Chapter 8 Solutions — Journey Inside the Atom

Revise, Reflect, Refine

1. 1.
   Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford's gold foil experiment:

   • (i) The experiment clearly showed the existence of neutrons in the nucleus.
   • (ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.
   • (iii) The large deflection of a few alpha particles indicated…
2. 2.
   Which of the following statements are correct or incorrect according to the Bohr's atomic model? Give a reason for each statement.

   • (i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.
   • (ii) Electrons can exist anywhere around the nucleus with no fixed energy.
   • (iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy. (iv)…
3. 3.
   The composition of the nuclei of three atomic species X, Y, and Z are given as follows. X: protons = 18, neutrons = 19. Y: protons = 17, neutrons = 18. Z: protons = 17, neutrons = 20. Explain the relation between the following:

   • (i) Y and Z,
   • (ii) Z and X.
4. 4.
   What conclusion did Rutherford draw about the position and characteristics of the atom's positively charged part based on the few alpha particles that bounced back or were deflected at large angles in the gold foil experiment?
5. 5.
   Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time.

   • (i) Bohr's model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.
   • (ii) Thomson's model depicted the atom as a 'plum pudding' with electrons embedded in a sphere of positive charge.
   • (iii) Rutherford's model proposed that…
6. 6.
   Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
7. 7.
   Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure. Reason (R): The number of electrons is equal to the number of protons in an atom. Choose the correct option:

   • (i) Both A and R are true, and R is the correct explanation of A.
   • (ii) Both A and R are true, but R is not the correct explanation of A.
   • (iii) A is true, but R is false.
   • (iv) A is false, but…
8. 8.
   Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of

   • (i) protons,
   • (ii) neutrons,
   • (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
9. 9.
   Find the following information for the elements shown in Fig. 8.17 (four atomic diagrams labelled a, b, c, d):

   • (i) Name of the element,
   • (ii) Symbol,
   • (iii) Total number of electrons,
   • (iv) Number of valence electrons,
   • (v) Valency of the element,
   • (vi) Number of protons,
   • (vii) Atomic number.
10. 10.
    Both Rutherford's and Bohr's models have electrons orbiting the nucleus. Why did Rutherford's model fail to explain atomic stability, while Bohr's model succeeded?
11. 11.
    An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?
12. 12.
    An atom has 79 protons and a mass number of 197. Calculate

    • (i) the number of neutrons, and
    • (ii) the number of electrons.
13. 13.
    Complete the Table 8.5 (columns: Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of element). Given rows: Row 1: Z=5, neutrons=6. Row 2: A=14, electrons=7, name=Nitrogen. Row 3: A=24, protons=12. Row 4: Z=15, neutrons=16. Row 5: A=1, neutrons=0.
14. 14.
    Aman was discussing the structure of atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35 and contains 18 neutrons. Based on this information, answer the following questions:

    • (i) How many electrons and protons does element X have?
    • (ii) What is its atomic number?
    • (iii) Identify the element X.
    • (iv) Write its electronic configuration.
    • (v) How many…
15. 15.
    In an atom, there are 12 protons and 12 neutrons in the nucleus. Now, imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500 times heavier. What effect will this replacement have on the atom's:

    • (i) Atomic number,
    • (ii) Atomic mass,
    • (iii) Mass number,
    • (iv) Overall charge.