Introduction to Periodic Table
Periodic table is an arrangement of elements with similar properties placed together, evolved from experimental observations.
Earlier Attempts to Classify Elements
Dobereiner’s Triads (1829)
Groups of three elements with similar properties
Atomic weight of middle = average of other two
e.g., Ca, Sr, Ba; Cl, Br, I
Telluric Screw/Helix (1862)
Proposed by Chancourtois
Newlands' Law of Octaves (1864)
Every eighth element has similar properties
Like eighth note in musical octave
Mendeleef’s Periodic Law (1869)
Properties are periodic functions of atomic weights
7 periods, 8 groups (I-VIII), subgroups A/B
Group VIII: 9 elements in 3 sets
Zero group for inert gases
Modern Periodic Law
“The properties of elements are periodic functions of their atomic number.” - Moseley
Basis: Atomic number, not atomic weight.
Anomalies Resolved
Position of Hydrogen
Dual behavior: Like alkali (H⁺) and halogens (H⁻)
Dissimilar Elements
Period lengths determine subgroups
Rare Earths
Same valency electrons, similar behavior
Anomalous Pairs
Corrected by atomic number order
Isotopes
Same atomic number, same position
Group VIII
Space for 3 elements in long periods
Transuranic Elements
Actinide series outside table
Classification Based on Electronic Configuration
According to Bohr, elements grouped into four types.
Inert Gases
ns² np⁶ (He: 1s²)
Stable, chemically inert
He, Ne, Ar, Kr, Xe, Rn
Representative Elements
Incomplete outermost shell
ns¹ to ns² np⁵
Metals, non-metals, metalloids
Group = valence electrons
Transition Elements
(n-1)d¹⁻⁹ ns⁰⁻²
IVB to VIIB, VIII groups
4 series, 9 elements each
Inner Transition Elements
(n-2)f¹⁻¹⁴ (n-1)d⁰⁻¹ ns²
Lanthanides: Ce (58) to Lu (71)
Actinides: Th (90) to Lr (103)
14 elements each, radioactive
Notes: Zn, Cd, Hg not transition; Lu, Lr not inner transition due to full f-shell.
Important Points to Remember
Key JEE Points
- Modern Law: Properties periodic in atomic number
- Mendeleef: Atomic weight basis, anomalies resolved by modern
- Triads: Arithmetic mean atomic weights
- Octaves: Every 8th element similar
- Groups: I-VII (A/B), VIII (3 triads), 0 (inert)
- Classification: Inert (ns²np⁶), Representative (incomplete outer), Transition ((n-1)d incomplete), Inner ((n-2)f incomplete)
- Lanthanides: 4f filling, Actinides: 5f filling
- Hydrogen: Dual position (Group 1/17)