Periodic Table: Groups, Periods, Trends & Classification

TAKEAWAYS:

• The periodic table arranges elements by atomic number and shows repeating patterns in their properties.
• Groups (vertical columns) have similar chemical behaviors; periods (horizontal rows) show increasing atomic mass.
• Trends like atomic radius, ionization energy, and electronegativity change predictably across groups and periods.
• Mendeleev’s table was the first to organize elements by atomic mass, but the modern table uses atomic number.
• Metals, nonmetals, and metalloids are classified based on their properties and positions in the table.
• Lanthanides and actinides are special series placed below the main table for simplicity.

1. Early Attempts to Classify Elements

Before the modern periodic table, scientists tried to organize elements based on their properties.

Döbereiner’s Triads (1829)

• Idea: Groups of three elements with similar properties, where the middle element’s atomic mass was the average of the other two.
• Example:
  • Li (6.9), Na (23), K (39) Atomic mass of Na ≈ (6.9 + 39)/2 = 22.95 (close to 23).
  • Ca (40), Sr (87.6), Ba (137.3) Atomic mass of Sr ≈ (40 + 137.3)/2 = 88.65 (close to 87.6).

Limitations:

• Only worked for a few elements.
• Could not explain all known elements.

Newlands’ Law of Octaves (1864)

• Idea: When elements are arranged in increasing atomic mass, their properties repeat every 8th element (like musical notes).
• Example:
  • Li, Be, B, C, N, O, F, Ne → Similar to Na, Mg, Al, Si, P, S, Cl, Ar.
• Limitations:
  • Only worked for the first 16 elements.
  • Failed for heavier elements (e.g., Co and Ni did not fit).

2. Mendeleev’s Periodic Table (1869)

Dmitri Mendeleev arranged elements in increasing atomic mass but left gaps for undiscovered elements.

Key Features:

• Horizontal rows (Periods): Elements with increasing atomic mass.
• Vertical columns (Groups): Elements with similar properties.
• Predicted missing elements:
  • Eka-boron (Scandium, Sc), Eka-aluminum (Gallium, Ga), Eka-silicon (Germanium, Ge) were later discovered with properties matching Mendeleev’s predictions.

Advantages of Mendeleev’s Table:

✅ Explained similarities in groups. ✅ Predicted new elements. ✅ Corrected atomic masses of some elements (e.g., Te and I).

Limitations:

❌ Did not explain why properties repeated. ❌ Could not place isotopes (elements with same atomic number but different mass).

3. Modern Periodic Table (Based on Atomic Number)

Henry Moseley (1913) discovered that elements should be arranged by atomic number (proton number), not atomic mass.

Key Features:

• Periods (7 rows): Show increasing atomic number and electron shells.
• Groups (18 columns): Elements in the same group have the same number of valence electrons and similar chemical properties.
• Blocks (s, p, d, f):
  • s-block: Groups 1-2 (Alkali and Alkaline Earth Metals).
  • p-block: Groups 13-18 (Nonmetals, Halogens, Noble Gases).
  • d-block: Transition Metals (Groups 3-12).
  • f-block: Lanthanides and Actinides (placed below).

flowchart TD
    A["Periodic Table"] --> B["Periods (7 Rows)"]
    A --> C["Groups (18 Columns)"]
    A --> D["Blocks: s, p, d, f"]
    B --> E["Increasing Atomic Number"]
    C --> F["Same Valence Electrons"]
    D --> G["s: Groups 1-2"]
    D --> H["p: Groups 13-18"]
    D --> I["d: Transition Metals"]
    D --> J["f: Lanthanides & Actinides"]

Classification of Elements:

4. Trends in the Periodic Table

Properties change predictably across periods and groups.

A. Atomic Radius

• Across a Period (Left → Right): Decreases (nuclear charge increases, pulling electrons closer).
• Down a Group (Top → Bottom): Increases (more electron shells).

B. Ionization Energy (Energy to remove an electron)

• Across a Period: Increases (nucleus holds electrons tighter).
• Down a Group: Decreases (outer electrons are farther, easier to remove).

C. Electronegativity (Ability to attract electrons)

• Across a Period: Increases (nuclear charge pulls bonding electrons more).
• Down a Group: Decreases (outer electrons are shielded).

D. Electron Affinity (Energy released when gaining an electron)

• General Trend: Increases across a period, decreases down a group (except noble gases, which have zero electron affinity).

5. Special Groups in the Periodic Table

A. Alkali Metals (Group 1)

• Properties:
  • Highly reactive (react with water to form alkaline solutions).
  • Soft, low density (can be cut with a knife).
  • Good conductors of heat and electricity.
• Reaction with Water:
  • 2Na + 2H₂O → 2NaOH + H₂↑ (explosive reaction).

B. Halogens (Group 17)

• Properties:
  • Highly reactive nonmetals.
  • Form salts with metals (e.g., NaCl, KCl).
  • Exist as diatomic molecules (F₂, Cl₂, Br₂, I₂).
• Reactivity Trend: Decreases down the group (F₂ is the most reactive).

C. Noble Gases (Group 18)

• Properties:
  • Unreactive (full valence shell).
  • Colorless, odorless gases.
  • Used in lighting and balloons (e.g., He in balloons, Ne in signs).

6. Lanthanides and Actinides

• Lanthanides (57-71): Rare Earth Metals (used in magnets, catalysts).
• Actinides (89-103): Radioactive (Uranium, Plutonium used in nuclear reactions).

flowchart TD
    A["Periodic Table"] --> B["Main Body"]
    A --> C["Lanthanides (57-71)"]
    A --> D["Actinides (89-103)"]
    C --> E["Used in electronics, magnets"]
    D --> F["Radioactive, nuclear fuel"]

Exam Tip: How to Score Full Marks in NEB Exams

1. Memorize Group Names:

   • Group 1: Alkali Metals
   • Group 2: Alkaline Earth Metals
   • Group 17: Halogens
   • Group 18: Noble Gases

2. Understand Trends:

   • Atomic radius → ↓ (Group), ← (Period)
   • Ionization Energy → ↑ (Period), ↓ (Group)
   • Electronegativity → ↑ (Period), ↓ (Group)

3. Compare Mendeleev vs. Modern Table:

   • Mendeleev: Atomic Mass
   • Modern: Atomic Number

4. Practical Questions:

   • "Why is Fluorine more reactive than Iodine?" → Smaller atomic size, higher electronegativity.
   • "Predict the formula of Sodium Oxide." → Na₂O (Group 1 + Group 16 → 2:1 ratio).

5. Diagrams Help!

   • Draw trend graphs (atomic radius, ionization energy).
   • Label groups and periods correctly.

NEB Board-Style Questions (Practice)

Short Answer (5 marks)

1. Explain Mendeleev’s periodic law and how it differs from the modern periodic law.
2. Why does atomic radius decrease across a period?
3. Write the electronic configuration of:
   • Sodium (Na)
   • Chlorine (Cl)
4. Why are noble gases unreactive?
5. Compare the reactivity of Fluorine and Iodine.

Long Answer (10 marks)

6. Describe the trends in ionization energy across a period and down a group. Explain with examples.
7. How did Mendeleev’s periodic table help in predicting new elements? Give two examples.
8. Draw a diagram of the periodic table and label:
   • Alkali Metals
   • Halogens
   • Noble Gases
   • Transition Metals

Final Advice:

• Revise trends daily (they are frequently asked).
• Practice drawing the periodic table (groups, periods, blocks).
• Relate theory to real life (e.g., why sodium reacts violently with water).

Good luck! 🚀