{"id":23728,"date":"2021-08-24T19:22:26","date_gmt":"2021-08-24T13:52:26","guid":{"rendered":"https:\/\/mcq-questions.com\/?p=23728"},"modified":"2022-03-02T10:21:15","modified_gmt":"2022-03-02T04:51:15","slug":"ncert-solutions-for-class-9-science-chapter-4","status":"publish","type":"post","link":"https:\/\/mcq-questions.com\/ncert-solutions-for-class-9-science-chapter-4\/","title":{"rendered":"NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom"},"content":{"rendered":"

These NCERT Solutions for Class 9 Science<\/a> Chapter 4 Structure of the Atom Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.<\/p>\n

Structure of the Atom NCERT Solutions for Class 9 Science Chapter 4<\/h2>\n

Class 9 Science Chapter 4 Structure of the Atom InText Questions and Answers<\/span><\/h3>\n

Question 1.
\nWhat are canal rays?
\nAnswer:
\nCanal rays are positively charged radiations. These rays consist of positively charged particles known as protons. They were discovered by Goldstein in 1886.<\/p>\n

Question 2.
\nIf an atom contains one electron and one proton, will it cany any charge or not?
\nAnswer:
\nAn electron is a negatively charged; particle, whereas a proton is a positively charged; particle. The magnitude of their charges is equal. Therefore, an atom containing one electron and one proton will not carry any charge. Thus, it will be a neutral atom.<\/p>\n

Question 3.
\nName the three sub-atomic particles of an atom.
\nAnswer:
\nThe three sub-atomic particles of an atom are:<\/p>\n

    \n
  1. Protons<\/li>\n
  2. Electrons<\/li>\n
  3. Neutrons<\/li>\n<\/ol>\n

    Question 4.
    \nHelium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
    \nAnswer:
    \nA helium atom has two neutrons. The mass of an atom is the sum of the masses of protons and neutrons present in its nucleus. Since helium atom has two protons, mass contributed by the two protons is (2 \u00d7 1) u = 2 u. Then, the remaining mass (4 – 2) u = 2 u is contributed \\(\\frac{2 \\mathrm{u}}{1 \\mathrm{u}}\\) = 2 by neutrons.<\/p>\n

    Question 5.
    \nDraw a sketch of Bohr’s model of an atom with three shells.
    \nAnswer:
    \nBohr’s model of an atom with three shells
    \n\"NCERT<\/p>\n

    Question 6.
    \nWhat do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold?
    \nAnswer:
    \nIf the a-scattering experiment is carried out using a foil of a metal rather than gold, there would be no change in the observation. In the a- scattering experiment, a gold foil was taken because gold is malleable and a thin foil of gold can be easily made. It is difficult to make such foils from other metals.<\/p>\n

    Question 7.
    \nWrite the distribution of electrons in carbon and sodium atoms?
    \nAnswer:
    \nThe total number of electrons in a carbon atom is 6. The distribution of electrons in carbon atom is given by:
    \nFirst orbit or K-shell = 2 electrons
    \nSecond orbit or L-shell = 4 electrons
    \nOr, we can write the distribution of electrons in a carbon atom as 2, 4.<\/p>\n

    The total number of electrons in a sodium atom is 11. The distribution of electrons in sodium atom is given by:
    \nFirst orbit or K-shell = 2 electrons
    \nSecond orbit or L-shell = 8 electrons
    \nThird orbit or M-shell = 1 electron
    \nOr, we can write the distribution of electrons in a sodium atom as 2, 8, 1.<\/p>\n

    Question 8.
    \nIf K and L shells of an atom are full, then what would be the total number of electrons in the atom?
    \nAnswer:
    \nThe maximum number of electrons that can occupy K and L-shells of an atom are 2 and 8 respectively. Therefore, if K and L-shells of an atom are full, then the total number of electrons in the atom would be (2 + 8) = 10 electrons.<\/p>\n

    Question 9.
    \nIf 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?
    \nAnswer:
    \n(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.
    \n(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.<\/p>\n

    Question 10.
    \nWith the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
    \nAnswer:
    \nMass number of oxygen = Number of protons +     Number of neutrons<\/a>
    \n= 8 + 8
    \n= 16
    \nMass number of sulphur=Number of protons + Number of neutrons
    \n= 16 + 16
    \n= 32<\/p>\n

    Question 11.
    \nFor the symbol H, D and T tabulate three sub-atomic particles found in each of them.
    \nAnswer:<\/p>\n\n\n\n\n\n\n
    Symbol<\/td>\nProton<\/td>\nNeutron<\/td>\nElectron<\/td>\n<\/tr>\n
    H<\/td>\n1<\/td>\n0<\/td>\n1<\/td>\n<\/tr>\n
    D<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n<\/tr>\n
    T<\/td>\n1<\/td>\n2<\/td>\n1<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Question 12.
    \nWrite the electronic configuration of any one pair of isotopes and isobars.
    \nAnswer:
    \nTwo isoto of carbon are \\(\\frac {12}{6}\\)C and \\(\\frac {14}{6}\\)C.
    \nThe electronic configuration of \\(\\frac {12}{6}\\)C is 2, 4.
    \nThe electronic configuration of \\(\\frac {14}{6}\\)C is 2, 4.
    \n[Isotopes have the same electronic configuration]
    \n\\(\\frac {40}{20}\\)Ca and \\(\\frac {40}{18}\\)Ar are a pair of isobars
    \nThe electronic configuration of \\(\\frac {40}{20}\\)Ca is 2, 8, 8, 2.
    \nThe electronic configuration of \\(\\frac {40}{18}\\)Ar is 2, 8, 8.<\/p>\n

    Class 9 Science Chapter 4 Structure of the Atom Textbook Questions and Answers<\/span><\/h3>\n

    Question 1.
    \nCompare properties of electrons, protons and neutrons.
    \nAnswer:<\/p>\n\n\n\n\n\n\n
    Electron<\/td>\nProton<\/td>\nNeutron<\/td>\n<\/tr>\n
    (i) Electrons are present outside the nucleus of an atom<\/td>\n(i) Protons are present in the nucleus of an atom.<\/td>\n(i) Neutrons are present in the of an atom<\/td>\n<\/tr>\n
    (ii) Electrons are negatively charged<\/td>\n(ii) Protons are positively charged<\/td>\n(ii) Neutrons are neutral<\/td>\n<\/tr>\n
    (iii) The mass of an electron is considered to negligible<\/td>\n(iii) The mass of a proton is approximately 2000 times as times as the mass of an of an electron<\/td>\n(iii) The mass of neutron is nearly equal to the mass of a proton<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Question 2.
    \nWhat are the limitations of J.J. Thomson’s model of the atom?
    \nAnswer:
    \nAccording to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere with electrons embedded in it. However, it was later found that the positively charged particles reside at the centre of the atom called the nucleus, and the electrons revolve around the nucleus.<\/p>\n

    Question 3.
    \nWhat are the limitations of Rutherford’s model of the atom?
    \nAnswer:
    \nAccording to Rutherford’s model of an atom, electrons revolve around the nucleus in fixed orbits. But, an electron revolving in circular orbits will not be stable because during revolution, it will experience acceleration. Due to acceleration, the electrons will lose energy in the form of radiation and fall into the nucleus. In such a case, the atom would be highly unstable and collapse.<\/p>\n

    Question 4.
    \nDescribe Bohr’s model of the atom.
    \nAnswer:
    \nBohr’s model of the atom:
    \nNiels Bohr proposed the following postulates regarding the model of the atom.<\/p>\n

      \n
    1. Only certain orbits known as discrete orbits of electrons are allowed inside the atom.<\/li>\n
    2. While revolving in these discrete orbits, the electrons do not radiate energy.<\/li>\n<\/ol>\n

      These discrete orbits or shells are shown in the following diagram.
      \n\"NCERT
      \nThe first orbit (i.e., for n = 1) is represented by letter K. Similarly, for n = 2, it is L – shell, for n = 3, it is M – shell and for n = 4, it is N – shell. These orbits or shells are also called energy levels.<\/p>\n

      Question 5.
      \nSummarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.
      \nAnswer:
      \nThe rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.
      \n(i) The maximum number of electrons that a shell can accommodate is given by the formula ‘2n2<\/sup>‘, where ‘n’ is the orbit number or energy level index (n = 1, 2,3..).<\/p>\n

      The maximum number of electrons present in an orbit of n = 1 is given by 2n2<\/sup> = 2 \u00d7 12 = 2
      \nSimilarly, for second orbit, it is 2n2<\/sup> = 2 \u00d7 22<\/sup> = 8
      \nFor third orbit, it is 2n2<\/sup> = 2 \u00d7 32<\/sup> = 18
      \nAnd so on ……
      \n(ii) The outermost orbit can be accommodated by a maximum number of 8 electrons.
      \n(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.<\/p>\n

      Question 6.
      \nDefine valency by taking examples of silicon and oxygen.
      \nAnswer:
      \nThe 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.<\/p>\n

      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.<\/p>\n

      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, tire valency of oxygen is(8 – 6) i.e., two.<\/p>\n

      Question 7.
      \nExplain with examples (i) Atomic number (ii) Mass number (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
      \nAnswer:
      \n(i) Atomic number:
      \nThe atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.<\/p>\n

      (ii) Mass number:
      \nThe mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, tire atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.<\/p>\n

      (iii) Isotopes:
      \nIsotopes are atoms of the same element having the same atomic number, but different mass numbers. For example, hydrogen has three isotopes. They are protium \\(\\left({ }_{1}^{1} \\mathrm{H}\\right)\\), deuterium \\(\\left({ }_{1}^{2} \\mathrm{H}\\right)\\), and tritium \\(\\left({ }_{1}^{3} \\mathrm{H}\\right)\\).<\/p>\n

      (iv) Isobars:
      \nIsobars 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, \\({ }_{20}^{40} \\mathrm{Ca}\\) and \\({ }_{18}^{40} \\mathrm{Ar}\\) andare isobars.<\/p>\n

      Two uses of isotopes are:<\/p>\n

        \n
      1. One isotope of uranium is used as a fuel in nuclear reactors.<\/li>\n
      2. One isotope of cobalt is used in the treatment of cancer.<\/li>\n<\/ol>\n

        Question 8.
        \nNa+<\/sup> has completely filled K and L shells. Explain.
        \nAnswer:
        \nAn atom of Na has a total of 11 electrons. Its electronic configuration is 2,8,1. But, Na+<\/sup> 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.<\/p>\n

        Question 9.
        \nIf bromine atom is available in tile form of, say, two isotopes \\({ }_{35}^{79} \\mathrm{Br}\\) (49.7%) and \\({ }_{35}^{81} \\mathrm{Br}\\) (50.3%), calculate the average atomic mass of bromine atom.
        \nAnswer:
        \nIt is given that two isotopes of bromine are \\({ }_{35}^{79} \\mathrm{Br}\\)(49.7%) and \\({ }_{35}^{81} \\mathrm{Br}\\) (50.3%). Then, the average atomic mass of bromine atom is given by:
        \n\"NCERT
        \n= 80.006 u
        \n= 80 u (approx)<\/p>\n

        Question 10.
        \nThe average atomic mass of a sample of an clement X is 16.2 u. What are the
        \npercentages of isotopes \\({ }_{8}^{16} \\mathrm{X}\\) and \\({ }_{8}^{18} \\mathrm{X}\\) in the sample?
        \nAnswer:
        \nIt is given that the average atomic mass of the sample of element X is 16.2 u.
        \nLet the percentage of isotope \\({ }_{8}^{18} \\mathrm{X}\\) be y%. Thus,
        \nthe percentage of isotope \\({ }_{8}^{16} \\mathrm{X}\\) will be (100 – y) %.<\/p>\n

        Therefore,
        \n\"NCERT
        \n\u21d2 18y + 1600 = 1620
        \n\u21d2 2y + 1600 = 1620
        \n\u21d2 2y +1620 – 1600
        \n\u21d2 2y = 20
        \n\u21d2 y = 10
        \nTherefore, the percentage of isotope \\({ }_{8}^{18} \\mathrm{X}\\) is 10%.
        \nAnd, the percentage of isotope \\({ }_{8}^{16} \\mathrm{X}\\) is (100 – 10) % = 90%.<\/p>\n

        Question 11.
        \nIf Z = 3, what would be the valency of the element? Also, name the element
        \nAnswer:
        \nBy Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron).<\/p>\n

        Therefore, the element with Z = 3 is lithium.<\/p>\n

        Question 12.
        \nComposition of the nuclei of two atomic species X and Y are given as under
        \n\"NCERT
        \nGive the mass numbers of X and Y. What is the relation between the two species?
        \nAnswer:
        \nMass number of X = Number of protons + Number of neutrons
        \n= 6 + 6
        \n= 12
        \nMass number of Y = Number of protons + Number of neutrons
        \n= 6 + 8
        \n= 14
        \nThese two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.<\/p>\n

        Question 13.
        \nFor the following statements, write T for ‘True’ and F for ‘False’.
        \n(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
        \n(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
        \n(c) The mass of an electron is about \\(\\frac{1}{2000}\\) times that of proton.
        \n(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
        \nAnswer:
        \n(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons: (F)
        \n(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral (F).
        \n(c) The mass of an electron is about \\(\\frac{1}{2000}\\) times that of proton. (T)
        \n(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (F)<\/p>\n

        Question 14.
        \nPut tick (\u221a) against correct choice and cross (\u00d7) against wrong choice in the following question:
        \nRutherford’s alpha-particle scattering experiment was responsible for the discovery of
        \n(a) Atomic nucleus
        \n(b) Electron
        \n(c) Proton
        \n(d) Neutron
        \nAnswer:
        \nRutherford’s alpha-particle scattering experiment was responsible for the discovery of
        \n(a) Atomic nucleus (\u221a)
        \n(b) Electron
        \n(c) Proton
        \n(d) Neutron<\/p>\n

        Question 15.
        \nPut tick (\u221a) against correct choice and cross (\u00d7) against wrong choice in the following question:
        \nIsotopes of an element have
        \n(a) the same physical properties
        \n(b) different chemical properties
        \n(c) different number of neutrons
        \n(d) different atomic numbers
        \nAnswer:
        \nIsotopes of an element have
        \n(a) the same physical properties
        \n(b) different chemical properties
        \n(c) different number of neutrons (\u221a)
        \n(d) different atomic numbers<\/p>\n

        Question 16.
        \nPut tick (\u221a) against correct choice and cross (\u00d7) against wrong choice in the following question:
        \nNumber of valence electrons in Cl- ion are:
        \n(a) 16
        \n(b) 8
        \n(c) 17
        \n(d) 18
        \nAnswer:
        \nNumber of valence electrons in Cl- ion are:
        \n(a) 16
        \n(b) 8(\u221a)
        \n(c) 17
        \n(d) 18<\/p>\n

        Question 17.
        \nWhich one of the following is a correct electronic configuration of sodium?
        \n(a) 2, 8
        \n(b) 8, 2, 1
        \n(c) 2, 1, 8
        \n(d) 2, 8, 1
        \nAnswer:
        \n(d) The correct electronic configuration of sodium is 2, 8, 1.<\/p>\n

        Class 9 Science Chapter 4 Structure of the Atom Additional Important Questions and Answers<\/span><\/h3>\n

        Very Short Answer Questions<\/span><\/p>\n

        Question 1.
        \nWhat are nucleons?
        \nAnswer:
        \nThe subatomic particles present in the nucleus of an atom are called nucleons. These include protons and neutrons.<\/p>\n

        Question 2.
        \nWhich element does not contain any neutron?
        \nAnswer:
        \nHydrogen<\/p>\n

        Question 3.
        \nWho discovered the presence of the protons in an atom?
        \nAnswer:
        \nGoldstein after his discovery of anode rays.<\/p>\n

        Question 4.
        \nWhat did Rutherford discover from his alpha particle scattering experiment?
        \nAnswer:
        \nRutherford from alpha particle scattering experiment had discovered the nucleus of an atom.<\/p>\n

        Question 5.
        \nWhich subatomic particles were not known when Rutherford performed his experiment?
        \nAnswer:
        \nNeutrons<\/p>\n

        Question 6.
        \nWhat is the number of protons and neutrons present in a helium atom?
        \nAnswer:
        \nIn a helium atom the number of both protons and neutrons is two.<\/p>\n

        Question 7.
        \nWhat is common in isotopes of an element?
        \nAnswer:
        \nAtomic number<\/p>\n

        Question 8.
        \nWhat was the major drawback of Rutherford\u2019s model of atom?
        \nAnswer:
        \nRutherford had failed to describe the stability of an atom.<\/p>\n

        Question 9.
        \nWhat are the fundamental building blocks of matter?
        \nAnswer:
        \nAtoms and molecules<\/p>\n

        Question 10.
        \nWhat is the mass and charge of a proton?
        \nAnswer:
        \nThe mass of a .proton is taken as one unit and so also its charge is taken as one positive unit.<\/p>\n

        Question 11.
        \nWhat is the charge on the nucleus of an atom?
        \nAnswer:
        \nThe nucleus of an atom is positively charged.<\/p>\n

        Question 12.
        \nWhat are valence electrons?
        \nAnswer:
        \nThe electrons present in the outermost shell of an atom are called valence electrons.<\/p>\n

        Question 13.
        \nIf an atom has atomic number-16, what are the number of valence electrons and what is its valency?
        \nAnswer:
        \nElectronic configuration: 2, 8, 6.
        \nValence electrons = 6
        \nValency = 2<\/p>\n

        Question 14.
        \nWhich isotope of uranium is used as nuclear fuel in nuclear reactor?
        \nAnswer:
        \nU-235.<\/p>\n

        Short Answer Questions<\/span><\/p>\n

        Question 1.
        \nHow will you prove that matter is electrical in nature but neutral?
        \nAnswer:
        \nWhen a glass rod rubbed with silk cloth is brought near an inflated balloon, the balloon gets attracted to the rod but when a glass rod without being rubbed to silk cloth is brought close to an inflated balloon nothing happens shows that matter is electrical in nature but neutral.<\/p>\n

        Question 2.
        \nWhat made the scientists to realize that atom is divisible after Dalton proposed his theory of atom in which he described atom as indivisible?
        \nAnswer:
        \nThe attraction of bits of paper by a comb after combing dry hair or gluing of a balloon to a wall after rubbing made the scientists think that atom is divisble.<\/p>\n

        Question 3.
        \nWhat is the charge and mass of an electron and a proton?
        \nAnswer:
        \nCharge on an electron = -1.6 x 10-19<\/sup>C
        \nMass of an electron = 1\/1840 times of a proton
        \nCharge on a proton = + 1.6 x 10-19<\/sup>C
        \nMass of an electron = mass of a hydrogen atom<\/p>\n

        Question 4.
        \n(i) What are the mass number (A) and atomic number (Z) of an element?
        \n(ii) How are they indicated in a symbol of an atom?
        \nAnswer:
        \n(i) Mass number refers to the sum of the numbers of protons and neutrons present in atom while the atomic number refers to the number of protons present in the nucleus of an atom.
        \n(ii) Mass number is always written in superscript and atomic number in the subscript as shown below.
        \n\"NCERT<\/p>\n

        Question 5.
        \n(i) What is the nucleus of atom?
        \n(ii) What charge does the nucleus of an atom carry?
        \nAnswer:
        \n(i) Nucleus is a small dense region present in the centre of an atom. It is 10s times smaller than an atom. It is like a football placed in the centre of football field, (ii) It contains positively charged protons and neutral neutrons, therefore it is positively charged.<\/p>\n

        Question 6.
        \nWhat did Thomson propose about the model of an atom?
        \nAnswer:
        \nThomson proposed that:
        \n\"NCERT<\/p>\n