Periodic Classification

Periodic Classification of Elements – Class 9

Introduction

The classification of elements is essential to understand their properties and chemical behavior. The Modern Periodic Table, based on atomic number, systematically arranges elements in periods and groups, revealing periodic trends.

Early Attempts of Classification

1. Döbereiner’s Triads

  • Grouped elements into sets of three with similar properties.
  • The atomic mass of the middle element was the average of the other two.
  • Example: Li (7), Na (23), K (39).
  • Limitation: Only applicable to a few elements.

2. Newlands’ Law of Octaves

  • Arranged elements in increasing atomic mass.
  • Every 8th element had similar properties to the 1st (like a musical octave).
  • Limitation: Failed after Calcium (Ca).

3. Mendeleev’s Periodic Table

  • Arranged elements by increasing atomic mass.
  • Left gaps for undiscovered elements.
  • Predicted properties of Gallium (Ga) and Germanium (Ge).
  • Limitation: Did not explain isotopes and hydrogen’s position.

Modern Periodic Table

  • Developed by Henry Moseley, based on atomic number (Z).
  • Modern Periodic Law: “The properties of elements are periodic functions of their atomic number.”
  • 7 Periods (Horizontal Rows): Indicate number of electron shells.
  • 18 Groups (Vertical Columns): Elements have similar properties.

Special Groups

  • Group 1 (Alkali Metals) → Li, Na, K → Highly reactive.
  • Group 17 (Halogens) → F, Cl, Br → Highly reactive nonmetals.
  • Group 18 (Noble Gases) → He, Ne, Ar → Inert gases.

Periodic Trends in the Modern Periodic Table

1. Atomic Size (Atomic Radius)

🔻 Decreases across a period (due to increasing nuclear charge).
🔺 Increases down a group (due to more electron shells).

2. Metallic and Non-metallic Character

🔹 Metals (Left side): Lose electrons (Form +ve ions).
🔹 Non-metals (Right side): Gain electrons (Form -ve ions).

3. Valency

  • Determined by the number of valence electrons.
  • Example: Na (1 valence electron) → Valency = 1; O (6 valence electrons) → Valency = 2.

4. Reactivity

🔺 Metals → Increases down a group (easier to lose electrons).
🔻 Non-metals → Decreases down a group (harder to gain electrons).

Practice Questions

1. Multiple Choice Questions (MCQs)

  1. Which scientist proposed the Modern Periodic Law?
    a) Mendeleev
    b) Newlands
    c) Moseley ✅
    d) Rutherford
  2. Which of the following elements belongs to Group 1?
    a) Magnesium
    b) Oxygen
    c) Sodium ✅
    d) Neon
  3. Noble gases are placed in Group 18 because:
    a) They are highly reactive.
    b) They are found in large amounts.
    c) They have a full outer shell of electrons. ✅
    d) They easily form compounds.
  4. As we move across a period, atomic size:
    a) Increases
    b) Decreases ✅
    c) Remains the same
    d) First increases, then decreases
  5. Which element has the smallest atomic radius?
    a) Na
    b) Cl
    c) He ✅
    d) K

2. Numerical Problems

Q1: Atomic Radius Calculation

The atomic radius of Sodium (Na) is 186 pm, while that of Chlorine (Cl) is 99 pm.
Which element has a smaller atomic radius and why?

💡 Hint: Atomic size decreases across a period due to increasing nuclear charge.

Q2: Valency Calculation

Determine the valency of the following elements:

  1. Sulfur (Z = 16)
  2. Magnesium (Z = 12)
  3. Fluorine (Z = 9)

💡 Hint: Valency = 8 – (Valence Electrons) for nonmetals, and Valence Electrons for metals.

3. Case Study-Based Questions

Case Study 1: Predicting Properties of an Unknown Element

A newly discovered element X is found in Period 3, Group 2.
Based on the periodic table, answer the following questions:

  1. What is the likely valency of element X?
  2. Will X be a metal or a non-metal?
  3. Predict the reaction of X with water.

💡 Hint: Elements in Group 2 (Alkaline Earth Metals) react with water to form hydroxides and hydrogen gas.

Case Study 2: Comparing Atomic Size and Reactivity

Two students argue about the reactivity of Sodium (Na) and Potassium (K).

  • Student A: “Sodium is more reactive because it has a smaller atomic radius.”
  • Student B: “Potassium is more reactive because it is lower in the group.”

📝 Question: Who is correct and why?
💡 Hint: Reactivity increases down Group 1 because electrons are lost more easily.

Conclusion

The Periodic Table is a powerful tool for understanding the properties and trends of elements. The Modern Periodic Table, arranged by atomic number, helps scientists predict the behavior of elements and their chemical reactions.

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