RICE

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). 
PERIODIC 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). 

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). 

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.

Smallest and Largest Atoms: 

  • Smallest atom: Helium (by van der Waals radius) / Hydrogen (by covalent radius). 
  • Largest atom (non-radioactive): Cesium (Cs). 

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. Which element of Group 17 has two isotopes of masses 35 and 37 amu with average abundance of 75.77% and 24.23%, respectively? SSC CGL 17/07/2023 (1st shift) 

(a) Chlorine  

(b) lodine 

(c) Fluorine  

(d) Astatine 

Ans: (a) Chlorine 

2. An image of which of the following melements is shown by the map of Cyprus from which the element got its name ? SSC MTS 13/09/2023 (2nd Shift)

(a) Silver  

(b) Cobalt 

(c) Copper 

 (d) Sulphur 

Ans: (c) Copper 

3. Which of the following statements is NOT correct regarding the alkali metals ?   SSC CPO 11/11/2022 (Afternoon) 

(a) All the alkali metals are silvery white, soft and light metals. 

(b) The ionization enthalpies of the alkali metals are considerably low and decrease down the group from. 

(c) Group 1 metals are known as alkali metals. 

(d) All the alkali metals have two valence electrons. 

Ans: (d) Alkali 

4. Which of the following metals is always a component in an amalgam? SSC MTS 15/06/2023 (Morning)

(a) Strontium 

 (b) Mercury 

(c) Zinc  

(d) Aluminium 

Ans: (b) Mercury. 

5. Which of the following statements is INCORRECT ? SSC MTS 15/07/2022 (Afternoon)

(a) Metals react with oxygen to form basic oxides. 

(b) Non-metals are malleable 

(c) Non-metals do not produce any sound 

(d) Metals have low ionisation enthalpy 

Ans: (b) Non-metals are malleable