NCERT Solutions for Class 9th Science
Chapter 4. Structure of The Atom
Additional And Important Questions with solutions
Ques.1 – Why does an atom have neutral charge?
Ans. – As an atom have equal protons and electrons. The protons carry positive charge and electrons carry negative charge. Thus an atom has neutral charge.
Ques.2 – What is an alpha particle?
Ans. – Alpha particle is a double charged helium particle which has positive charge.
Ques.3 – Write the name of sub-atomic particles of an atom.
- Proton – it has positive charge.
- Electron – it has negative charge.
- Neutron – it has no charge.
Ques.4 – Name the founder of electron.
Ans. – J. J. Thomson
Ques.5 – What is canal rays?
Ans. – Canal rays were positively charged radiations of a gas discharged from the anode of discharged canal which ultimately led to the discovery of another sub-atomic particle.
Ques.6 – Whom does credit for discovery of proton go?
Ans – E. Goldstein.
Ques.7 – What are the limitations of J.J. Thomson’s model of the atom?
Ans. – Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model,
Ques.8 – What are the limitations of Rutherford’s model of the atom?
Ans. – According to electromagnetic principle Rutherford’s model of the atom makes unstable to an atom while an atom is stable.
Ques.9 – What postuletes Neils Bohr put about the model of an atom?
Ans. – Neils Bohr put forward the following postulates about the model of an atom:
- Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
- While revolving in discrete orbits the electrons do not radiate energy.
Ques.10 – Describe Rutherford’s model of the atom.
Ans. – Rutherford put forward the nuclear model of an atom, which had the following features:
- There is a positively charged centre in an atom called the nucleus.
- Nearly all the mass of an atom resides in the nucleus.
- The electrons revolve around the nucleus in well-defined orbits.
- The size of the nucleus is very small as compared to the size of the atom.