Study Materials

# NCERT Solutions for Class 9th Science

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## 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.

Ans. –

1. Proton – it has positive charge.
2. Electron – it has negative charge.
3. 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:

1.  Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
2. 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:

1.  There is a positively charged centre in an atom called the nucleus.
2. Nearly all the mass of an atom resides in the nucleus.
3.  The electrons revolve around the nucleus in well-defined orbits.
4. The size of the nucleus is very small as compared to the size of the atom.

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