1Revise, Reflect, Refine15 questions
Q.1Choose 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 that most of the mass of the atom and positive charge are packed into a tiny centre. (iv) The way alpha particles were deflected showed that electrons move around the nucleus.v
Answer:(ii) and (iii) are correct.
Q.2Which 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) Electrons can be found between energy levels as they move around the nucleus.v
Q.3The composition of the nuclei of three atomic species X, Y, and Z are given as follows. X Y Z 17 17 18 20 Explain the relation between the following: (i) Y and Z (ii) Z and Xv
Answer:(i) Y and Z are isotopes. (ii) Z and X are isobars.
Q.4What 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?v
Answer:Rutherford concluded that the atom’s positive charge and most of its mass are concentrated in a very small, dense central nucleus, while most of the atom is empty space.
Q.5Explain 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 ʻplum puddingʼ with electrons embedded in a sphere of positive charge. (iii) Rutherford’s model proposed that atoms have a dense central nucleus. (iv) Dalton’s model described atoms as indivisible particles.v
Answer:Chronological order: Dalton’s indivisible atom model, Thomson’s plum-pudding model, Rutherford’s nuclear model, and then Bohr’s fixed-energy orbit model.
Q.6Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.v
Answer:Electrons are negatively charged and the nucleus is positively charged, so electrostatic attraction keeps electrons bound to the atom instead of flying away.
Q.7Assertion (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 R is true.v
Answer:(ii) Both A and R are true, but R is not the correct explanation of A.
Q.8Magnesium 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.v
Answer:Protons = 12, neutrons = 12, electrons = 12; electronic configuration = 2, 8, 2.
Q.9Find the following information for the elements shown in Fig. 8.17: (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 (a) (b) (c) (d) Fig. 8.17v
Answer:Fig. 8.17 corresponds to: lithium Li, 3 electrons, 1 valence electron, valency 1, 3 protons, atomic number 3; nitrogen N, 7 electrons, 5 valence electrons, valency 3, 7 protons, atomic number 7; aluminium Al, 13 electrons, 3 valence electrons, valency 3, 13 protons, atomic number 13; fluorine F, 9 electrons, 7 valence electrons, valency 1, 9 protons, atomic number 9.
Q.10Both 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?v
Answer:Rutherford’s orbiting electrons should continuously lose energy and fall into the nucleus, so his model could not explain stability. Bohr proposed fixed energy levels where electrons do not radiate energy, making atoms stable.
Q.11electrons. How many neutrons are there in its nucleus?v
Q.12protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.v
Answer:(i) 118 neutrons (ii) 79 electrons.
Q.13Complete Table 8.5: Atomic number, mass number, number of neutrons, number of protons, number of electrons and name of the element for the given entries.v
Answer:Boron: Z=5, A=11, neutrons 6, protons 5, electrons 5. Nitrogen: Z=7, A=14, neutrons 7, protons 7, electrons 7. Magnesium: Z=12, A=24, neutrons 12, protons 12, electrons 12. Phosphorus: Z=15, A=31, neutrons 16, protons 15, electrons 15. Hydrogen: Z=1, A=1, neutrons 0, protons 1, electrons 1.
Q.14Aman 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 valence electrons does it have? (vi) What will be the mass number if two neutrons are added to its nucleus? (vii) What will be the relation of X with the new atom?v
Answer:(i) Electrons 17; protons 17. (ii) Atomic number 17. (iii) Chlorine. (iv) 2, 8, 7. (v) 7 valence electrons. (vi) Mass number 37. (vii) Isotopes.
Q.15protons 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 chargev
Answer:Atomic number remains 12 because proton number is unchanged. Atomic mass increases slightly because the replacement particles are much heavier than electrons. Mass number remains 24 because it counts only protons and neutrons. Overall charge remains neutral if 12 negative particles replace 12 electrons.