Class 9 Science Important Questions and Answers: Structure of Atom
Premium NCERT, Board Exam, Exemplar and Olympiad-level question answers for Class 9 Science Structure of Atom. Learn subatomic particles, atomic models, Rutherford experiment, Bohr model, electronic configuration, valency, isotopes, isobars and important numericals.
Short Introduction
The chapter Structure of Atom explains the internal structure of an atom. Earlier, atoms were considered indivisible, but later experiments proved that atoms are made up of smaller particles called electrons, protons and neutrons.
This chapter helps students understand atomic models, arrangement of electrons in shells, atomic number, mass number, valency, isotopes and isobars.
Scoring area: Rutherford’s experiment, Bohr’s model, electronic configuration, valency, isotopes, isobars and numerical questions based on atomic number and mass number are very important for CBSE exams.
Chapter Overview
1. Charged Particles
Atoms contain electrons, protons and neutrons.
2. Thomson’s Model
Atom was compared with a sphere of positive charge having electrons embedded in it.
3. Rutherford’s Model
Atom has a small, dense and positively charged nucleus at the centre.
4. Bohr’s Model
Electrons revolve in fixed energy shells around the nucleus.
5. Atomic Number and Mass Number
Atomic number is number of protons. Mass number is sum of protons and neutrons.
6. Isotopes and Isobars
Isotopes have same atomic number. Isobars have same mass number.
Important Keywords
Important Rules and Formulas
Atomic Number
Z = Number of protons
Mass Number
A = Protons + Neutrons
Number of Neutrons
n = A – Z
Neutral Atom
Protons = Electrons
Maximum Electrons in Shell
2n2
Shell Capacity
K = 2, L = 8, M = 18, N = 32
Valency Rule
If valence electrons ≤ 4, valency = valence electrons
Valency Rule
If valence electrons > 4, valency = 8 – valence electrons
Symbols: Z = atomic number, A = mass number, n = shell number or number of neutrons depending on context.
Important Very Short Answer Questions
Q1. Name the three subatomic particles of an atom.
Answer: The three subatomic particles are electron, proton and neutron.
Q2. Which subatomic particle has a negative charge?
Answer: Electron has a negative charge.
Q3. Which subatomic particle has a positive charge?
Answer: Proton has a positive charge.
Q4. Which subatomic particle has no charge?
Answer: Neutron has no charge. It is neutral.
Q5. Who discovered the electron?
Answer: J. J. Thomson discovered the electron.
Q6. Who discovered the nucleus of an atom?
Answer: Ernest Rutherford discovered the nucleus of an atom.
Q7. What is atomic number?
Answer: Atomic number is the number of protons present in the nucleus of an atom.
Q8. What is mass number?
Answer: Mass number is the sum of number of protons and neutrons present in the nucleus.
Q9. What is the charge on the nucleus?
Answer: The nucleus is positively charged because it contains protons.
Q10. What are valence electrons?
Answer: Electrons present in the outermost shell of an atom are called valence electrons.
Q11. What is valency?
Answer: Valency is the combining capacity of an atom.
Q12. What are isotopes?
Answer: Isotopes are atoms of the same element having the same atomic number but different mass numbers.
Q13. What are isobars?
Answer: Isobars are atoms of different elements having the same mass number but different atomic numbers.
Q14. What is the maximum number of electrons in the K shell?
Answer: The K shell can hold a maximum of 2 electrons.
Short Answer Questions
Q1. Write the properties of electrons, protons and neutrons.
Answer:
| Particle | Charge | Location | Mass |
|---|---|---|---|
| Electron | Negative | Outside nucleus | Very small |
| Proton | Positive | Inside nucleus | About 1 atomic mass unit |
| Neutron | No charge | Inside nucleus | About 1 atomic mass unit |
Q2. Why is an atom electrically neutral?
Answer: An atom is electrically neutral because the number of positively charged protons is equal to the number of negatively charged electrons.
Q3. Write two important features of Thomson’s model of atom.
Answer:
- According to Thomson, an atom is a sphere of positive charge.
- Electrons are embedded in this positively charged sphere.
- The positive and negative charges balance each other, making the atom neutral.
Q4. What were the main conclusions of Rutherford’s gold foil experiment?
Answer:
- Most alpha particles passed straight through the gold foil, showing that most of the atom is empty space.
- Some alpha particles were deflected, showing that positive charge is concentrated at the centre.
- Very few alpha particles bounced back, showing that the nucleus is very small, dense and positively charged.
Q5. Write two drawbacks of Rutherford’s atomic model.
Answer:
- It could not explain the stability of an atom.
- According to classical theory, revolving electrons should lose energy and fall into the nucleus.
- It could not explain the arrangement of electrons in different energy levels.
Q6. Write the main postulates of Bohr’s model of atom.
Answer:
- Electrons revolve around the nucleus in fixed circular paths called shells or orbits.
- Each shell has a fixed energy.
- Electrons do not lose energy while revolving in these fixed shells.
- Shells are represented as K, L, M, N or 1, 2, 3, 4.
Q7. Differentiate between atomic number and mass number.
Answer:
| Atomic Number | Mass Number |
|---|---|
| It is the number of protons in an atom. | It is the sum of protons and neutrons. |
| It is represented by Z. | It is represented by A. |
| For a neutral atom, atomic number is also equal to number of electrons. | Mass number gives approximate atomic mass. |
Q8. Differentiate between isotopes and isobars.
Answer:
| Isotopes | Isobars |
|---|---|
| Atoms of the same element. | Atoms of different elements. |
| They have the same atomic number. | They have different atomic numbers. |
| They have different mass numbers. | They have the same mass number. |
| Example: Protium, deuterium and tritium. | Example: Calcium-40 and Argon-40. |
Q9. Why do isotopes have similar chemical properties?
Answer: Isotopes have similar chemical properties because they have the same atomic number and the same electronic configuration.
Q10. Why do isotopes have different physical properties?
Answer: Isotopes have different physical properties because they have different mass numbers due to different number of neutrons.
Long Answer Questions
Q1. Explain Rutherford’s gold foil experiment with observations and conclusions.
Answer:
Experiment: Rutherford bombarded a very thin gold foil with fast-moving alpha particles.
Observations:
- Most alpha particles passed straight through the gold foil without any deflection.
- Some alpha particles were deflected through small angles.
- Very few alpha particles bounced back.
Conclusions:
- Most of the atom is empty space.
- The positive charge of the atom is concentrated in a very small region called the nucleus.
- The nucleus is small, dense and positively charged.
- Electrons revolve around the nucleus.
Q2. Explain Bohr’s atomic model and how it improved Rutherford’s model.
Answer:
- Bohr proposed that electrons revolve around the nucleus only in certain fixed orbits called shells.
- These shells have fixed energy and are represented as K, L, M, N or 1, 2, 3, 4.
- Electrons do not radiate energy while moving in these fixed orbits.
- This explained why electrons do not fall into the nucleus.
- Thus, Bohr’s model explained the stability of an atom, which Rutherford’s model could not explain.
Q3. Explain the rules for writing electronic configuration.
Answer:
- The maximum number of electrons in a shell is given by the formula 2n2.
- The first shell K can hold a maximum of 2 electrons.
- The second shell L can hold a maximum of 8 electrons.
- The third shell M can hold a maximum of 18 electrons.
- The outermost shell cannot have more than 8 electrons.
- Electrons are filled step by step from lower energy shells to higher energy shells.
Q4. Explain valency with examples.
Answer: Valency is the combining capacity of an atom.
- If the number of valence electrons is 1, 2, 3 or 4, valency is equal to the number of valence electrons.
- If the number of valence electrons is more than 4, valency = 8 – number of valence electrons.
- Sodium has electronic configuration 2, 8, 1. Its valency is 1.
- Magnesium has electronic configuration 2, 8, 2. Its valency is 2.
- Chlorine has electronic configuration 2, 8, 7. Its valency is 8 – 7 = 1.
- Neon has electronic configuration 2, 8. Its valency is 0 because its outermost shell is complete.
Q5. Write the uses of isotopes.
Answer: Important uses of isotopes are:
- Uranium isotope is used as fuel in nuclear reactors.
- Cobalt isotope is used in the treatment of cancer.
- Iodine isotope is used in the treatment of goitre.
- Radioactive isotopes are used in medical diagnosis and scientific research.
Important Numericals with Solutions
Numerical 1: An atom has atomic number 11 and mass number 23. Find the number of protons, electrons and neutrons.
Solution:
Atomic number Z = 11
Mass number A = 23
Number of protons = Z = 11
Number of electrons = 11 because the atom is neutral
Number of neutrons = A – Z = 23 – 11 = 12
Answer: Protons = 11, Electrons = 11, Neutrons = 12
Numerical 2: Calculate the number of neutrons in chlorine atom having atomic number 17 and mass number 35.
Solution:
Z = 17, A = 35
Number of neutrons = A – Z
Number of neutrons = 35 – 17 = 18
Numerical 3: Write the electronic configuration and valency of oxygen. Atomic number of oxygen is 8.
Solution:
Atomic number = 8
Number of electrons = 8
Electronic configuration = 2, 6
Valence electrons = 6
Valency = 8 – 6 = 2
Numerical 4: Write the electronic configuration and valency of magnesium. Atomic number of magnesium is 12.
Solution:
Atomic number = 12
Number of electrons = 12
Electronic configuration = 2, 8, 2
Valence electrons = 2
Valency = 2
Numerical 5: An atom has 13 protons and 14 neutrons. Find its atomic number and mass number.
Solution:
Number of protons = 13
Number of neutrons = 14
Atomic number = Number of protons = 13
Mass number = Protons + Neutrons = 13 + 14 = 27
Numerical 6: An element has electronic configuration 2, 8, 7. Find its valency.
Solution:
Valence electrons = 7
Since valence electrons are more than 4,
Valency = 8 – 7 = 1
Numerical 7: An element has atomic number 10. Write its electronic configuration and valency.
Solution:
Atomic number = 10
Number of electrons = 10
Electronic configuration = 2, 8
Outermost shell is complete.
Valency = 0
Numerical 8: Two atoms have mass number 40 but atomic numbers 18 and 20. What are they called?
Solution:
Both atoms have the same mass number = 40
They have different atomic numbers = 18 and 20
Atoms of different elements having the same mass number are called isobars.
Case-Study Based Questions
Case Study 1: Rutherford’s Experiment
Rutherford directed fast-moving alpha particles towards a thin gold foil. Most alpha particles passed straight through the foil, some were deflected and very few bounced back.
Q1. What did the passing of most alpha particles show?
Answer: It showed that most of the atom is empty space.
Q2. What did the deflection of some alpha particles show?
Answer: It showed that positive charge is concentrated in a small region at the centre of the atom.
Q3. What is the central part of an atom called?
Answer: The central part of an atom is called the nucleus.
Q4. Why did very few alpha particles bounce back?
Answer: Very few alpha particles bounced back because the nucleus is very small, dense and positively charged.
Case Study 2: Electronic Configuration
An atom has atomic number 17. Its electrons are arranged in shells according to Bohr-Bury rules. The outermost shell contains 7 electrons.
Q1. Write the electronic configuration of the atom.
Answer: Electronic configuration = 2, 8, 7.
Q2. How many valence electrons are present?
Answer: There are 7 valence electrons.
Q3. Find its valency.
Answer: Valency = 8 – 7 = 1.
Q4. Is this atom chemically stable?
Answer: No. Its outermost shell is not complete, so it is chemically reactive.
Case Study 3: Isotopes
Three atoms of hydrogen have the same atomic number 1 but different mass numbers 1, 2 and 3. They have the same number of protons but different number of neutrons.
Q1. What are these atoms called?
Answer: These atoms are called isotopes of hydrogen.
Q2. Why do they have the same chemical properties?
Answer: They have the same chemical properties because they have the same atomic number and same electronic configuration.
Q3. Why do they have different mass numbers?
Answer: They have different mass numbers because they have different number of neutrons.
Q4. Give one use of an isotope.
Answer: Cobalt isotope is used in the treatment of cancer.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
Q1. Assertion: An atom is electrically neutral.
Reason: In a neutral atom, number of protons is equal to number of electrons.
Answer: A. Both are true and Reason correctly explains Assertion.
Q2. Assertion: Most of the atom is empty space.
Reason: In Rutherford’s experiment, most alpha particles passed straight through the gold foil.
Answer: A. Both are true and Reason correctly explains Assertion.
Q3. Assertion: The nucleus is negatively charged.
Reason: The nucleus contains protons and neutrons.
Answer: D. Assertion is false but Reason is true.
Q4. Assertion: Isotopes have the same chemical properties.
Reason: Isotopes have the same electronic configuration.
Answer: A. Both are true and Reason correctly explains Assertion.
Q5. Assertion: Isobars have the same atomic number.
Reason: Isobars are atoms of different elements having the same mass number.
Answer: D. Assertion is false but Reason is true.
Q6. Assertion: The maximum number of electrons in the K shell is 2.
Reason: Maximum number of electrons in a shell is given by 2n2.
Answer: A. Both are true and Reason correctly explains Assertion.
Exam Tips
- Learn the charge, location and mass of electron, proton and neutron.
- Write Rutherford’s experiment in three parts: experiment, observations and conclusions.
- Do not confuse isotopes with isobars.
- For numericals, always write A = Z + n and n = A – Z.
- For neutral atoms, remember that protons are equal to electrons.
- Use 2n2 rule carefully while writing electronic configuration.
- Valency depends on the number of electrons in the outermost shell.
- Use tables for difference-based questions to score better marks.
Quick Revision Box
Electron
Negatively charged particle present outside the nucleus.
Proton
Positively charged particle present inside the nucleus.
Neutron
Neutral particle present inside the nucleus.
Nucleus
Small, dense and positively charged centre of an atom.
Atomic Number
Number of protons present in an atom.
Mass Number
Sum of protons and neutrons.
Valency
Combining capacity of an atom.
Isotopes
Same atomic number but different mass numbers.
Isobars
Same mass number but different atomic numbers.
FAQ Section
What is the main focus of Class 9 Science Structure of Atom?
The main focus is to understand subatomic particles, atomic models, electronic configuration, valency, isotopes and isobars.
What are the three subatomic particles?
The three subatomic particles are electrons, protons and neutrons.
What is atomic number?
Atomic number is the number of protons present in the nucleus of an atom.
What is mass number?
Mass number is the sum of protons and neutrons present in the nucleus.
What is the formula to find number of neutrons?
Number of neutrons = Mass number – Atomic number.
What did Rutherford’s experiment prove?
Rutherford’s experiment proved that most of the atom is empty space and positive charge is concentrated in a small nucleus.
Why was Rutherford’s model not fully accepted?
Rutherford’s model could not explain the stability of atom because revolving electrons should lose energy and fall into the nucleus.
What did Bohr propose?
Bohr proposed that electrons revolve around the nucleus in fixed energy shells without losing energy.
What are isotopes?
Isotopes are atoms of the same element having the same atomic number but different mass numbers.
What are isobars?
Isobars are atoms of different elements having the same mass number but different atomic numbers.
Final Conclusion
Structure of Atom is a very important chapter because it builds the foundation for higher classes in Chemistry. Students should focus on atomic models, subatomic particles, electronic configuration, valency, isotopes and isobars.
To score high marks, practise diagrams, learn NCERT keywords and solve numerical questions based on atomic number, mass number and number of neutrons. A clear understanding of this chapter will help in CBSE board exams, Exemplar questions and Olympiad-level reasoning.
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