Chapter 5: Periodic Classification of Elements (Notes with questions and answers)

Class 10 Science - Chapter 5: Periodic Classification of Elements

Class 10 Science - Chapter 5: Periodic Classification of Elements

1. Introduction

The periodic table is a tabular arrangement of elements based on their atomic number, electronic configuration, and chemical properties.

The classification of elements helps in understanding the trends and periodic properties of elements.

2. Early Attempts at Classification

1. Dobereiner's Triads:

  • Grouped elements into triads (groups of three) with similar properties.
  • The atomic mass of the middle element was approximately the average of the other two.
  • Example: Lithium (Li), Sodium (Na), Potassium (K).

2. Newlands' Law of Octaves:

  • Arranged elements in increasing order of atomic mass.
  • Observed that every 8th element had properties similar to the first.
  • Limitations: Only worked for lighter elements.

3. Mendeleev's Periodic Table:

  • Arranged elements in increasing order of atomic mass and grouped them based on similar properties.
  • Left gaps for undiscovered elements and predicted their properties.
  • Example: Predicted the properties of Gallium (Ga) and Germanium (Ge).

3. Modern Periodic Table

Henry Moseley proposed that the atomic number (number of protons) should be the basis for classification.

The modern periodic table is based on the periodic law: "The properties of elements are a periodic function of their atomic numbers."

4. Features of the Modern Periodic Table

1. Periods:

  • Horizontal rows in the periodic table.
  • There are 7 periods in the modern periodic table.
  • Each period represents the number of electron shells in an atom.

2. Groups:

  • Vertical columns in the periodic table.
  • There are 18 groups in the modern periodic table.
  • Elements in the same group have the same number of valence electrons and similar chemical properties.

3. Blocks:

  • The periodic table is divided into 4 blocks based on the subshell being filled:
    • s-block: Groups 1 and 2.
    • p-block: Groups 13 to 18.
    • d-block: Groups 3 to 12 (transition metals).
    • f-block: Lanthanides and Actinides.

5. Trends in the Periodic Table

Property Trend Across a Period Trend Down a Group
Atomic Size Decreases Increases
Metallic Character Decreases Increases
Non-Metallic Character Increases Decreases
Electronegativity Increases Decreases
Ionization Energy Increases Decreases
Electron Affinity Increases Decreases

6. Position of Elements in the Periodic Table

1. s-Block Elements:

  • Groups 1 (Alkali metals) and 2 (Alkaline earth metals).
  • Highly reactive metals.

2. p-Block Elements:

  • Groups 13 to 18.
  • Includes metals, non-metals, and metalloids.

3. d-Block Elements:

  • Groups 3 to 12.
  • Transition metals with variable valency.

4. f-Block Elements:

  • Lanthanides and Actinides.
  • Inner transition metals.

7. Importance of Periodic Table

  • Helps in predicting the properties of elements.
  • Provides a systematic arrangement of elements.
  • Useful in understanding the chemical behavior of elements.

8. Previous Year Questions with Answers

Question 1: What is the basis of the modern periodic table?

Answer: The modern periodic table is based on the atomic number of elements.

Question 2: Who proposed the modern periodic law?

Answer: Henry Moseley proposed the modern periodic law.

Question 3: What are periods and groups in the periodic table?

Answer: Periods are horizontal rows (7 in total), and groups are vertical columns (18 in total).

Question 4: Why did Mendeleev leave gaps in his periodic table?

Answer: Mendeleev left gaps for undiscovered elements and predicted their properties.

Question 5: What is the trend of atomic size across a period?

Answer: Atomic size decreases across a period due to increased nuclear charge.

Question 6: Which element has the smallest atomic size in the periodic table?

Answer: Helium (He) has the smallest atomic size.

Question 7: What is the trend of metallic character down a group?

Answer: Metallic character increases down a group.

Question 8: Which block elements are known as transition metals?

Answer: d-block elements are known as transition metals.

Question 9: What is electronegativity?

Answer: Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond.

Question 10: Which element has the highest electronegativity?

Answer: Fluorine (F) has the highest electronegativity.

Question 11: What is ionization energy?

Answer: Ionization energy is the energy required to remove an electron from an atom.

Question 12: What is the trend of ionization energy across a period?

Answer: Ionization energy increases across a period.

Question 13: Which group elements are called alkali metals?

Answer: Group 1 elements are called alkali metals.

Question 14: What is the valency of group 18 elements?

Answer: The valency of group 18 elements is 0 (they are inert gases).

Question 15: What is the trend of electron affinity down a group?

Answer: Electron affinity decreases down a group.

Question 16: Which element is the most reactive metal?

Answer: Francium (Fr) is the most reactive metal.

Question 17: What is the trend of non-metallic character across a period?

Answer: Non-metallic character increases across a period.

Question 18: Which element is the most reactive non-metal?

Answer: Fluorine (F) is the most reactive non-metal.

Question 19: What is the valency of group 17 elements?

Answer: The valency of group 17 elements is 1.

Question 20: What is the trend of atomic size down a group?

Answer: Atomic size increases down a group.

Question 21: Which element has the highest ionization energy?

Answer: Helium (He) has the highest ionization energy.

Question 22: What is the trend of metallic character across a period?

Answer: Metallic character decreases across a period.

Question 23: Which block elements are known as inner transition metals?

Answer: f-block elements are known as inner transition metals.

Question 24: What is the trend of electron affinity across a period?

Answer: Electron affinity increases across a period.

Question 25: Which element is the first member of the lanthanide series?

Answer: Lanthanum (La) is the first member of the lanthanide series.

Question 26: What is the trend of electronegativity down a group?

Answer: Electronegativity decreases down a group.

Question 27: Which element is the first member of the actinide series?

Answer: Actinium (Ac) is the first member of the actinide series.

Question 28: What is the trend of non-metallic character down a group?

Answer: Non-metallic character decreases down a group.

Question 29: Which element has the lowest ionization energy?

Answer: Francium (Fr) has the lowest ionization energy.

Question 30: What is the trend of atomic size in the periodic table?

Answer: Atomic size decreases across a period and increases down a group.

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