Study Materials

# NCERT Solutions for Class 9th Science

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## Chapter 4. Structure of The Atom

### Additional And Important Questions With solutions:

Q – Write the results of Rutherford’s particle scattering experiment.

A – The following observations were made: Most of the fast moving particles passed straight through the gold foil.

1. Some of the α-particles were deflected by the foil by small angles.
2. Surprisingly one out of every 12000 particles appeared to rebound.

5. Compare all the proposed models of an atom given in this

chapter.

6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Q – Describe Thomson’s model of the atom.

A – Thomson proposed that:

1.  An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Q – Define valency by taking examples of silicon and oxygen.

A – The electrons present in the outermost shell of an atom are known as the valence electrons. This number of valence electrons is called valency.

Example of silicon and oxygen

Number of electrons in silicon = 14

Electronic configuration = 2, 8, 4

The number of valence electrons = 4

Hence valency = 4

Example of oxygen

Number of electrons in oxygen = 8

Electronic configuration = 2, 6

The number of valence electrons = 8 – 6 = 2

Hence valency = 2

Q – Define atomic number.

A – The number of protons presence in nucleus of an atom is known as atomic number.

Q – Define atomic mass.

A – The sum of number of protons and neutrons presence in nucleus of an atom is known as atomic mass.

Q – Define isotopes.

A – The atoms of an element are called isotopes of that element which has equal atomic number but different atomic mass.

Q – Define isobar.

A – The atoms of an element are called isobar of that element which

Q – Differentiate between isotopes and isobars.

A -

 isotopes isobars It has equal atomic number but different atomic mass. Example Hydrogen has three isotopes : Protium (), deuterium (  or D) and tritium  ( ) It has equal atomic mass but different atomic number. Calcium, atomic number 20, and argon, atomic number 18. The number of electrons in these atoms is different, but the mass number of both these elements is 40.

Q – Write three applications of isotopes.

A – Three applications of isotopes.

1.  An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.
3. An isotope of iodine is used in the treatment of goitre.

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