PERIODIC TABLE

Introduction: Unlocking the Periodic Table

The Periodic Table of Elements is arguably the most powerful and elegant tool in chemistry. It organizes all known chemical elements in the universe based on their atomic numbers, electron configurations, and recurring chemical properties.

From the early empirical attempts of Dobereiner’s triads and Newlands’ law of octaves to Dmitri Mendeleev’s visionary arrangement and Henry Moseley’s modern atomic number law, the periodic table transforms a vast, seemingly chaotic collection of elements into a predictable, logical system. Understanding it is essential for explaining chemical bonding, reaction dynamics, and the physical behavior of matter.

• Dobereiner’s Triads (1817): Atomic mass of the middle element is roughly the arithmetic mean of the other two (e.g., Lithium, Sodium, Potassium; Chlorine, Bromine, Iodine; Calcium, Strontium, Barium).
• Newlands’ Law of Octaves (1864): Every eighth element has properties similar to the first, analogous to musical notes. Limitation: Valid only up to Calcium (Z=20).
• Mendeleev’s Periodic Law (1869): Properties of elements are periodic functions of their atomic masses.
  • Predicted unknown elements: Eka-Boron (Scandium), Eka-Aluminium (Gallium), Eka-Silicon (Germanium).
• Modern Periodic Law (Henry Moseley, 1913): Properties of elements are periodic functions of their atomic numbers (Z).
  • The modern table contains 7 horizontal periods and 18 vertical groups (columns).

Master Table: Periodic Trends (Across a Period vs. Down a Group)

Property	Along a Period (Left to Right)	Down a Group (Top to Bottom)	Scientific Reason for WBPSC Mains
Atomic Radius / Size	Decreases	Increases	Left to right: Effective nuclear charge increases, pulling electrons closer.   Top to bottom: New electron shells are added.
Valence Electrons	Increases (from 1 to 8)	Remains Same	Number of valence electrons determines group number.
Valency	Increases from 1 to 4, then decreases to 0	Remains Same	Group 1 = 1, Group 14 = 4, Group 17 = 1, Group 18 = 0.
Ionization Energy / Potential	Increases	Decreases	Harder to remove electrons as nuclear attraction grows across a period; easier down a group due to increased shielding.
Electronegativity	Increases	Decreases	Tendency of an atom to attract shared electrons increases toward the top right.
Electron Affinity	Increases (becomes more negative)	Decreases	Energy released on gaining an electron increases across a period.
Metallic Character (Electropositive)	Decreases	Increases	Tendency to lose electrons drops across a period, rises down a group.
Non-Metallic Character	Increases	Decreases	Tendency to gain electrons rises toward the halogen group.
Nature of Oxides	Basic  Amphoteric  Acidic	More Basic	Sodium oxide is strongly basic, Aluminium oxide is amphoteric, Sulfur oxides are acidic.
Reducing Property	Decreases	Increases	Strongest reducing agents are alkali metals (Group 1).
Oxidizing Property	Increases	Decreases	Strongest oxidizing agents are halogens (Group 17).

High-Yield “Exception” Questions in WBPSC-CLERKSHIP

1. Highest Electronegativity vs. Highest Electron Affinity:
   • Highest Electronegativity: Fluorine (F).
   • Highest Electron Affinity: Chlorine (Cl) (Chlorine releases more energy than Fluorine because Fluorine’s tiny 2p subshell has high electron-electron repulsion).
2. Liquid Elements at Room Temperature (25°C):
   • Mercury (Hg}): Only liquid metal.
   • Bromine (Br}): Only liquid non-metal.
   • (Gallium and Cesium melt just above room temperature, around 28–30°C).
3. Smallest and Largest Atoms:
   • Smallest atom: Helium (by van der Waals radius) / Hydrogen (by covalent radius).
   • Largest atom (non-radioactive): Cesium (Cs).
4. Transition Elements (d-block) & Inner Transition (f-block):
   • d-block (Groups 3–12): Exhibit variable valency, form coloured ions, and act as industrial catalysts.
   • Lanthanides: 14 elements from Cerium (Z=58) to Lutetium (Z=71).
   • Actinides: 14 elements from Thorium (Z=90) to Lawrencium (Z=103); all are radioactive.
   • Elements beyond Uranium (Z=92) are synthetic and termed Transuranic elements.

PYQS

1. Who formulated the “Modern Periodic Law”, which states that the physical and chemical properties of elements are periodic functions of their atomic numbers? (RRB NTPC (Graduate 2025))

(a) Dmitri Mendeleev

(b) Henry Moseley

(c) John Newlands

(d) Johann Wolfgang Döbereiner

Correct Answer: (b) Henry Moseley

2. According to Newlands’ Law of Octaves, the properties of every eighth element were similar to those of the first. Up to which element was this law applicable? (RRB NTPC (UG 2025))

(a) Potassium

(b) Calcium

(c) Iron

(d) Magnesium

Correct Answer: (b) Calcium

3. How many elements were known and arranged in Dmitri Mendeleev’s original Periodic Table published in 1869? (RRB ALP 2025)

(a) 56

(b) 63

(c) 72

(d) 118

Correct Answer: (b) 63

4. Which of the following is the most electronegative element in the entire Modern Periodic Table? (RRB NTPC (UG)2025)

(a) Chlorine

(b) Fluorine

(c) Oxygen

(d) Caesium

Correct Answer: (b) Fluorine

5. In Mendeleev’s periodic table, the missing elements predicted as “Eka-Aluminium” and “Eka-Silicon” were later discovered and named respectively as: (RRB NTPC (Graduate 2024))

(a) Scandium and Gallium

(b) Gallium and Germanium

(c) Germanium and Scandium

(d) Technetium and Gallium

Correct Answer: (b) Gallium and Germanium