NCERT Solutions for Class 9th Science
Chapter 4 – STRUCTURE OF THE ATOM
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
Q.1 What are canal rays?
Ans Canal rays are the positively charged radiations due to the presence of positively charged sub-atomic particles called protons. They were discovered by E. Goldstein in 1886.
Q.2 If an atom contains one electron and one proton, will it carry any charge or not?
Ans Atom containing one electron and one proton will not carry any charge because electron has negative charge whereas proton has an equal and opposite charge (positive charge), which will cancel out each other. Thus the atom will be electrically neutral.
Q.3 On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Ans According to Thomson’s model of the atom, an atom consists of a positively charged sphere and the electrons are embedded in it. The negative and positive charges are equal in magnitude and cancel out each other. So, the atom as a whole is electrically neutral.
Q.4 On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans On the basis of Rutherford’s model of an atom, protons (positively charged sub-atomic particles) are present in the nucleus of an atom.
Q.5 Draw a sketch of Bohr’s model of an atom with three shells.
Q.6 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?
Q.7 Name the three sub-atomic particles of an atom.
Q.8 Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Q.9 Write the distribution of electrons in carbon and sodium atoms.
Q.10 If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Q.11 How will you find the valency of chlorine, sulphur and magnesium?
Ans Valency may be defined as the definite combining capacity of an atom of the element. For atoms of the elements having 4 or less than 4 electrons in the outermost shell, the valency is equal to the electrons in the outermost shell (valence electrons). On the other hand for atoms of the elements having more than 4 electrons in the outermost shell, the valency is determined by subtracting electrons in the outermost shell (valence electrons) from 8.
The electrons distribution and valency of the given elements is given by:
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Chlorine (Cl) – atomic no. = 17; e- distribution = 2,8,7
» Valency = 8 – 7 = 1
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Sulphur (S) – atomic no. = 16; e- distribution = 2,8,6
» Valency = 8 – 6 = 2
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Magnesium (Mg) – atomic no. = 12; e- distribution = 2,8,2
» Valency = 2
Q.12 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?
Q.13 With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Q.14 For the symbol H,D and T tabulate three sub-atomic particles found in each of them.
Q.15 Write the electronic configuration of any one pair of isotopes and isobars.
EXERCISES
Q.1 Compare the properties of electrons, protons and neutrons.
Q.2 What are the limitations of J.J. Thomson’s model of the atom?
Ans Limitations of J.J. Thomson’s model of the atom are:
1) The model failed to explain how the positively charged particles are shielded from the negatively charged electrons without getting neutralized. According to Thomson’s model, electrons are embedded in the positively charged sphere. But it was later proved through experiments that the protons exist in the nucleus whereas electrons revolve around the nucleus.
2) The model failed to explain the results of experiments carried out by other scientists like Rutherford’s scattering experiment etc.
Q.3 What are the limitations of Rutherford’s model of the atom?
Ans Rutherford’s model of the atom could not explain the stability of the atom and a proper atomic spectra. According to Rutherford’s model of the atom, electrons revolve around nucleus in fixed orbits. But electrons in a circular orbit would undergo acceleration and during acceleration, they would radiate energy. Thus, the revolving electrons would lose energy and finally fall into the nucleus causing the atom to collapse. But actually the atom is quite stable.
Q.4 Describe Bohr’s model of the atom.
Q.5 Compare all the proposed models of an atom given in this chapter.
Q.6 Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Q.7 Define valency by taking examples of silicon and oxygen.
Q.8 Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and iv) Isobars. Give any two uses of isotopes.
Q.9 Na+ has completely filled K and L shells. Explain.
Q.10 If bromine atom is available in the form of, say, two isotopes 79Br35 (49.7%) and 81Br35 (50.3%), calculate the average atomic mass of bromine atom.
Q.11 The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 16X8 and 18X8 in the sample?
Q.12 If Z = 3, what would be the valency of the element? Also, name the element.
Q.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?
Q.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 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Ans (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. (F)
(c) The mass of an electron is about 1/2000 times that of proton. (T)
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)
Put tick (√) against correct choice and cross (×) against wrong choice in questions 15, 16 and 17
Q.15 Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus (b) Electron (c) Proton (d) Neutron
Ans (a) Atomic Nucleus
Q.16 Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
Q.17 Number of valence electrons in Cl- ion are:
(a) 16 (b) 8 (c) 17 (d) 18
Q.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.
Q.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 -