SEBA Class 9 Science Important Chapter 4 Structure of the Atom

SEBA Class 9 Science Important Chapter 4 Structure of the Atom Solutions English Medium As Per SEBA New Syllabus to each chapter is provided in the list so that you can easily browse through different chapters SEBA Class 9 General Science Additional Solutions and select need one. NCERT Important Solutions For Class 9 General Science Download PDF. Important Notes for Class 9 Science.

SEBA Class 9 Science Important Chapter 4 Structure of the Atom

Also, you can read the NCERT book online in these sections Solutions by Expert Teachers as per Central Board of Secondary Education (CBSE) Book guidelines. Assam SEBA Board Class 9 Science Additional Question Answer are part of All Subject Solutions. Here we have given CBSE Class 9 Science Important Notes for All Chapters, You can practice these here.

Structure of the Atom

Chapter: 4

IMPORTANT QUESTION ANSWER

Answer the Following Question:

1. Who discovered the electron?

Ans: J.J. Thomson.

2. Which sub-atomic particle was discovered by E. Goldstein?

Ans: Proton (via canal rays).

3. What is the valency of helium?

Ans: Zero.

4. Which experiment led to the discovery of the nucleus?

Ans: Rutherford’s alpha-particle scattering experiment.

5. What are the limitations of Rutherford’s atomic model?

Ans: It couldn’t explain the stability of the atom. Accelerating electrons should lose energy and collapse into the nucleus.

6. Define isotopes and give two examples.

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

Examples: ¹H, ²H, ³H (Hydrogen), ³⁵Cl and ³⁷Cl (Chlorine)

7. What is atomic number?

Ans: The total number of protons present in the nucleus of an atom of an element is called the atomic number of the element.

8. What is mass number?

Ans: The total number of protons and neutrons present in the nucleus of an atom of an element is called its mass number.

9. What are isotopes?

Ans: The atoms of the same element having the same atomic number but different mass numbers are called isotopes.

10. How was proton discovered?

Ans: In 1886, E. Goldstein discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which led to the discovery of another sub-atomics particle. This sub-atomic particle had a charge, equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 times as that of the electron. It was given the name of proton.

11. Describe the alpha-particle scattering experiment designed by Rutherford. Also mention the conclusions made from it.

Ans: Rutherford chose a thin gold foil for his experiment and fast moving alpha-particles were made to fall on it. 

He observed that:

(i) Most of the alpha-particles passed straight through the gold foil. 

(ii) Some of the alpha-particles were deflected by the foil by small angles.

(iii) One out of every 12000 particles appeared to rebound. 

Rutherford concluded from this experiment that:

(i) Most of the space inside the atom is empty, because most of the alpha-particles passed through the gold foil without getting deflected.

(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

(iii) A very small fraction of alpha-particles were deflected by 180°, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

12. How will you determine the valency of carbon?

Ans: Atomic number of carbon is 6 and hence its electronic configuration is 2.4. Thus an atom of carbon will share its 4 valence electrons with 4 electrons of other atoms. Hence carbon has valency 4.

13. An element is represented by the symbol ₁₅X. Calculate the number of protons, electrons and neutrons in an atom of the element. Also name the element.

Ans: (i) Number of protons = 15

(ii) Number of electrons = 15

(iii) Number of neutrons = 31-15 = 16.

14. Write the applications of isotopes.

Ans: (i) An isotope of uranium is used as a fuel in nuclear reactors.

(ii) An isotope of cobalt is used in the treatment of cancer.

(iii) An isotope of iodine is used in the treatment of goitre.

15. Describe canal rays. What did their discovery lead to?

Ans: Canal rays are positively charged rays discovered by E. Goldstein.

(i) They led to the discovery of protons.

(ii) Canal rays travel in the opposite direction to cathode rays in a discharge tube.