NCERT Solutions for Class 9 Science Chapter 4 - Structure of Atom

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    Q. What are canal rays?

    Ans. When high voltage is passed through a gas at low pressure a beam of positively charged rays is emitted from cathode and travel towards anode in a discharge tube these rays are known as canal rays. They were discovered by Goldstein in 1886.

    Q. Write any two observations which support the fact that atoms are divisible.

    Ans. Discovery of electrons and discovery of protons support the fact that atoms are divisible. During a chemical reaction, there is either the transfer of electrons or sharing of electrons between different atoms which leads to the rearrangement of atoms.

    Q. If an atom contains one electron and one proton, will it carry any charge or not?

    Ans. An electron is negatively charged particle whereas a proton is a positively charged particle. The magnitude of their charges is equal thus, there will not be remaining charge over the atom and it is considered electrically neutral.

    Q. Why do Helium, Neon and Argon have a zero valency?

    Ans. Helium has two electrons in its K-shell i.e., energy shell. Argon and Neon have 8 electrons each in their valence shells. The maximum number of electrons that can be accommodated in energy shells is given by the formula 2n2. These elements have all their energy shell fully filled i.e., they have completely filled octet configuration. Thus, they do not have tendency to combine with other elements. Hence, Helium, Neon and Argon have zero valency.

    Q. An element ‘X’ has mass number 4 and atomic number 2. Write the valency of this element. Will it react with other atoms of different elements?

    Ans. We know that only valence electrons take part in bond formation with different atoms. In the atom of ‘X’ element there are only two electrons since atomic number is 2. Thus, K shell is fully filled for this atom. Hence, its valency is zero. It will not react with other atoms of different elements.

    Q. What were the drawbacks of Rutherford’s model of an atom?

    OR

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

    Ans. Rutherford proposed a model in which the electrons were revolving around the nucleus in well-defined orbits. However, the orbital revolution of the electron was not expected to be stable. Any particle in a circular orbit would undergo acceleration and the charged particles would radiate energy. Thus, the revolving electrons would lose energy and finally fall into the nucleus. Revolution of electrons around the nucleus would make the atom highly unstable and matter would not exist. But this is contrary to our common observations found by other experiments.

    Q. What are the postulates of Bohr’s model of an atom?

    Ans. The postulates put forward by Niels Bohr about the model of an atom are as follows:

    1. Only certain special orbits known as discrete orbits are allowed inside the atom. These discrete orbits are called energy levels. Energy levels in an atom are shown by concentric circles. These orbits are represented by the letters K, L, M, N, or the numbers, n = 1, 2, 3, 4,
    2. While revolving in the discrete orbits, the electrons do not radiate any energy.

    Q. In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?

    Ans. Rutherford’s model concluded that electrons revolved around the nucleus in well-defined orbits. There is a positively charged center(later called “nucleus”) in an atom. According to his model, the size of the nucleus was very small as compared to the size of the atom and nearly all the mass of an atom is centered in the nucleus.
    Thomson had proposed the model of an atom to be similar to a Christmas pudding. The electrons are stuffed like currants in a positively charged sphere of pudding and the mass of the atom was supposed to be uniformly distributed.

    Q. Compare the properties of electron, proton and neutron.

    Ans.

    Proton Electron Neutron
    It is positively charged particle and carries charge equal to +1.6 × 10 –19 coulomb. It is negatively charged particle and carries charge equal to –1.6 × 10 –19 coulomb. It is neutral.
    Its mass is equivalent to the mass of hydrogen atom i.e., 1 amu. Its mass is 1/1840 of the mass of a proton. It is equal in mass to a proton.
    It is present inside the nucleus. It is found outside the nucleus of the atom. It is found inside the nucleus of the atom.

    Q. (ii) Compare all the proposed models of an atom given in the chapter (Structure of Atom).

    Ans.

    Thomson's model Rutherford's moled Bohr's model
    An atom consist of positively charged sphere with negatively charged electrons are embedded in it. The positive and the negative charges in an atom are equal in magnitude due to which an atom as a whole is electrically neutral. An atom consisting of positively charged particles concentrated in the centre of the nucleus. The size of nucleus is very small as compared to the size of atom. The electron revolves around the nucleus in well-defined circular orbits. Only certain orbits known as discrete orbits inside an atom in which electrons revolve around the nucleus. Electrons do not radiate energy while revolving.

    Q. (i) Helium atom has atomic mass of 4 u and two protons in its nucleus. How many electrons does it have?

    (ii) Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.

    $$(iii) \space \text{The average atomic mass of the sample of the element X is 16.2 u. Calculate the percentage of two isotopes}{\space_8 ^{16} X}\space and{\space_8 ^{18} X}\space in the sample.$$

    Ans. (i) The mass of an atom is the sum of the masses of protons and neutrons present in its nucleus.
    Given,
    Atomic mass of Helium = 4 u
    Number of protons = 2
    As atomic mass = number of protons + number of neutrons
    Number of neutrons = Atomic mass – number of protons
    = 4 – 2 = 2
    Hence, the Helium atom has 2 neutrons.

    (ii)Some rules for writing the distribution of electrons in various shells for the first eighteen elements are as follows:
    1. The maximum number of electrons that a shell can accommodate is given by the formula 2n2 where n is the orbit number or energy level index (n = 1, 2, 3….).
    2. The maximum number of electrons present in the orbit of n = 1 given by: 2n2 = 2 × 12 = 2 Similarly for second orbit, 2n2 = 2 × 22 = 8 For third shell, 2n2 = 2 x 32 = 18…..
    3. The outermost orbit can accommodate maximum number of 8 electrons.
    4. Shells are filled with electrons in a stepwise manner, i.e., the outer shell is not occupied with electrons until inner shells are completely filled with electrons.

    $$(iii) \space \text{ Since, average atomic mass of x is 16.24, thus}\\ X=\frac{16x+18(100-x)}{100}\\ 16.2=\frac{16x+1800-18x}{100}\\ 1620 = – 2x + 1800\\ 2x = 1800 – 1620\\ x=\frac{180}{2}= 90\\ Thus, 90\%\space is\space _8^{16} X and\space 10\%\space _8^{18}X$$

    Q. (i) How will you find the valency of chlorine, sulphur and magnesium? 
    (ii) Isotopes have special properties which are useful in various fields. Name the isotope used in the fields mentioned below:
    (a) Used in treatment of goiter.
    (b) Used as a fuel in nuclear reactors.
    (c) Used in treatment of cancer.
    (d) Used in treatment of blood cancer.

    Ans. (i) If the number of electrons in the outermost shell of an atom is less than or equal to 4, then its valency is equal to the number of electrons in the outermost shell. On the other hand if the number of electrons in the outermost shell are greater than 4 then its valency is determined by subtracting the number of electrons in its outermost shell from 8.
    Thus, Chlorine: Electronic configuration: 2, 8, 7
    Sulphur: Electronic configuration: 2, 8, 6
    Magnesium: Electronic configuration: 2, 8, 2
    So, valency is given as:
    Chlorine: 8 – 7 = 1
    Sulphur: 8 – 6 = 2
    Magnesium:2
    (ii)(a) Isotope of iodine (I –131)
    (b) Isotope of uranium (U – 235)
    (c) Isotope of cobalt (Co – 60)
    (d) Isotope of phosphorus (P – 32)

    Q. In the Thomson’s model of atom, which of the following statements are correct?
    (i)The mass of the atom is assumed to be uniformly distributed over the atom.
    (ii) The positive charge is assumed to be uniformly distributed over the atom.
    (iii)The electrons are uniformly distributed in the positively charged sphere.
    (iv) The electrons attract each other to stabilise the atom.

    Ans. (a) i, ii and iii
    (b) i and iii
    (c) i and iv
    (d) i, iii and iv

    Q. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

    Q. (ii) Show diagrammatically, the electron distributions in a sodium atom and a sodium ion and also give their atomic numbers.

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