Chapter 4. Structure of the Atom

Class 9 Science Chapter 4. Structure of the Atom Internal Questions and Answers :

Internal Questions :

1. What are canal rays?

Answer: Canal rays are the positively charged radiation which ultimately led to the discovery of another sub-atomic particles .
2. If an atom contains one electron and one proton, will it carry any charge or not ?

Answer: Yes, an atom containing one electron and one proton will have a net charge of zero. So, the atom will be electrically neutral .

Internal Questions :

1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Answer: According to Thomson's model of the atom,

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

2. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?

Answer: According to Rutherford's model of the atom, the nucleus of an atom contains positively charged protons and uncharged neutrons. Electrons orbit around the nucleus in specific energy levels or shells.
3. Draw a sketch of Bohr’s model of an atom with three shells.

Answer:

4. What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?

Answer: If the α-particle scattering experiment is conducted using a foil of a metal other than gold, the observations would likely be similar but with variations. The scattering pattern and angles of deflection would depend on the atomic structure and density of the metal used.

Internal Questions :

1. Name the three sub-atomic particles of an atom.

Answer: Name of the three sub-atomic particles of an atom are proton , electron and neutrons .
2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Answer:  We know that ,

  Atomic mass = Number of protons + Number of neutrons

The atomic mass of helium = 4 u

Number of protons = 2

Number of neutrons = Atomic mass - Number of protons             

 Number of neutrons = 4 – 2 = 2

Internal Questions :

1. Write the distribution of electrons in carbon and sodium atoms.

Answer:  Atomic number of carbon =  6  

Carbon has 6 electrons since its atomic number is 6.

Electron configuration = 2. 4

The K shell (n=1) = 2 electrons

The L shell (n=2) = 4 electrons

Atomic number of sodium = 11

 Sodium has 11 electrons since its atomic number is 11.

Electron configuration = 2.8.1

The K shell (n=1) = 2 electrons

The L shell (n=2) = 8 electrons

The K shell (n=3) = 1 electrons

2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Answer:  We know that ,

Maximum number of electrons in a shell = 2n²  , where 'n' is number the shell.

 For the K shell (n = 1), the maximum capacity = 2 × 1² = 2 electrons.

 For the L shell (n = 2), the maximum capacity = 2 × 2² = 8 electrons.

Total number of electrons = Number of electrons in K shell + Number of electrons in L shell

Total number of electrons = 2 + 8

Total number of electrons = 10

So, if both K and L shells of an atom are full, the atom would have a total of 10 electrons.

Internal Questions :

1. How will you find the valency of chlorine, sulphur and magnesium?

Answer: Atomic number of chlorine = 17

Electron configuration (Cl) = 2.8.7

The valency of Cl = 8 – 7 = 1

Atomic number of sulphur = 16

Electron configuration = 2.8.6

The valency of S = 8 – 6 = 2

Atomic number of magnesium  = 12

Electron configuration = 2.8.2

The valency of Mg = 2

Internal Questions :

1. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? and (ii) what is the charge on the atom?

Answer: (i) The atomic number of an atom is equal to the number of protons in its nucleus. In this case, the number of protons is given as 8. Therefore, the atomic number of the atom is also 8.

(ii) The charge on an atom is determined by comparing the number of protons (positive charge) and electrons (negative charge). If the number of protons and electrons are equal, the atom is neutral .

2. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.

Answer:  Here, The number of proton in oxygen atom = 8

The number of neutrons in oxygen atom = 8

The mass of atom = Proton + neutron

So, the mass number of oxygen atom = 8 + 8 = 16 u

The number of proton in sulphur atom = 16

The number of neutrons in sulphur = 16

So, the mass number of sulphur atom = 16 + 16 = 32 u

Internal Questions :

1. For the symbol H,D and T tabulate three sub-atomic particles found in each of them.

Answer:

Name (Symbol)	Protons	Neutrons	Electrons
Hydrogen(H)	1	0	1
Deuterium (D)	1	1	1
Tritium (T)	1	2	1

2. Write the electronic configuration of any one pair of isotopes and isobars.

Answer: Isotopes are atoms of the same element, which have different mass numbers.

(i) Chlorine :  and

Atomic number of Cl = 17

Electronic configuration of Cl = 2.8.7 

Isobars are atoms having the same mass number but different atomic numbers.

(i) Argon  and Calcium

Atomic number of Ar = 18

Electronic configuration of Ar = 2.8.8 

Atomic number of Ca = 20

Electronic configuration of Ca = 2.8.8.2 

Class 9 Science Chapter 4. Structure of the Atom Exercise Questions and Answers :

1. Compare the properties of electrons, protons and neutrons.

Answer: Electrons, protons, and neutrons are subatomic particles with distinct properties:

(i) Electrons are negatively charged and orbit the nucleus. They have a negligible mass compared to protons and neutrons.

(ii) Protons are positively charged particles located in the nucleus. They have a mass of approximately 1 atomic mass unit.

(iii) Neutrons are neutral particles found in the nucleus. They have a mass similar to protons but carry no charge.             

2. What are the limitations of J.J. Thomson’s model of the atom?

Answer: The limitations of J.J. Thomson's model of the atom, known as the "plum pudding model," include its failure to account for the nucleus, subatomic particles (protons and neutrons), and the quantization of energy levels in atoms. It also couldn't explain the scattering of alpha particles or the discrete spectral lines observed in atomic spectra.

3. What are the limitations of Rutherford’s model of the atom?

Answer: The limitations of Rutherford’s model of the atom are :

(i) The revolution of the electron in a circular orbit is not expected to be stable.

(ii)  During acceleration, charged particles would radiate energy.

 (iii) The revolving electron would lose energy and finally fall into the nucleus.

4. Describe Bohr’s model of the atom.

Answer:  The  Bohr’s model of an atom are :

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy .

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

Answer: The proposal of the models of an atom given below :

J.J. Thomson’s model :

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Rutherford’s model :

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

(ii) The electrons revolve around the nucleus in circular paths.

(iii) The size of the nucleus is very small as compared to the size of the atom.

Bohr’s model :

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy.

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

Answer: The distribution of electrons in various shells for the first eighteen elements follows specific rules based on the principles of electron configuration:

The maximum number of electrons present in a shell = 2n² , where n is number of shell (orbit) .

The K shell (n = 1) can hold a maximum of 2 electrons (= 1.1²).

The second shell (n = 2) can hold a maximum of 8 electrons (= 2.2² ).

The third shell (n = 3) can hold a maximum of 18 electrons (= 2.3² ).

The fourth shell (n = 4) can hold a maximum of 32 electrons (= 2. 4²).

The elements fill their electron shells in increasing order of atomic number.

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

Answer: Valency is the combining capacity of an atom.

Atomic number of silicon = 14

Electronic configuration of silicon = 2.8.4

Atomic number of oxygen = 8

Electronic configuration of oxygen = 2.6

Silicon has a valency of 4, while oxygen has a valency of 2 (= 8 – 6 ).         

8. Explain with examples :

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

Answer: (i) The atomic number of an element is the same as the number of protons in the nucleus of its atom.

(ii) Mass number The mass number of an atom is equal to the number of nucleons (proton + neutron) in its nucleus.
(iii)  Isotopes are atoms of the same element, which have different mass numbers.

(iv)  Isobars are atoms having the same mass number but different atomic numbers.

 Two uses of isotopes are :

(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 goiter .

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

Answer:  The atomic number of Na = 11

The electronic configuration of Na = 2.8.1

The electronic configuration of  = 2.8

         

10. If bromine atom is available in the form of, say, two isotopes  (49.7%) and  (50.3%), calculate the average atomic mass of bromine atom.

Answer: Given, two isotopes of bromine are  (49.7%) and  (50.3%) .

The isotopes of the mass of  bromine are 79 u and 81 u .

The average atomic mass of bromine atom

 u

The average atomic mass of bromine (Br) atom is 80.006 atomic mass units (u)

11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes  and  in the sample?

Answer: let, the percentages of   be  .

And the percentages of   be  .

A/Q, 

 

 

 

 

Therefore, the percentages of   is  .

And the percentages of   is  .

12. If Z = 3, what would be the valency of the element? Also, name the element.

Answer: The atomic number (Z) of an element = 3

The electron configuration Z = 2.1

The valency of lithium (Z = 3) is 1 .

The element with atomic number 3 is lithium (Li).

13. Composition of the nuclei of two atomic species X and Y are given as under
                               X                    Y
      Protons =           6                    6
    Neutrons =           6                    8
Give the mass numbers of X and Y. What is the relation between the two species?

Answer:  We know that, Atomic mass = proton + neutron

The mass numbers of X = 6+ 6 = 12 u

 The mass numbers of Y = 6 + 8 = 14 u

The only difference between the two species is the number of neutrons.

Species X has 6 neutrons, and Species Y has 8 neutrons.

Since they both have the same number of protons (6), they are isotopes of the same element.

14. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about  times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Answer: (a) False , (J.J. Thomson did not propose the existence of the nucleus; he proposed the plum pudding model where positive charge and electrons are evenly distributed in a uniform sphere.)

(b) False,  (A neutron is formed by the combination of three quarks and is neutral due to the balance of positive and negative charges within it.)

(c) True , (The mass of an electron is approximately 1/2000 times that of a proton.)  

(d) True,  (An isotope of iodine, iodine-127, is used for making tincture iodine, which is used as a medicine.)

Put tick (right Sign) against correct choice and cross (×) against wrong choice in questions 15, 16 and 17
15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus  (b) Electron   (c) Proton   (d) Neutron

Answer: (a) Atomic Nucleus .
16. Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.

Answer: (c) different number of neutrons .
17. Number of valence electrons in ion are:
  (a) 16           (b) 8          (c) 17           (d) 18

Answer: (d) 18 (= 17 +1)

18. Which one of the following is a correct electronic configuration of sodium  ?
    (a) 2,8      (b) 8,2,1    (c) 2,1,8   (d) 2,8,1.

Answer:  (d) 2,8,1.

[  Atomic number of Na = 11

The electronic configuration of sodium = 2. 8 .1 ]
19. Complete the following table.

Atomic       Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of the Atomic Species
9	-	10	-	-	-
16	32	-	-	-	Sulphur
-	24	-	12	-	-
-	2	-	1	-	-
-	1	0	1	0	-

Answer:

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of the Atomic Species
9	19	10	9	9	Fluorine
16	32	16	16	16	Sulphur
12	24	12	12	12	Magnesium
1	2	1	1	1	Deuterium
1	1	0	1	1	Hydrogen