Azeotropes

STRUCTURE OF ATOM

INTRODUCTION

The chapter Structure of Atom is one of the most important topics in Class 9 Chemistry. It helps students understand the basic building blocks of matter and how atoms are formed. In this post, we provide important questions and answers based on this chapter to help students revise concepts like protons, neutrons, electrons, atomic models, and atomic number in an easy and clear way.

This post includes
✓ Very short question- answer
✓ Short question-answer
✓ Long question-answer
✓ Case - study
✓ Assertion - reason

VERY SHORT QUESTION - ANSWER

BASIC CONCEPTS

1. What is an atom?

Ans: Smallest unit of an element.


2. Who discovered the electron?

Ans: J. J. Thomson


3. What is the charge of an electron?

Ans: Negative


4. What is the charge of a proton?

Ans: Positive


5. What is the charge of a neutron?

Ans: Neutral


6. Who discovered the proton?

Ans: Ernest Rutherford


7. Who discovered the neutron?

Ans: James Chadwick


8. Where is the nucleus located?

Ans: At the center

ATOMIC STRUCTURE

1. What is atomic number?

Ans: Number of protons


2. What is mass number?

Ans: Protons + Neutrons


3. What is nucleus made of?

Ans: Protons and neutrons


4. What are electrons?

Ans: Negatively charged particles


5. What are protons?

Ans: Positively charged particles


6. What are neutrons?

Ans: Neutral particles


7. What is a shell?

Ans: Energy level of electrons

ATOMIC MODELS

1. Who proposed plum pudding model?

Ans: J. J. Thomson


2. Who proposed nuclear model?

Ans: Ernest Rutherford


3. Who proposed Bohr’s model?

Ans: Niels Bohr


4. What did Rutherford discover?

Ans: Nucleus


5. What is Bohr’s orbit?

Ans: Fixed path of electrons

ELECTROSTATIC CONFIGURATION

1. Maximum electrons in K shell?

Ans: 2


2. Maximum electrons in L shell?

Ans: 8


3. Formula for maximum electrons?

Ans: 2n²


4. What are valence electrons?

Ans: Outermost electrons


5. What is valency?

Ans: Combining capacity


6. Valency of oxygen?

Ans: 2


7. Valency of sodium?

Ans: 1

ISOTOPES & IONS

1. What are isotopes?

Ans: Same atomic number, different mass number


2. Give example of isotopes.

Ans: Protium, Duteriumand Tritium are isotopes of Hydrogen.


3. What is an ion?

Ans: Charged atom


4. What is cation?

Ans: Positively charged ion


5. What is anion?

Ans: Negatively charged ion

IMPORTANT FACTS

1. Why atom is neutral?

Ans: Equal protons and electrons


2. What is atomic mass?

Ans: Sum of protons and neutrons


3. What is nucleus charge?

Ans: Positive


4. What is electron mass?

Ans: Very small


5. What is proton mass?

Ans: ≈ 1 amu


6. What is neutron mass?

Ans: ≈ 1 amu

APPLICATIONS & EXAMPLES

1. Which isotope is used in carbon dating?

Ans: Carbon-14


2. Which isotope is used in cancer treatment?

Ans: Cobalt-60


3. Which isotope is used as fuel?

Ans: Uranium-235

EXTRA PRACTICE

1. Number of electrons in neutral atom?

Ans: Equal to protons


2. What is energy level?

Ans: Shell of electrons


3. What is atomic structure?

Ans: Arrangement of particles


4. What is outermost shell?

Ans: Valence shell


5. What is inner shell?

Ans: Shell close to nucleus


6. What is electron distribution?

Ans: Arrangement of electrons


7. What is stability of atom?

Ans: Balanced structure


8. What is Bohr-Bury rule?

Ans: Rules for electron filling


9. What is maximum electrons in outer shell?

Ans: 8


10. What is matter made of?

Ans: Atoms


11. What is subatomic particle?

Ans: Particle inside atom

SHORT QUESTION - ANSWER

1. Name the three subatomic particles of an atom.
   Ans : (i )Electrons (negatively charged)
   (ii) Protons(positively charged)
   (iii) Neutrons(no charge)
   
2. What are canal rays ?
   Ans : Canal rays are positively charged radiations which led to the discovery of positively charged sub- atomic particle called proton.


3. If an atom contains one electron and one proton, will it carry any charge or not?
   Ans : The atom will be electrically neutral as one negative charge balances one positive charge.


4. On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
   Ans: According to Thomson's model of an atom-
   (i) An atom consists of a positively charged sphere and the electrons are embedded in it.
   (ii) The negative and positive charges are equal in magnitude. So, the atom is electrically neutral.


5. On the basis of Rutherford's model of an atom, which sub-atomic particle is present in the neucleus of an atom ?
   Ans :- Proton is present in the neucleus.


6. Draw a sketch of Bohr's model of an atom with three shells
   Ans :-
   









7. What do you think would be the observation if the α - particle scattering experiment is carried out using a foil of a metal other than gold?
   Ans :- On using any metal foil, the observations of the α - particle scattering experiment would remain the same as all atoms would hav same structure.


8. What do you mean by atomic number?
   Ans :- Atomic Number (Z) :- The total number of protons present in a nucleus of an atom is called its atomic number.


9. What do you mean by atomic number?
   Ans :- Mass number (A) :- The sum of total number of protons and neutrons present in the nucleus of an atom is called its mass number.


10. How the atomic number(Z), mass number (A) and symbol of an element are represented?
    Ans :-
11. Write the name and symbols of the elements from the atomic number 1 to 10. Also write the number of protons and electrons also.


12. Write the name and symbols of the elements from the atomic number 11 to 20. Also write the number of protons and electrons.


13. Write the name and symbols of the elements from the atomic number 21 to 30. Also write the number of protons and electrons .


14. Write the name and symbols of the elements from the atomic number 31 to 40. Also write the number of protons and electrons .


15. What do you mean by isotopes?
    Ans :- Isotopes: The atoms of same element having same atomic number but different mass numbers are called isotopes of that element.


16. What are the isotopes of hydrogen?
    Ans: There are three isotopes of hydrogen.
    (i) Protium (ii) Deuterium (iii) Tritium


17. Show the number of electrons, protons and neutrons of isotopes of hydrogen.



18. What subatomic particles are different in number in isotopes of the same element?
    Ans: neutrons


19. What do you mean by isobars?
    Ans:- Isobars : The atoms of different elements having same mass number but different atomic numbers are called isobars.


20. Give example of isobars.
    Ans :


21. Write the symbols of elements from atomic number 1 to 20 with the representation of atomic number and mass numbers.


22. Write the number of protons , electrons and neutrons of Al and Mg.
23. Write the number of neutrons of O and N.
24. Atomic number of an element is p and its number of neutrons is q and its symbol is A. How to represent this element symbolically?

LONG QUESTION-ANSWER

1. Describe Thomson’s model of an atom. What were its limitations?

Answer:

J. J. Thomson proposed that an atom is a positively charged sphere in which negatively charged electrons are embedded, like raisins in a pudding. This is also known as the plum pudding model.

Limitations:

• It could not explain the scattering results of α-particles.
• It failed to describe the presence of a nucleus.
• It did not explain how electrons are arranged in an atom.

2. Explain Rutherford’s model of an atom with its observations and conclusions.

Answer:

Ernest Rutherford conducted the famous gold foil experiment.

Observations:

• Most α-particles passed straight through the foil.
• Some were slightly deflected.
• Very few were deflected back.

Conclusions:

• Most of the atom is empty space.
• A small, dense, positively charged nucleus is present at the center.
• Electrons revolve around the nucleus.

Limitations:

• Could not explain the stability of the atom.
• Did not explain electron arrangement.

3. What are the postulates of Bohr’s model of atom?

Answer:

Niels Bohr improved Rutherford’s model and proposed:

• Electrons revolve in fixed circular orbits called shells or energy levels.
• Each orbit has a fixed energy.
• Electrons do not lose energy while moving in these orbits.
• Energy is emitted or absorbed when electrons jump between orbits.

4. Define atomic number and mass number. Explain their significance.

Answer:

Atomic Number (Z): Number of protons in the nucleus.

Mass Number (A): Sum of protons and neutrons.

Significance:

• Atomic number determines the identity of an element.
• Mass number helps in calculating atomic mass.
• It helps to determine number of neutrons:
  Neutrons = Mass Number – Atomic Number

5. What are isotopes? Give examples and uses.

Answer:

Isotopes are atoms of the same element having the same atomic number but different mass numbers.

Examples:

Hydrogen: Protium, Deuterium, Tritium

Uses:

• Carbon-14 is used in carbon dating.
• Uranium-235 is used in nuclear reactors.
• Cobalt-60 is used in cancer treatment.

6. Write the distribution of electrons in different shells according to Bohr and Bury rules.

Answer:

Bohr-Bury Rules:

• Maximum electrons in a shell = 2n² (where n = shell number).
• Outer shell can have a maximum of 8 electrons.
• Electrons fill inner shells first

Example:

K shell (n=1) → 2 electrons

L shell (n=2) → 8 electrons

M shell (n=3) → 18 electrons

7. Explain valency. How is it determined?

Answer:

Valency is the combining capacity of an atom.

Determination:

• If outer shell electrons ≤ 4 → Valency = number of electrons
• If outer shell electrons > 4 → Valency = 8 – number of electrons

Example:

1. Oxygen (6 electrons in outer shell) → Valency = 2
2. Sodium (1 electron) → Valency = 1

8. Describe the structure of an atom with a neat explanation.

Answer:

An atom consists of:

• Nucleus: Contains protons (+) and neutrons (0)
• Electrons: Negatively charged particles revolving in shells

Key Points:

• Nucleus is very small but contains most of the mass
• Electrons are arranged in different energy levels
• Atom is electrically neutral (protons = electrons)

CASE STUDY

Case Study 1: Discovery of Subatomic Particles

In early atomic theory, scientists believed atoms were indivisible. However, experiments by scientists like J. J. Thomson and Ernest Rutherford proved that atoms are made up of smaller particles.

Questions:

1. Who discovered the electron?
2. What was the charge of the electron?
3. Which experiment led to the discovery of the nucleus?
4. What conclusion was drawn from Rutherford’s experiment?

Answers:

1. J. J. Thomson discovered the electron.
2. Electron has a negative charge.
3. Gold foil experiment.
4. Atom has a small, dense, positively charged nucleus and mostly empty space.

Case Study 2: Atomic Models

Different scientists proposed different models to explain atomic structure. Niels Bohr improved earlier models by explaining electron distribution in fixed orbits.

Questions:

1. What was the main idea of Bohr’s model?
2. What are energy levels in an atom?
3. Why do electrons not lose energy in fixed orbits?
4. Name one limitation of Rutherford’s model.

Answers:

1. Electrons revolve in fixed circular orbits with definite energy.
2. Fixed shells where electrons move.
3. Because they are stable orbits with fixed energy.
4. It could not explain atomic stability.

Case Study 3: Atomic Number and Mass Number

Every element is identified by its atomic number and mass number. These values help in understanding the composition of atoms.

Questions:

1. What does atomic number represent?
2. How is mass number calculated?
3. Find neutrons if atomic number = 11 and mass number = 23.
4. What is the number of electrons in a neutral atom?

Answers:

1. Number of protons.
2. Protons + Neutrons.
3. Neutrons = 23 – 11 = 12
4. Equal to number of protons.

Case Study 4: Isotopes and Their Uses

Atoms of the same element can have different mass numbers. These are called isotopes and have various applications.

Questions:

1. Define isotopes.
2. Name one isotope used in medicine.
3. Why do isotopes have similar chemical properties?
4. Give one example of isotopes.

Answers:

1. Same atomic number but different mass number.
2. Cobalt-60
3. Same number of electrons.
4. Hydrogen isotopes (Protium, Deuterium, Tritium).

Case Study 5: Electronic Configuration

Electrons are arranged in different shells according to certain rules.

Questions:

1. What is the maximum number of electrons in K shell?
2. State the formula for maximum electrons in a shell.
3. Write electronic configuration of Oxygen (Atomic number 8).
4. What are valence electrons?

Answers:

1. 2
2. 2n²
3. 2, 6
4. Electrons in outermost shell.

ASSERTION - REASONING

Instructions:

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.

C. Assertion is true, but Reason is false.

D. Assertion is false, but Reason is true.

Q1

Assertion (A): An atom is electrically neutral.

Reason (R): Number of protons is equal to number of electrons.

Answer: ✅ A
Explanation: Equal positive and negative charges make the atom neutral.

Q2

Assertion (A): J. J. Thomson model could not explain the stability of an atom.

Reason (R): It did not include a nucleus in the atom.

Answer: ✅ A

Q3

Assertion (A): Ernest Rutherford concluded that most of the atom is empty space.

Reason (R): Most α-particles passed through the gold foil without deflection.

Answer: ✅ A

Q4

Assertion (A): Electrons revolve in fixed orbits around the nucleus.

Reason (R): Niels Bohr proposed stable energy levels.

Answer: ✅ A

Q5

Assertion (A): Atomic number determines the identity of an element.

Reason (R): Atomic number is equal to number of neutrons.

Answer: ❌ C
Explanation: Atomic number = number of protons, not neutrons.

Q6

Assertion (A): Isotopes have similar chemical properties.

Reason (R): They have the same number of electrons.

Answer: ✅ A

Q7

Assertion (A): Mass number is always equal to atomic number.

Reason (R): Mass number is the sum of protons and neutrons.

Answer: ❌ D
Explanation: Assertion is false, but Reason is true.

Q8

Assertion (A): Valency of an element depends on its outermost electrons.

Reason (R): Chemical properties are determined by valence electrons.

Answer: ✅ A

Q9

Assertion (A): Maximum number of electrons in K shell is 2.

Reason (R): Maximum electrons in a shell is given by 2n².

Answer: ✅ A

Q10

Assertion (A): Neutrons have no charge.

Reason (R): They are present outside the nucleus.

Answer: ❌ C
Explanation: Neutrons are neutral but present inside the nucleus.

CONCLUSION

Understanding the structure of an atom is essential for building a strong foundation in chemistry. By practicing these questions and answers, students can improve their knowledge and prepare effectively for exams. Regular revision of key concepts will make this topic easier and more interesting to learn.

Also you might like

Structure of atom
Newton's laws of motion
Motion
Momentum
Science Practice Questions class 9
Mathematics MCQ