Class 9 › Science › Exploration Journey Inside the Atom Class 9 Science · Exploration · Chapter 8
Practice all → Summary Questions & answers Practise this chapter → This chapter traces how our picture of the atom developed and what atoms are made of. The idea of indivisible particles is ancient — Acharya Kanada called them parmanus and the Greeks called them atomos — but Dalton (1808) made it scientific. Then J. J. Thomson discovered the negatively charged electron from cathode rays and proposed the plum-pudding model (electrons embedded in a positive sphere). Rutherford’s gold-foil (alpha-scattering) experiment showed most of the atom is empty space with a tiny, dense, positively charged nucleus, giving the planetary model; he also identified the proton. Because an orbiting electron should radiate energy and spiral in, Rutherford’s model could not explain stability, so Bohr proposed fixed energy levels or shells (K, L, M, N… or n = 1, 2, 3…) in which electrons move without losing energy. Chadwick later discovered the neutral neutron, explaining atomic mass. The three subatomic particles are electrons (−1), protons (+1) and neutrons (0). Atomic number Z is the number of protons (it defines the element), and mass number A is protons plus neutrons (nucleons). Electrons fill shells by the Bohr–Bury rule (maximum \(2n^2\) per shell, outermost ≤ 8), giving the electronic configuration; a complete octet (or duplet for the K-shell) makes an atom stable, and valency is the combining capacity. Isotopes have the same Z but different A (e.g. protium, deuterium, tritium), and average atomic mass is the abundance-weighted mean of isotope masses (chlorine ≈ 35.5 u); isobars have the same A but different Z.
Atomic number (Z)
The number of protons in the nucleus of an atom; it identifies the element and equals the number of electrons in a neutral atom.
Mass number (A)
The total number of protons and neutrons (nucleons) in the nucleus of an atom.
Electron, proton, neutron
The three subatomic particles, with relative charges of −1, +1 and 0 respectively; protons and neutrons lie in the nucleus and electrons revolve around it.
Bohr’s shells (energy levels)
Fixed orbits K, L, M, N… (n = 1, 2, 3…) in which electrons move without losing energy, each holding up to \(2n^2\) electrons.
Valency
The combining capacity of an atom, equal to the number of electrons it gains, loses or shares to complete its octet.
Isotopes
Atoms of the same element with the same atomic number but different mass numbers, e.g. protium, deuterium and tritium of hydrogen.
Q1 What did Thomson discover and what model did he propose? Answer Q2 What did the gold-foil experiment reveal about the atom? Answer Q3 Why did Rutherford’s model fail to explain stability? Answer Q4 How did Bohr’s model account for stability? Answer Q5 Why was the neutron’s discovery important? Answer Q6 Differentiate atomic number and mass number. Answer Q7 State the rules for filling electrons in shells. Answer Q8 How is the valency of an element determined from its configuration? Answer Q9 What are isotopes? Give an example. Answer Q10 Why is the average atomic mass of chlorine 35.5 u? Answer Q1 Are atoms the smallest indivisible particles? Answer Q2 Why do electrons not fall into the nucleus even though they are attracted to protons in it? Answer Q3 Why did scientists keep modifying atomic models? Answer Q1 1. Suppose you made up your own ‘atom’, as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if: (i) the positive charge on the clay is lesser than the total negative charge of the beads? Answer Q2 1. Suppose you made up your own ‘atom’, as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if: (ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom? Answer Q3 2. Could an orange or a lemon, which also contain seeds inside soft pulp, be a good comparison? In what ways does it match Thomson’s idea and where does it fall short? Answer Q4 3. Why did Thomson conclude that electrons are present in all atoms? Answer Q1 Observe Fig. 8.4 of the gold foil experiment. Predict the observations you would expect if the gold foil in the experiment were made thicker. Also, draw a simple diagram to show the observations you expect. Hint: Compare thin foil vs thick foil. How does the thickness affect the chances of hitting a nucleus? Answer Q1 4. What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford’s gold foil experiment? Answer Q2 5. Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding model’ of the atom? Answer Q3 6. If you could ask Rutherford one question about his work, what would it be? Answer Q1 7. Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus. Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere. 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. Answer Q1 8. Imagine you are a scientist who has discovered a new element. Name this element after yourself and justify that the symbol you have chosen follows the IUPAC rules. Answer Q2 9. What problems could arise if every scientist used different symbols for the same element? Answer Q1 10. An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons. Answer Q2 11. The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it. Answer Q3 12. An atom has 18 neutrons and an atomic number of 17. What is its mass number? Answer Q4 13. An atom \(^{23}A\) has 11 electrons. Find the number of neutrons in it. Answer Q1 14. Identify the number of electrons in the outermost shell of the following elements: (i) \(^{12}_{6}C\) Answer Q2 14. Identify the number of electrons in the outermost shell of the following elements: (ii) \(^{19}_{9}F\) Answer Q3 14. Identify the number of electrons in the outermost shell of the following elements: (iii) \(^{28}_{14}Si\) Answer Q4 15. Write the electronic configuration of the elements having atomic numbers 12, 16 and 18. Answer Q5 16. Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I and how many neutrons do I have? You can also create one such riddle. Answer Q1 17. Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements? Answer Q2 18. If a bromine atom is available in the form of, say two isotopes, \(^{79}_{35}Br\) (49.7%) and \(^{81}_{35}Br\) (50.3%), calculate the average atomic mass of the bromine atom. Answer Q1 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. Answer Q2 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: (ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom. Answer Q3 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: (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. Answer Q4 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: (iv) The way alpha particles were deflected showed that electrons move around the nucleus. Answer Q5 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. Answer Q6 2. Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (ii) Electrons can exist anywhere around the nucleus with no fixed energy. Answer Q7 2. Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy. Answer Q8 2. Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (iv) Electrons can be found between energy levels as they move around the nucleus. Answer Q9 3. The composition of the nuclei of three atomic species X, Y, and Z are given as follows. X Y Z Number of protons 18 17 17 Number of neutrons 19 18 20 Explain the relation between the following: (i) Y and Z Answer Q10 3. The composition of the nuclei of three atomic species X, Y, and Z are given as follows. X Y Z Number of protons 18 17 17 Number of neutrons 19 18 20 Explain the relation between the following: (ii) Z and X Answer Q11 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? Answer Q12 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 atoms have a dense central nucleus. (iv) Dalton’s model described atoms as indivisible particles. Answer Q13 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. Answer Q14 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 R is true. Answer Q15 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, Answer Q16 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 (ii) neutrons, Answer Q17 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 (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom. Answer Q18 9. Find the following information for the elements shown in Fig. 8.17: (i) Name of the element Answer Q19 9. Find the following information for the elements shown in Fig. 8.17: (ii) Symbol Answer Q20 9. Find the following information for the elements shown in Fig. 8.17: (iii) Total number of electrons Answer Q21 9. Find the following information for the elements shown in Fig. 8.17: (iv) Number of valence electrons Answer Q22 9. Find the following information for the elements shown in Fig. 8.17: (v) Valency of the element Answer Q23 9. Find the following information for the elements shown in Fig. 8.17: (vi) Number of protons Answer Q24 9. Find the following information for the elements shown in Fig. 8.17: (vii) Atomic number Answer Q25 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? Answer Q26 11. An atom \(^{70}X\) has 31 electrons. How many neutrons are there in its nucleus? Answer Q27 12. An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, Answer Q28 12. An atom has 79 protons and a mass number of 197. Calculate (ii) the number of electrons. Answer Q29 13. Complete the Table 8.5: Atomic number 5, Mass number –, Number of neutrons 6, Number of protons –, Number of electrons –, Name of the elements – Answer Q30 13. Complete the Table 8.5: Atomic number –, Mass number 14, Number of neutrons –, Number of protons –, Number of electrons 7, Name of the elements Nitrogen Answer Q31 13. Complete the Table 8.5: Atomic number –, Mass number 24, Number of neutrons –, Number of protons 12, Number of electrons –, Name of the elements – Answer Q32 13. Complete the Table 8.5: Atomic number 15, Mass number –, Number of neutrons 16, Number of protons –, Number of electrons –, Name of the elements – Answer Q33 13. Complete the Table 8.5: Atomic number –, Mass number 1, Number of neutrons 0, Number of protons –, Number of electrons –, Name of the elements – Answer Q34 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? Answer Q35 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: (ii) What is its atomic number? Answer Q36 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: (iii) Identify the element X. Answer Q37 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: (iv) Write its electronic configuration. Answer Q38 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: (v) How many valence electrons does it have? Answer Q39 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: (vi) What will be the mass number if two neutrons are added to its nucleus? Answer Q40 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: (vii) What will be the relation of X with the new atom? Answer Q41 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 Answer Q42 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: (ii) Atomic mass Answer Q43 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: (iii) Mass number Answer Q44 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: (iv) Overall charge Answer
Which subatomic particle did J. J. Thomson discover?
A Nucleus B Proton C Neutron D Electron
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