The Periodic Table

🔹 1. What is the Periodic Table?

The Periodic Table is a systematic arrangement of all known elements based on their atomic number (number of protons).

It shows patterns in chemical properties and electron arrangements and is a useful tool for predicting element behavior.

🔹 2. History of the Periodic Table

• Dmitri Mendeleev (1869): First organized the table based on atomic mass and predicted missing elements.
• Modern Periodic Table: Organized by atomic number, not atomic mass, which corrected the order and explained the patterns better.

🔹 3. Structure of the Periodic Table

🔹 4. Group Trends and Periodic Trends

A. Group Trends

• Group I (Alkali Metals):
  • 1 outer electron. Very reactive (especially with water). Reactivity increases down the group. Soft metals, low melting points, form +1 ions (e.g., Na⁺, K⁺)
  Example:
  • 2Na + 2H₂O → 2NaOH + H₂ (g)
• Group VII (Halogens):
  • 7 outer electrons. Non-metals form diatomic molecules (e.g., Cl₂, Br₂). Reactivity decreases down the group Form -1 ions (e.g., Cl⁻, Br⁻)
  Example of Displacement Reaction:
  • Cl₂ + 2KBr → 2KCl + Br₂
    (Chlorine displaces bromine from salt)
• Group 0 or 18 (Noble Gases):
  • Full outer shell
  • Very unreactive
  • Used in lighting (Ne), balloons (He), etc.

B. Period Trends

As you go left to right across a period:

• Atomic number increases
• Metallic → Non-metallic character
• Electronegativity increases
• Atomic size decreases

🔹 5. Transition Metals (Group 3 to 12)

• Hard, strong metals with high melting points
• Form coloured compounds
• Often used as catalysts
• Show variable oxidation states. Examples:
  • Fe²⁺ (green), Fe³⁺ (yellow/brown)
  • Cu²⁺ (blue)

🔹 6. Predicting Properties Using the Table

You can predict:

• Chemical reactivity: by group number
• Number of electrons: from atomic number
• Ion formation: Group 1 → +1, Group 17 → -1
• Physical properties: metals vs. non-metals

🔹 7. Metallic vs. Non-Metallic Elements

🔹 8. Examples for Practice

1. Which group does magnesium belong to?
   • Group 2 → 2 outer electrons
2. Predict the formula of the compound between calcium (Group 2) and chlorine (Group 17):
   • Ca²⁺ + 2Cl⁻ → CaCl₂
3. Why is argon unreactive?
   • It has a full outer shell of electrons.
4. Order these elements by increasing reactivity: Li, Na, K
   • Li < Na < K (down Group I, reactivity increases)

🔹 9. Summary Table of Key Groups

🧠 Tips for Exams

• Always link the electronic structure to the group and period.
• Know trends across a period and within a group.
• Be able to compare two elements based on their position.
• Practice displacement reactions and ionic compound formulas.

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