Class 9 Chemistry - Structure of the Atom | Willer Academy

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Class 9 Chemistry - NCERT Supplement

Chapter 4: Structure of the Atom

Comprehensive Notes and Practice Test

Chapter 4: Structure of the Atom - Detailed Notes

1. Subatomic Particles

Atoms consist of three fundamental particles:

Electrons - Negatively charged particles (discovered by J.J. Thomson)
Protons - Positively charged particles (discovered by E. Goldstein)
Neutrons - Neutral particles with no charge (discovered by James Chadwick)

Key Concept: Protons and neutrons are located in the nucleus, while electrons revolve around the nucleus in specific orbits.

2. Atomic Models

Thomson's Model

Plum pudding model - electrons embedded in positive sphere

Rutherford's Model

Planetary model - nucleus at center, electrons orbiting

Bohr's Model

Electrons in discrete energy levels

3. Atomic Number and Mass Number

Term	Symbol	Definition
Atomic Number	Z	Number of protons in an atom
Mass Number	A	Sum of protons and neutrons
Neutron Number	N	Number of neutrons (A - Z)

Definition: Isotopes are atoms of the same element with same atomic number but different mass numbers.

Example: Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H)

4. Electron Distribution

Electrons are arranged in shells around the nucleus:

Shell capacities (Bohr-Bury scheme):

K shell (n=1): 2 electrons
L shell (n=2): 8 electrons
M shell (n=3): 18 electrons
N shell (n=4): 32 electrons

Example: Sodium (Na) atomic number 11: K=2, L=8, M=1

5. Valency and Chemical Bonding

Valency is the combining capacity of an atom. It is determined by the number of electrons in the outermost shell.

Important: Atoms tend to achieve a stable electron configuration with 8 electrons in the outermost shell (octet rule).

Element	Atomic Number	Valency
Hydrogen	1	1
Oxygen	8	2
Sodium	11	1
Chlorine	17	1

Multiple Choice Questions (20 Questions)

Select the correct answer for each question. Each question carries 1 mark.

Test Progress: 0/20 questions answered

Short Answer Questions (5 Questions - 2 Marks Each)

Answer these questions in 30-50 words.

1. State the main postulates of Bohr's model of the atom.

2. What are isotopes? Give one example.

3. Define valency. What is the valency of magnesium (atomic number 12)?

4. Write the electronic configuration of chlorine (atomic number 17).

5. What is the difference between atomic number and mass number?

Subjective Questions (5 Questions - 5 Marks Each)

Answer these questions in detail (100-150 words). After submitting, model answers are shown for self-evaluation.

1. Describe Rutherford's alpha particle scattering experiment. What conclusions were drawn from it?

2. Explain the distribution of electrons in different shells with the help of Bohr-Bury scheme.

3. What are isotopes? Write any two applications of isotopes.

4. The atomic number of an element is 20. Write its electronic configuration and find its valency.

5. Differentiate between isotopes and isobars with suitable examples.

Performance Feedback & Ranking

MCQ Test

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Multiple Choice

Short Answers

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5 Questions

Subjective

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5 Questions

Total Score

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Overall

Overall Performance Summary

You have completed the Chapter 4: Structure of the Atom test. Here's your performance analysis:

Section	Questions	Score
MCQ Test	20	0/20
Short Answer	5	0/10
Subjective	5	0/25
Total	30	0/55

Strengths:

Excellent understanding of atomic models
Strong grasp of electron configuration
Accurate calculation of valency

Areas for Improvement:

Need more clarity on isobars vs isotopes
Review applications of isotopes in real life
Work on explaining Rutherford's experiment in detail

Recommendations:

Practice more questions on electronic configuration. Review the Bohr-Bury scheme for electron distribution. Focus on understanding the significance of atomic number and mass number in determining isotopes.

Your Performance Rank

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Focus on the improvement areas to achieve better results.