighth element are similar to those of the first.

Lothar Meyer's Atomic Volume Curve (1869)

Plotted atomic volume against atomic weight and observed that elements with similar properties occupied similar positions on the curve.

Mendeleev's Periodic Table (1869)

Stated that physical and chemical properties of elements are periodic functions of their atomic weights. Organized elements into 7 periods and 8 groups.

Modern Periodic Law & Table Structure

Cause of Periodicity: Repetition of similar outer electronic configurations at regular intervals.

Blocks of Periodic Table

Electronic Configuration

Alkali Metals (Group 1)

General configuration: ns1

Example: Na (1s2 2s2 2p6 3s1)

Alkaline Earth Metals (Group 2)

General configuration: ns2

Example: Mg (1s2 2s2 2p6 3s2)

p-Block Elements (Groups 13-18)

• Group 13 (Boron family): ns2np1
• Group 14 (Carbon family): ns2np2
• Group 15 (Nitrogen family): ns2np3
• Group 16 (Oxygen family): ns2np4
• Group 17 (Halogen family): ns2np5
• Group 18 (Noble gases): ns2np6 (except He: 1s2)

d-Block Elements (Transition Metals)

Four series: 3d (Sc-Zn), 4d (Y-Cd), 5d (La-Hg), 6d (Ac-Cn)

Example: Fe: [Ar] 3d6 4s2

f-Block Elements (Inner Transition)

Lanthanoids: Atomic numbers 58-71, 4f series

Actinoids: Atomic numbers 90-103, 5f series

Periodic Properties

Atomic Radius

• Covalent Radius: Half the distance between nuclei of two covalently bonded atoms
• Metallic Radius: Half the internuclear distance between adjacent metal ions in metallic lattice
• Van der Waals Radius: Half the distance between nuclei of two non-bonded isolated atoms

Order: Van der Waals radius > Metallic radius > Covalent radius

Ionization Energy (IE)

IE1 < IE2 < IE3 ... (Successive ionization energies)

Electron Affinity (EA)

Halogens have highest electron affinity. Order: Cl > F > Br > I

Electronegativity (EN)

Pauling scale: F = 4.0 (highest), Cs = 0.7 (lowest among natural elements)

Special Concepts & Relationships

Diagonal Relationship

Elements of 2nd period show similarity with diagonally placed elements of 3rd period due to similar ionic potential (φ).

Lanthanoid Contraction

Poor shielding by 4f electrons causes atomic radii of post-lanthanoid elements to be smaller than expected.

Inert Pair Effect

Tendency of s-electrons to remain inert in heavier elements of p-block, leading to stability of lower oxidation states.

Example: Pb2+ more stable than Pb4+

Transuranic Elements

Elements with atomic number > 92 (Uranium). All are radioactive and artificial (man-made).

Bridge Elements

Modern periodic table: 2nd period elements (Li, Be, B)

Mendeleev's table: 3rd period elements (Na, Mg, Al)

Chemical Behavior & Trends

Acidic/Basic Nature

Across a period: Basic → Amphoteric → Acidic

Down a group: Basic character increases

Oxides

• Basic oxides: Metal oxides (Na2O, MgO)
• Acidic oxides: Non-metal oxides (CO2, SO3)
• Amphoteric oxides: React with both acid and base (Al2O3, ZnO)
• Neutral oxides: Neither acidic nor basic (CO, NO, H2O)

Hydrides

Acidic character: HF < HCl < HBr < HI

Important Formulas & Relationships

Effective Nuclear Charge (Zeff)

Zeff = Z - σ (where σ = shielding constant)

Slater's Rules for σ Calculation

• For ns/np electrons: σ = 0.35 (same shell) + 0.85 (n-1 shell) + 1.0 (n-2 or lower)
• For nd/nf electrons: σ = 0.35
• For 2-electron species: σ = 0.3

Bond Length Formula

dA-B = rA + rB - 0.09|ΔEN| (Schoemaker & Stevenson)

Ionic Character

% Ionic character = 16|ΔEN| + 3.5|ΔEN|2 (Henry-Smith equation)

Classification & Identification

Group Identification from Atomic Number

• For atomic number > 100: Last two digits give group number (e.g., Z=115 → Group 15)
• Lanthanoids: Group 3, Period 6, f-block
• Actinoids: Group 3, Period 7, f-block

From Electronic Configuration

• s-block: Group = number of outermost s electrons
• p-block: Group = 12 + number of outermost p electrons
• d-block: Group = (outermost s electrons) + (penultimate d electrons)
• f-block: Always Group 3

Bohr's Classification

1. Inert gases: Outermost orbit complete (ns2np6)
2. Normal/Representative elements: Last orbit incomplete (s & p-block except inert gases)
3. Transition elements: Last two orbits incomplete (d-block except group 12)
4. Inner transition elements: Last three orbits incomplete (f-block)

Key Facts & Summary

• Total natural elements: 90 (atomic numbers 1-90)
• First man-made element: Technetium (Tc, Z=43)
• First man-made lanthanoid: Promethium (Pm, Z=61)
• Liquid elements at room temperature: Hg, Br
• Highest electronegativity: Fluorine (F) = 4.0
• Lowest ionization energy: Cesium (Cs)
• Highest ionization energy: Helium (He)
• Magic numbers: 2, 8, 8, 18, 18, 32
• Metalloids: B, Si, Ge, As, Sb, Te, Po, At