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General Science: Periodic Classification of Elements
📅 Topic: General Science (Chemistry)
You are now tackling the "Map of Chemistry." The Periodic Table is the organizational system for everything we have studied so far. For competitive exams, questions here are rarely about memorizing the whole table. Instead, they focus on Trends (What happens to size/valency as you move left to right?) and Historical Evolution.
1. Early Attempts at Classification (The History)
Before the modern table, scientists tried various methods. You need to know the Name, the Method, and the Failure.
A. Döbereiner’s Triads (1817)
- Method: Grouped elements into Triads (groups of 3).
- Logic: Atomic mass of the middle element was roughly the average of the other two. (e.g., Li, Na, K).
- Failure: He could only identify 3 triads. It didn't work for all known elements.
B. Newlands’ Law of Octaves (1866)
- Logic: Every 8th element had properties similar to the 1st (like musical notes: Sa, Re, Ga, Ma...).
- Failure: Worked only up to Calcium. He assumed only 56 elements existed. He placed unlike elements together (Iron far from Cobalt/Nickel).
C. Mendeleev’s Periodic Table (The Breakthrough)
Method: Arranged by increasing Atomic Mass and similarity of Chemical Properties.
He left gaps for undiscovered elements and predicted their properties correctly:
- Eka-Boron → became Scandium.
- Eka-Aluminium → became Gallium.
- Eka-Silicon → became Germanium.
Limitations: Could not explain Isotopes, Hydrogen's position, or Atomic Mass anomalies (Cobalt before Nickel).
2. The Modern Periodic Table (1913)
Scientist: Henry Moseley.
The Shift: He proved that Atomic Number (Z) is more fundamental than Atomic Mass.
Structure: 18 Vertical Columns (Groups) and 7 Horizontal Rows (Periods).
3. Trends in the Modern Periodic Table
This is the most critical section for logical questions. You must understand how properties change as you move Left to Right (Period) and Top to Bottom (Group).
1. Atomic Size
- Period (L→R): Decreases (Nuclear pull increases, pulling shell closer).
- Group (T→B): Increases (New shells are added).
- Period (L→R): Increases 1 to 4, then decreases to 0.
- Group (T→B): Remains Same (Valence electrons are same).
- Period (L→R): Decreases (Harder to lose electrons).
- Group (T→B): Increases (Easier to lose electrons).
- Period (L→R): Increases (Nucleus pulls shared electrons stronger).
- Group (T→B): Decreases (Nucleus pull is weaker due to distance).
Zig-Zag Line: Separates Metals (Left) from Non-Metals (Right). Borderline elements are Metalloids (Boron, Silicon, Germanium, Arsenic).
4. Position of Hydrogen
It is still an anomaly.
Resembles Alkali Metals (Group 1): Has 1 valence electron, forms oxides/halides.
Resembles Halogens (Group 17): Diatomic molecule (H2), gains electron to form hydride.
5. Mentor’s Final Drill (Exam-Ready Questions)
A: Henry Moseley.
Q: Eka-Silicon was the name given by Mendeleev to which modern element?
A: Germanium. (Hint: Silicon Valley → Technology → German Engineering → Germanium).
Q: Which element has the largest atomic size in the second period?
A: Lithium (Size decreases left to right).
Q: Why does Atomic Size increase down a group?
A: Because new shells are being added.
Q: Identify the element with electronic configuration 2, 8, 2.
A: Magnesium (Atomic number 12).
Action Plan: Memorize the Mendeleev Predictions (Eka-Boron, Eka-Aluminium, Eka-Silicon) and the Trends Table. If you master the Trends, you can answer almost any logical question in this chapter. You have now completed the Chemistry module!