Journey Inside the Atom — Complete
Think It Over
Q1. Are atoms the smallest indivisible particles?
Answer:
No, atoms are not indivisible. Later discoveries showed that atoms are composed of even smaller subatomic particles – electrons, protons, and neutrons. The atom itself can be divided into these particles.
Q2. Why do electrons not fall into the nucleus even though they are attracted to protons in it?
Answer:
This was a major puzzle that Rutherford's model could not explain. Bohr solved it by proposing that electrons revolve in fixed energy shells (orbits). In these allowed orbits, electrons do not lose energy and do not spiral into the nucleus. They are stable due to the balance between electrostatic attraction and their motion.
Q3. Why did scientists keep modifying atomic models?
Answer:
Scientists kept modifying atomic models because new experimental evidence emerged that could not be explained by existing models. Science is a dynamic process – as new discoveries are made (like the gold foil experiment, discovery of neutrons, etc.), models are refined and improved to better match observations.
Pause and Ponder
Q1. 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? (ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?
Answer:
(i) If the positive charge is lesser than the negative charge, the atom would have a net negative charge. It would be an ion, not a neutral atom.
(ii) If the clay itself carries negative charge, the total negative charge would increase further, making the atom even more negatively charged. It would not represent a neutral atom.
A neutral atom requires equal positive and negative charges to balance each other.
Q2. 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:
Matches Thomson's idea: The pulp (soft matter) represents the positive charge spread throughout, and seeds represent electrons embedded in it – similar to the plum pudding model.
Falls short: In an atom, the positive charge is not uniformly spread – it is concentrated in the nucleus (as Rutherford later showed). The orange/lemon model cannot explain why alpha particles bounced back sharply.
Q3. Why did Thomson conclude that electrons are present in all atoms?
Answer:
Thomson observed that cathode rays (streams of electrons) were produced regardless of the material of the cathode or the gas filled in the tube. This showed that electrons are a fundamental component of all atoms, present in every element.
Q4. What do you think would happen if alpha particles were replaced with negatively charged particles in Rutherford's gold foil experiment?
Answer:
If negatively charged particles were used instead of alpha particles (which are positive), they would be attracted to the positively charged nucleus instead of being repelled. They would bend towards the nucleus, and many would be absorbed or captured by the gold atoms. The scattering pattern would be completely different.
Q5. Rutherford found that a few alpha particles bounced back sharply. How does this single surprising result completely rule out Thomson's 'plum pudding model' of the atom?
Answer:
If Thomson's model were correct, the positive charge would be spread evenly throughout the atom. Alpha particles (positive) would pass straight through or be deflected only slightly. The fact that some alpha particles bounced back (almost 180°) means they must have hit a dense, concentrated positive mass – the nucleus. A spread-out positive charge could not produce such a sharp deflection.
Q6. If you could ask Rutherford one question about his work, what would it be?
Answer:
Sample question: "Sir, if the nucleus is so dense and positively charged, what holds the protons together in such a tiny space despite their repulsion?"
This question relates to the discovery of the nuclear force and the neutron, which Rutherford did not fully explain.
Q7. 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:
Answer: (ii) Both A and R are true, but R is not the correct explanation of A.
Rutherford concluded that mass is concentrated in the nucleus based on the gold foil experiment results (alpha particles bouncing back). Thomson's model is a separate historical model and does not explain Rutherford's conclusion.
Q8. 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:
Sample answer: Let the element be named Sharmium (after Sharma).
Symbol: Sh (First letter capital S, second letter small h).
Justification: The symbol follows IUPAC rules – it is derived from the name, starts with a capital letter, and has a second letter in lowercase.
Q9. What problems could arise if every scientist used different symbols for the same element?
Answer:
If every scientist used different symbols:
- There would be confusion and miscommunication in scientific work.
- Research papers, textbooks, and chemical formulas would be inconsistent and difficult to understand.
- International collaboration would be nearly impossible.
- It would be difficult to standardise chemical knowledge globally.
Q10. An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.
Answer:
Given: Atomic number (Z) = 26, Mass number (A) = 56
Protons: Z = 26 protons
Electrons: In a neutral atom, electrons = protons = 26
Neutrons: A = p⁺ + n − 56 = 26 + n → n = 30
So, p⁺ = 26, e⁻ = 26, n = 30
Q11. The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
Answer:
Given: Protons (p⁺) = 20, Mass number (A) = 41
A = p⁺ + n − 41 = 20 + n → n = 21
Q12. An atom has 18 neutrons and an atomic number of 17. What is its mass number?
Answer:
Given: Neutrons (n) = 18, Atomic number (Z) = 17 (protons)
A = p⁺ + n = 17 + 18 = 35
Q13. An atom ²³A has 11 electrons. Find the number of neutrons in it.
Answer:
Given: Mass number (A) = 23, Electrons = 11
In a neutral atom, protons = electrons = 11
Neutrons = Mass number − Protons = 23 − 11 = 12
Q14. Identify the number of electrons in the outermost shell of the following elements: (i) ¹²₆C (ii) ¹⁹₉F (iii) ²⁸₁₄Si
Answer:
(i) ¹²₆C (Carbon): Atomic number = 6. Electronic configuration: 2, 4. Valence electrons = 4
(ii) ¹⁹₉F (Fluorine): Atomic number = 9. Electronic configuration: 2, 7. Valence electrons = 7
(iii) ²⁸₁₄Si (Silicon): Atomic number = 14. Electronic configuration: 2, 8, 4. Valence electrons = 4
Q15. Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Answer:
Atomic number 12 (Magnesium): 2, 8, 2
Atomic number 16 (Sulfur): 2, 8, 6
Atomic number 18 (Argon): 2, 8, 8
Q16. 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:
Answer: The element is Sodium (Na).
Neutrons = Mass number − Protons = 23 − 11 = 12
Riddle created: "I have 6 protons and 6 neutrons. I am found in all living things. Who am I?"
Answer: Carbon (¹²₆C)
Q17. 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:
Atomic number: Both have 11 protons, so both have atomic number Z = 11 – they are the same element (sodium).
Mass numbers:
- First atom: A = 11 + 12 = 23
- Second atom: A = 11 + 13 = 24
They are isotopes of sodium.
Q18. If a bromine atom is available in the form of, say two isotopes, ₇₉Br (49.7%) and ₈₁Br (50.3%), calculate the average atomic mass of the bromine atom.
Answer:
Average atomic mass = (mass₁ × abundance₁ + mass₂ × abundance₂) / 100
= (79 × 49.7 + 81 × 50.3) / 100
= (3926.3 + 4074.3) / 100
= 8000.6 / 100 = 80.006 u
So, the average atomic mass of bromine is approximately 80 u.
Revise, Reflect, Refine
Q1. 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 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.
Answer:
Correct options: (ii) and (iii)
Explanation:
- (i) Incorrect: The gold foil experiment did not show neutrons. Neutrons were discovered by Chadwick in 1932, much later.
- (ii) Correct: Rutherford's results disproved Thomson's plum pudding model and established the nuclear model.
- (iii) Correct: The large deflection of some alpha particles indicated a dense, positively charged nucleus with most of the mass.
- (iv) Incorrect: The deflection of alpha particles showed the presence of a positive nucleus, not the motion of electrons.
Q2. 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) Electrons can be found between energy levels as they move around the nucleus.
Answer:
(i) Incorrect: In Bohr's model, electrons do not lose energy while in fixed orbits – they remain stable.
(ii) Incorrect: Electrons have fixed energy levels and cannot exist anywhere; they occupy specific shells.
(iii) Correct: Electrons revolve in fixed orbits (shells) with definite energy and do not lose energy.
(iv) Incorrect: Electrons cannot exist between energy levels – they can only jump from one shell to another.
Q3. The composition of the nuclei of three atomic species X, Y, and Z are given as follows: Explain the relation between the following: (i) Y and Z (ii) Z and X
Answer:
Table – Composition of Nuclei (X, Y, Z)
(i) Y and Z – Isotopes
Y and Z have the same atomic number (17) but different mass numbers (35 and 37). Therefore, they are isotopes of chlorine (Cl-35 and Cl-37).
(ii) Z and X – Isobars
Z and X have the same mass number (37) but different atomic numbers (17 and 18). Therefore, they are isobars (Chlorine-37 and Argon-37).
Q4. 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:
Rutherford concluded that:
- The positive charge of an atom is concentrated in a tiny central region called the nucleus.
- The nucleus is dense and contains most of the mass of the atom.
- Most of the atom is empty space (since most alpha particles passed through undeflected).
Q5. 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:
Correct chronological order:
- (iv) Dalton's model – atoms as indivisible particles (1808).
- (ii) Thomson's model – plum pudding model (1897).
- (iii) Rutherford's model – dense central nucleus (1911).
- (i) Bohr's model – fixed energy orbits (1913).
Q6. 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:
Electrons are attracted to the positively charged nucleus due to the electrostatic force of attraction (opposite charges attract). This force acts as the centripetal force that keeps electrons in their orbits. The balance between the electrostatic attraction and the electron's motion prevents it from flying away.
Q7. 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:
Answer: (ii) Both A and R are true, but R is not the correct explanation of A.
The discovery of subatomic particles (electrons, protons, neutrons) indeed helped understand atomic structure. The equality of protons and electrons in a neutral atom is true, but it is not the reason why subatomic particles helped understand atomic structure.
Q8. 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.
Answer:
Given: Z = 12, A = 24
(i) Protons: Z = 12
(ii) Neutrons: A − Z = 24 − 12 = 12
(iii) Electrons: In a neutral atom, electrons = protons = 12
Electronic configuration: 2, 8, 2
Q9. Find 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
Answer:
Here is the complete information for each element shown in Fig. 8.17:
Element (a)
- (i) Name: Lithium
- (ii) Symbol: Li
- (iii) Total electrons: 3
- (iv) Valence electrons: 1
- (v) Valency: 1
- (vi) Number of protons: 3
- (vii) Atomic number: 3
Element (b)
- (i) Name: Nitrogen
- (ii) Symbol: N
- (iii) Total electrons: 7
- (iv) Valence electrons: 5
- (v) Valency: 3 (since 8 − 5 = 3)
- (vi) Number of protons: 7
- (vii) Atomic number: 7
Element (c)
- (i) Name: Aluminium
- (ii) Symbol: Al
- (iii) Total electrons: 13
- (iv) Valence electrons: 3
- (v) Valency: 3
- (vi) Number of protons: 13
- (vii) Atomic number: 13
Element (d)
- (i) Name: Fluorine
- (ii) Symbol: F
- (iii) Total electrons: 9
- (iv) Valence electrons: 7
- (v) Valency: 1
- (vi) Number of protons: 9
- (vii) Atomic number: 9
Q10. 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:
Rutherford's model failed because according to classical physics, an accelerating charged particle (electron in circular motion) should radiate energy and spiral into the nucleus, making atoms unstable.
Bohr's model succeeded because he introduced the concept of stationary states (fixed orbits) – electrons in these orbits do not radiate energy and remain stable. Electrons can only jump between orbits by absorbing or releasing energy.
Q11. An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?
Answer:
Given: Mass number (A) = 70, Electrons = 31
In a neutral atom, protons = electrons = 31
Neutrons = A − p = 70 − 31 = 39
Q12. An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
Answer:
(i) Neutrons: A − p = 197 − 79 = 118
(ii) Electrons: In a neutral atom, electrons = protons = 79
Q13. Complete the Table 8.5:
Answer:
Q14. 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 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?
Answer:
(i) Protons = Mass number − Neutrons = 35 − 18 = 17. Electrons = 17 (neutral atom).
(ii) Atomic number = Number of protons = 17.
(iii) Element with Z = 17 is Chlorine (Cl).
(iv) Electronic configuration: 2, 8, 7.
(v) Valence electrons = 7.
(vi) New mass number = 35 + 2 = 37.
(vii) The new atom (³⁷Cl) is an isotope of chlorine (same atomic number, different mass number).
Q15. 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
Answer:
(i) Atomic number: No change – atomic number depends only on protons.
(ii) Atomic mass: Increases – because electrons become 500 times heavier.
(iii) Mass number: No change – mass number counts protons and neutrons only (electrons are not counted in mass number).
(iv) Overall charge: No change – charge depends on the balance of positive and negative charges, and the hypothetical particles have the same charge as electrons.
The Journey Beyond
Q1. Create an 'Atomic Prediction Board' game based on atomic number, mass number, number of electrons, protons, neutrons and valency. Students may predict elements using atomic clues.
Answer:
This is a project/game activity.
Game Idea – "Atomic Clues":
- Provide clues like: "I have 6 protons, 6 neutrons, and 6 electrons."
- Players guess the element (Carbon, ¹²₆C).
- Include valency clues: "I have 4 valence electrons and can form 4 bonds."
- Players earn points for correct answers.
Q2. Prepare a report on how the properties of atoms impact us in everyday life across fields, such as healthcare, energy, agriculture and technology.
Answer:
This is a research/project activity.
Key points to include:
- Healthcare: Radioactive isotopes (⁶⁰₂₇Co for cancer, ¹³¹I for thyroid).
- Energy: ²³⁵₉₂U in nuclear power plants.
- Agriculture: Isotopes for tracking plant growth, pest control.
- Technology: Semiconductors (silicon, germanium) in electronics.
Q3. Create a role-play, stage play or story about the Journey Inside the Atom, and the scientists who discovered and contributed to the identification of atomic structure.
Answer:
This is a creative project.
Suggested characters:
- John Dalton – indivisible atoms
- J.J. Thomson – electron
- Ernest Rutherford – nucleus
- Niels Bohr – fixed orbits
- James Chadwick – neutron
Q4. Use selected software or digital tools and try to create animations or simulations of various atomic models, and share them in the class.
Answer:
This is a digital activity.
Use PhET simulations:
- Rutherford Scattering: https://phet.colorado.edu/en/simulations/rutherford-scattering
- Isotopes and Atomic Mass: https://phet.colorado.edu/en/simulations/isotopes-and-atomic-mass
Q5. Watch a film or documentary about the structure of the atom and write a report answering the following questions: Which film or documentary did you watch, and what was its main idea or topic? What did the film or documentary teach you about the structure of the atom and the atomic model(s)? Which scientists were mentioned in the film or documentary, and what were their contributions? What part of the film or documentary did you find most interesting, and what question do you still have?
Answer:
This is a film/documentary review activity.
Suggested documentaries:
- "The Atom" (BBC)
- "Atom: The Illusion of Reality" (Jim Al-Khalili)
- "The Secrets of the Atom"
Q6. Draw a bar graph showing the number of electrons in each energy level for any three elements.
Answer:
Sample data:
- Carbon (C): K=2, L=4
- Neon (Ne): K=2, L=8
- Sodium (Na): K=2, L=8, M=1
Q7. To learn more about atoms, you can explore the links given below: https://phet.colorado.edu/en/simulations/rutherford-scattering https://phet.colorado.edu/en/simulations/isotopes-and-atomic-mass
Answer:
This is an online exploration activity.
Visit the PhET simulations to interactively explore Rutherford scattering and isotopes.
The Quest Continues
Q1. Is it possible to completely understand everything that happens inside an atom?
Answer:
This is a philosophical/scientific reflection question.
Science is an ongoing process. While we have learned a great deal about atomic structure, the quantum nature of particles means there are limits to what we can know (Heisenberg's uncertainty principle). New discoveries continue to emerge (e.g., quantum mechanics, subatomic particles like quarks). The journey of understanding the atom is far from over.