Olive Defence
Chemistry  ·  CDS

CC03 — Classification of Elements & Periodicity

📖 CC03  ·  CDS General Science — Chemistry ★ High Priority

The periodic table organises all 118 known elements by atomic number, revealing patterns in their properties. CDS tests periodic trends, important groups, and comparison of Mendeleev's vs Modern periodic law directly and frequently.

📌 CDS Focus: Modern periodic law (properties are a periodic function of atomic number — not atomic mass as Mendeleev thought); blocks of the periodic table (s,p,d,f); periodic trends (atomic radius increases down and decreases across; ionisation energy and electronegativity decrease down and increase across); important group properties (alkali metals, halogens, noble gases).

1. History of Periodic Classification

Fig. 1 — Mendeleev vs Modern Periodic Law: What Changed?
MENDELEEV vs MODERN PERIODIC LAW MENDELEEV (1869) Arranged by Atomic Mass Law: Properties are a periodic function of atomic mass. ✓ Achievements: Left gaps for undiscovered elements Predicted Ga, Ge, Sc correctly ✗ Could not explain anomalies (Co/Ni, Ar/K) MODERN (Moseley, 1913) Arranged by Atomic Number Law: Properties are a periodic function of atomic number (Z). ✓ Solved all anomalies 18 groups, 7 periods 118 elements placed correctly Noble gases placed as Group 18

2. Structure of the Modern Periodic Table

Fig. 2 — Blocks of the Periodic Table (s, p, d, f) with Group and Period Details
BLOCKS OF THE MODERN PERIODIC TABLE s-block Groups 1 & 2 Group 1: Alkali metals Li, Na, K, Rb, Cs, Fr Most reactive metals Group 2: Alkaline earth Be, Mg, Ca, Sr, Ba, Ra 2 valence electrons Valence e⁻: ns¹ or ns² All metals (except H) Highly reactive 2 elements wide 7 periods each p-block Groups 13–18 Group 17: Halogens F, Cl, Br, I, At Most reactive non-metals Group 18: Noble gases He, Ne, Ar, Kr, Xe, Rn Completely filled shells Has metals, metalloids AND non-metals Valence: np¹ to np⁶ 6 elements wide Periods 2–7 d-block Groups 3–12 Transition metals Fe, Co, Ni, Cu, Zn Cr, Mn, Ti, V, Sc Key properties: Variable oxidation states Coloured compounds Good catalysts High melting points 10 elements wide Periods 4–7 f-block Lanthanides & Actinides Lanthanides (Z=58–71) La to Lu; Period 6 Rare earth elements Actinides (Z=90–103) Th to Lr; Period 7 Mostly radioactive Inner transition metals Placed below main table to save space 14 elements wide 2 rows below table

3. Periodic Trends

Fig. 3 — Four Key Periodic Trends: Direction of Increase Across Period and Down Group
PERIODIC TRENDS — Direction of Increase Across Period and Down Group Property Across Period → Down Group ↓ Reason Atomic Radius (size of atom) DECREASES → More protons pull electrons inward INCREASES ↓ New electron shells are added Nuclear charge vs shielding Ionisation Enthalpy INCREASES → Smaller atom; electrons more tightly held DECREASES ↓ Larger atom; outermost electrons farther away Energy to remove outermost electron Electro- negativity INCREASES → More nuclear attraction for bonding electrons DECREASES ↓ Increased shielding effect by inner shells Attraction for bonding electrons Metallic Character DECREASES → Right side = non-metals (halogens, noble gases) INCREASES ↓ Electrons lost more easily as size grows Ease of losing electrons. F = most electronegative
💡 Most Electronegative & Most Electropositive: Fluorine (F) is the most electronegative element in the periodic table (top-right of p-block). Caesium (Cs) is the most electropositive/most reactive metal (bottom-left of s-block). Noble gases (Group 18) are the least reactive. These are direct CDS recall questions.

📝 CDS PYQs — Periodic Table

Q1. The Modern Periodic Law states that properties of elements are a periodic function of their: CDS PYQ
(a) Atomic mass(b) Atomic number(c) Mass number(d) Valence electrons
✔ Answer: (b) Atomic number
The Modern Periodic Law (Moseley, 1913) states that the physical and chemical properties of elements are periodic functions of their atomic number (Z). Mendeleev used atomic mass, which led to anomalies (e.g., Ar and K). Moseley's X-ray experiments showed atomic number is the more fundamental quantity. This is a top-tested CDS fact.
Q2. Which group in the periodic table contains the halogens? CDS PYQ
(a) Group 1(b) Group 2(c) Group 16(d) Group 17
✔ Answer: (d) Group 17
Halogens (F, Cl, Br, I, At) form Group 17 in the modern periodic table. They are highly reactive non-metals with 7 valence electrons, and they readily gain one electron to form −1 ions. Group 1 = Alkali metals; Group 2 = Alkaline earth metals; Group 18 = Noble gases; Group 16 = Chalcogens (O, S, Se, Te).
Q3. As we move across a period from left to right, atomic radius: CDS PYQ
(a) Increases(b) Decreases(c) First increases then decreases(d) Remains constant
✔ Answer: (b) Decreases
Moving left to right across a period, the number of protons increases while electrons are added to the same shell. The increased nuclear charge pulls electrons closer, so atomic radius decreases. Conversely, going down a group, new shells are added, so atomic radius increases. The largest atoms are in the bottom-left; smallest in the top-right (excluding noble gases).
Q4. Which element has the highest electronegativity? CDS PYQ
(a) Oxygen(b) Chlorine(c) Fluorine(d) Nitrogen
✔ Answer: (c) Fluorine
Fluorine (F) has the highest electronegativity of all elements (Pauling scale value = 3.98 ≈ 4.0). It is the most reactive non-metal and most powerful oxidising agent. Electronegativity increases across a period (left to right) and decreases down a group. Fluorine is at the top-right of the p-block, so it has maximum nuclear attraction with minimum shielding.

🧠 Quick Memory Chart — CC03

📈 Periodic Trends
  • Atomic radius: ↓ across; ↑ down
  • IE: ↑ across; ↓ down
  • Electronegativity: ↑ across; ↓ down
  • Metallic: ↓ across; ↑ down
  • F = most electronegative
🏠 Key Groups
  • Group 1: Alkali metals (Li to Fr)
  • Group 2: Alkaline earth metals
  • Group 17: Halogens (F to At)
  • Group 18: Noble gases (inert)
  • Groups 3–12: Transition metals
⚖ Key Facts
  • Modern law: atomic number (not mass)
  • 7 periods, 18 groups
  • s,p,d,f blocks explained by configuration
  • Period 3: Na to Ar
  • Noble gases: completely filled shells

📝 Practice Exercise

E1. Noble gases are present in:
(a) Group 17(b) Group 1(c) Group 18(d) Group 16
E2. Down a group in the periodic table, metallic character:
(a) Decreases(b) Increases(c) Stays same(d) Fluctuates
E3. Mendeleev arranged elements by:
(a) Atomic number(b) Atomic mass(c) Valence electrons(d) Melting point
Answers: E1 → (c) Group 18  |  E2 → (b) Increases (easier to lose electrons as size increases)  |  E3 → (b) Atomic mass
📝 Mock Tests 🎯 Subject Quizzes ✈️ Telegram
This material is for personal CDS exam preparation only.
Unauthorised reproduction or distribution is prohibited.
All rights reserved. · ODEA.Classes@gmail.com · OliveDefence.com