# NCERT Class 9 Science Solutions: Chapter 4 – Structure of the Atom Part 3

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Question 5:

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

A.

 Thomson’s model Rutherford’s model Bohr’s model An atom consists of a positively charged sphere with electrons embedded in it. An atom consists of a positively charged particles concentrated at the centre known as the nucleus. The size of the nucleus is very small as compared to the size of the atom. The electrons revolve around the nucleus in well-defined orbits. There are only certain orbits known as discrete orbits inside the atom in which electrons revolve around the nucleus. Electrons do not radiate energy while revolving.

Question 6:

Q. Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.

A. The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.

(i) The maximum number of electrons that a shell can accommodate is given by the formula, where ‘n’ is the orbit number or energy level index the maximum number of electrons present in an orbit of is given by

Similarly, for second orbit, it is

For third orbit, it is

And so on……

(ii) The outermost orbit can be accommodated by a maximum number of 8 electrons.

(iii) Shells are filled with electrons in a stepwise manner i.e., the outer shell is not occupied with electrons unless the inner shells are completely filled with electrons.

Question 7:

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

A. The valency of an element is the combining capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element.

If the number of valence electrons of the atom of an element is less than or equal to four, then the valency of that element is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons. Thus, the valency of silicon is four.

On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. Thus, the valency oxygen is i.e., two.

Question 8:

Q. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.

(i) Atomic number

A. The atomic number of an element is the total number of protons in its atom. Thus, the atomic number of nitrogen is 7.

(ii) Mass number

A. The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and neutrons. So, the mass number of boron is

(iii)Isotopes

A. Isotopes are atoms of the same element having the same atomic number, but different mass number. For example, hydrogen has three isotopes. They are protium, deuterium and tritium.

(iv)Isobars

A. Isobars are atoms having the same mass number, but different atomic numbers i.e., isobars are atoms of different elements having the same mass number. For example, and are isobars.

Two uses of isotopes are:

(i) One isotope of uranium is used as a fuel in nuclear reactors.

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

Question 9:

Q. has completely filled K and L shells. Explain.

A. An atom of Na has a total of 11 electrons. Its electronic configuration is But, ion has one electron less than Na atom i.e., it has 10 electrons. Therefore, 2 electrons go to K-Shell and 8 electrons go to L-shell, thereby completely filling K and L shells.

Question 10:

Q. If bromine atom is available in the form of say; two isotopes calculate the average atomic mass of bromine atom.

A. It is given that two isotopes of bromine are. Then, the average atomic mass of bromine atom is given by:

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