NCERT Solutions for Class 9 Science Chapter 4 – Structure of the Atom
Class 9 Science Chapter 4 – Structure of the Atom takes students through the fascinating journey of how scientists gradually discovered the internal structure of the atom. The chapter covers the atomic models proposed by J.J. Thomson, Ernest Rutherford, and Niels Bohr, the discovery of electrons, protons, and neutrons, the concept of atomic number and mass number, isotopes and isobars, and the distribution of electrons in shells. It builds on the foundation laid in Chapter 3 and gives students a clear understanding of how modern atomic theory developed through a series of experiments and revisions.
This chapter is an important part of the CBSE Class 9 Science syllabus and is regularly tested in school examinations. Questions from this chapter cover atomic models, electronic configuration, isotopes, isobars, valency, and numerical problems based on atomic number and mass number. A strong understanding of this chapter is also essential for Class 10 and 11 Chemistry and for competitive examinations like JEE and NEET.
NCERT Solutions for Class 9 Science Chapter 4 – Structure of the Atom
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Question 1. What are canal rays?
Answer:
Canal rays, also called anode rays, were discovered by E. Goldstein in 1886. When high voltage electricity was passed through a discharge tube containing a perforated cathode, positively charged rays were observed moving from the anode towards the cathode and passing through the holes in the cathode. These rays were called canal rays because they passed through the canals or holes in the cathode. The discovery of canal rays led to the identification of the proton as a positively charged subatomic particle.
Question 2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer:
An atom that contains one electron and one proton will carry no charge — it will be electrically neutral. This is because the negative charge of the electron and the positive charge of the proton are equal and opposite, and they cancel each other out. The atom of hydrogen, which has one proton and one electron, is a perfect example of this electrically neutral state.
Question 3. On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
Answer:
According to Thomson's model, an atom is a positively charged sphere in which electrons are embedded like seeds in a watermelon or like plums in a pudding — which is why this model is also called the plum pudding model. Thomson proposed that the positive charge of the sphere is exactly equal to the total negative charge of all the embedded electrons. Since the positive and negative charges are equal and opposite, they cancel each other out, making the atom electrically neutral as a whole.
Question 4. On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?
Answer:
On the basis of Rutherford's model, the nucleus of an atom contains protons — the positively charged subatomic particles. Rutherford concluded from his gold foil experiment that the positive charge of an atom is concentrated in a very small, dense region at the centre of the atom called the nucleus. The electrons revolve around the nucleus in circular paths called orbits. It was later discovered that the nucleus also contains neutrons, but Rutherford's original model focused on the concentration of positive charge in the nucleus.
Question 5. Draw a sketch of Bohr's model of an atom with three shells.
Answer:
Bohr's model of an atom with three shells can be described as follows. At the centre is the nucleus containing protons and neutrons. Around the nucleus, electrons revolve in fixed circular paths called shells or energy levels, labelled K, L, and M. The K shell is the innermost and closest to the nucleus, followed by the L shell, and then the M shell which is the outermost of the three. Electrons in each shell have a fixed amount of energy, and they do not lose energy as long as they remain in that shell.
Question 6. What do you think would be the observation if the alpha particle scattering experiment is carried out using a foil of a metal other than gold?
Answer:
If the alpha particle scattering experiment were carried out using a foil of another metal — such as silver, copper, or aluminium — the basic observations would remain similar, since the fundamental structure of the atom is the same for all elements. Most alpha particles would pass straight through the foil, a small number would be deflected at various angles, and a very few would bounce straight back. However, the extent of deflection might differ slightly depending on the atomic number and nuclear charge of the metal used. Gold was specifically chosen because it can be beaten into very thin foils without breaking, making the experiment easier to conduct.
Question 7. Name the three subatomic particles of an atom.
Answer:
The three subatomic particles of an atom are the electron, the proton, and the neutron. The electron was discovered by J.J. Thomson, the proton by Ernest Rutherford, and the neutron by James Chadwick in 1932. The electron carries a negative charge, the proton carries a positive charge, and the neutron carries no charge — it is electrically neutral. The proton and neutron are found in the nucleus of the atom, while electrons revolve around the nucleus in fixed energy levels called shells.
Question 8. Helium atom has an atomic mass of 4 u and has two protons in its nucleus. How many neutrons does it have?
Answer:
The atomic mass of helium = 4 u Number of protons = 2 Number of neutrons = Atomic mass – Number of protons = 4 – 2 = 2 neutrons
Therefore, the helium atom has 2 neutrons in its nucleus.
Question 9. Write the distribution of electrons in carbon and sodium atoms.
Answer:
The atomic number of carbon is 6, which means it has 6 electrons. Distribution: K shell = 2, L shell = 4 Electronic configuration of carbon: 2, 4
The atomic number of sodium is 11, which means it has 11 electrons. Distribution: K shell = 2, L shell = 8, M shell = 1 Electronic configuration of sodium: 2, 8, 1
Question 10. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer:
The maximum number of electrons in the K shell = 2 The maximum number of electrons in the L shell = 8 Total number of electrons = 2 + 8 = 10 electrons
An atom with completely filled K and L shells would have 10 electrons. This corresponds to the element neon (Ne), which has atomic number 10 and is a noble gas with a completely stable electronic configuration.
Question 11. How will you find the valency of chlorine, sulphur, and magnesium?
Answer:
The valency of an element is determined by the number of electrons in the outermost shell (valence shell). If the outermost shell has 1 to 4 electrons, the valency equals the number of valence electrons. If it has 5 to 8 electrons, the valency equals 8 minus the number of valence electrons.
Chlorine has atomic number 17. Electronic configuration: 2, 8, 7. Valence electrons = 7. Valency = 8 – 7 = 1
Sulphur has atomic number 16. Electronic configuration: 2, 8, 6. Valence electrons = 6. Valency = 8 – 6 = 2
Magnesium has atomic number 12. Electronic configuration: 2, 8, 2. Valence electrons = 2. Valency = 2
Question 12. What is the mass number? How is it calculated?
Answer:
The mass number of an atom is the total number of protons and neutrons present in the nucleus of the atom. It is denoted by the letter A.
Mass number (A) = Number of protons (Z) + Number of neutrons (N)
Since electrons have negligible mass, they are not included in the calculation of mass number. For example, the mass number of carbon is 12 because it has 6 protons and 6 neutrons (6 + 6 = 12).
Question 13. What are isotopes? Give two examples.
Answer:
Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons). Since isotopes have the same atomic number, they have the same chemical properties but different physical properties.
Two examples of isotopes are the isotopes of hydrogen — protium (¹H), deuterium (²H), and tritium (³H) — all having atomic number 1 but different mass numbers. The isotopes of carbon — carbon-12 (¹²C) and carbon-14 (¹⁴C) — both have atomic number 6 but differ in their number of neutrons.
Question 14. What are isobars? Give one example.
Answer:
Isobars are atoms of different elements that have the same mass number but different atomic numbers. Since they have different atomic numbers, they have different chemical properties and belong to different elements.
An example of isobars is calcium (Ca) and argon (Ar). Both have a mass number of 40 — calcium has atomic number 20 and argon has atomic number 18. They have the same mass number but are completely different elements.
Question 15. What was Rutherford's model of the atom? What were its drawbacks?
Answer:
Rutherford proposed his nuclear model of the atom in 1911 based on the results of his gold foil experiment. According to this model, the atom has a very small, dense, positively charged centre called the nucleus which contains most of the mass of the atom. The electrons revolve around the nucleus in circular orbits, much like planets revolve around the sun. Most of the atom is empty space.
The drawbacks of Rutherford's model were significant. According to classical physics, a charged particle moving in a circular path should continuously lose energy by emitting radiation and eventually spiral into the nucleus, making the atom unstable. However, atoms are known to be stable. Rutherford's model also could not explain the arrangement of electrons in the atom or the line spectrum of hydrogen.
FAQs – Chapter 4 Structure of the Atom
Q1. What is the difference between Thomson's and Rutherford's models of the atom? Thomson's model proposed that the atom is a uniform sphere of positive charge with electrons embedded in it like plums in a pudding. Rutherford's model, based on his gold foil experiment, proposed that the atom has a small, dense, positively charged nucleus at the centre with electrons revolving around it in circular orbits and most of the atom being empty space. Rutherford's model replaced Thomson's as it was supported by experimental evidence.
Q2. What is Bohr's model of the atom? Bohr's model proposed that electrons revolve around the nucleus in fixed circular paths called shells or orbits, each having a fixed energy level. Electrons do not emit energy as long as they remain in their orbit. Energy is emitted or absorbed only when an electron jumps from one orbit to another. Bohr's model successfully explained the stability of the atom and the line spectrum of hydrogen.
Q3. What is the difference between atomic number and mass number? The atomic number (Z) is the number of protons in the nucleus of an atom and determines the identity of the element. The mass number (A) is the total number of protons and neutrons in the nucleus. For a neutral atom, the atomic number also equals the number of electrons. The number of neutrons = mass number – atomic number.
Q4. Why are noble gases unreactive? Noble gases are unreactive because their outermost electron shells are completely filled. Helium has 2 electrons filling its K shell, while neon, argon, and other noble gases have 8 electrons in their outermost shell. A completely filled outermost shell gives the atom maximum stability, so it does not need to gain, lose, or share electrons with other atoms. This makes noble gases chemically inert.
Q5. What is the significance of isotopes in real life? Isotopes have many important applications. Carbon-14 is used in radiocarbon dating to determine the age of ancient organic materials. Uranium-235 is used as fuel in nuclear reactors. Cobalt-60 is used in cancer treatment through radiotherapy. Iodine-131 is used in the treatment of thyroid disorders. The isotopes of hydrogen — deuterium and tritium — are used in nuclear fusion research.