Chapter 4: Structure of the Atom - NCERT Solutions
In-Text Questions (Page No- 39)
Q1. What are canal rays?
Answer - Canal rays are streams of positively charged particles observed in discharge tubes.
They are composed of positive ions and move in the opposite direction to cathode rays (electrons).
They are composed of positive ions and move in the opposite direction to cathode rays (electrons).
Q2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer - If an atom contains one electron (negative charge) and one proton (positive charge), the charges cancel each other.
Hence, the atom will carry no net electric charge and will be electrically neutral.
Hence, the atom will carry no net electric charge and will be electrically neutral.
In-Text Questions (Page No- 41)
Q1. 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, the atom is like a positively charged sphere with electrons embedded in it. The positive charge of the sphere balances the negative charge of the electrons, making the atom electrically neutral overall.
Q2. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Answer - In Rutherford’s model, the nucleus contains protons, which are positively charged subatomic particles. Electrons orbit around the nucleus.
Q3. Draw a sketch of Bohr’s model of an atom with three shells.
Answer - Bohr's model shows a small, dense nucleus surrounded by electrons moving in three distinct circular orbits or shells. The first shell holds 2 electrons, the second holds 8, and the third shell holds electrons depending on the atom. (Visual sketch not included here.)
Q4. What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer - If α-particles are scattered using a foil of another metal, the basic observations would be similar because the nucleus is dense and positively charged in all atoms. However, the scattering pattern might vary slightly depending on the metal’s atomic number and thickness of the foil.
In-Text Questions (Page No- 41)
Q1. Name the three sub-atomic particles of an atom.
Answer - The three sub-atomic particles of an atom are protons (positively charged), neutrons (neutral), and electrons (negatively charged).
Q2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer - The number of neutrons in helium = Atomic mass - Number of protons = 4 - 2 = 2 neutrons.
In-Text Questions (Page No- 42)
Q1. Write the distribution of electrons in carbon and sodium atoms.
Answer - Electron distribution in carbon (atomic number 6): 2 electrons in K shell, 4 electrons in L shell.
Electron distribution in sodium (atomic number 11): 2 electrons in K shell, 8 electrons in L shell, 1 electron in M shell.
Electron distribution in sodium (atomic number 11): 2 electrons in K shell, 8 electrons in L shell, 1 electron in M shell.
Q2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer - K shell can hold 2 electrons and L shell can hold 8 electrons. So, total number of electrons = 2 + 8 = 10 electrons.
In-Text Questions (Page No- 44)
Q1. How will you find the valency of chlorine, sulphur and magnesium?
Answer - Valency of an element is the combining capacity of its atoms and is usually equal to the number of electrons an atom can lose, gain, or share to complete its outermost shell.
Chlorine: Atomic number = 17; electronic configuration = 2, 8, 7. It needs 1 electron to complete its outermost shell. So, valency = 1.
Sulphur: Atomic number = 16; electronic configuration = 2, 8, 6. It needs 2 electrons to complete its outermost shell. So, valency = 2.
Magnesium: Atomic number = 12; electronic configuration = 2, 8, 2. It tends to lose 2 electrons to attain stable configuration. So, valency = 2.
Chlorine: Atomic number = 17; electronic configuration = 2, 8, 7. It needs 1 electron to complete its outermost shell. So, valency = 1.
Sulphur: Atomic number = 16; electronic configuration = 2, 8, 6. It needs 2 electrons to complete its outermost shell. So, valency = 2.
Magnesium: Atomic number = 12; electronic configuration = 2, 8, 2. It tends to lose 2 electrons to attain stable configuration. So, valency = 2.
In-Text Questions (Page No- 44)
Q1. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? and (ii) what is the charge on the atom?
Answer - (i) The atomic number is 8, since atomic number equals the number of protons in an atom.
(ii) The atom has no charge because the number of electrons (negative charge) equals the number of protons (positive charge), so charges cancel out.
(ii) The atom has no charge because the number of electrons (negative charge) equals the number of protons (positive charge), so charges cancel out.
Q2. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Answer - The mass number is the sum of protons and neutrons in the nucleus.
Oxygen: Atomic number = 8, Neutrons = 8, so mass number = 8 + 8 = 16.
Sulphur: Atomic number = 16, Neutrons = 16, so mass number = 16 + 16 = 32.
Oxygen: Atomic number = 8, Neutrons = 8, so mass number = 8 + 8 = 16.
Sulphur: Atomic number = 16, Neutrons = 16, so mass number = 16 + 16 = 32.
In-Text Questions (Page No- 45)
Q1. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Answer -
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| H (Hydrogen) | 1 | 0 | 1 |
| D (Deuterium) | 1 | 1 | 1 |
| T (Tritium) | 1 | 2 | 1 |
Q2. Write the electronic configuration of any one pair of isotopes and isobars.
Answer - Isotopes have the same number of protons but different numbers of neutrons. For example, Hydrogen-1 (H) and Deuterium (D) both have 1 electron: electronic configuration is 1.
Isobars have the same mass number but different atomic numbers. For example, Argon-40 (18 protons, 22 neutrons) and Calcium-40 (20 protons, 20 neutrons). Electronic configurations:
Argon: 2, 8, 8
Calcium: 2, 8, 8, 2.
Isobars have the same mass number but different atomic numbers. For example, Argon-40 (18 protons, 22 neutrons) and Calcium-40 (20 protons, 20 neutrons). Electronic configurations:
Argon: 2, 8, 8
Calcium: 2, 8, 8, 2.
Exercises
Q1. Compare the properties of electrons, protons and neutrons.
Answer -
| Property | Electron | Proton | Neutron |
|---|---|---|---|
| Charge | Negative (-1) | Positive (+1) | Neutral (0) |
| Mass | Very small (9.11 × 10⁻³¹ kg) | Much heavier than electron (1.67 × 10⁻²⁷ kg) | Similar to proton (1.67 × 10⁻²⁷ kg) |
| Location in atom | Outside nucleus (in electron cloud) | Inside nucleus | Inside nucleus |
| Role | Determines chemical properties and bonding | Determines atomic number and identity | Contributes to atomic mass and stability |
Q2. What are the limitations of J.J. Thomson’s model of the atom?
Answer - J.J. Thomson's model, also called the 'plum pudding' model, suggested that electrons were scattered within a positively charged sphere. However, it had limitations: (1) It could not explain how electrons are arranged or why atoms are stable. (2) It did not account for the existence of the nucleus. (3) It failed to explain the results of Rutherford's gold foil experiment.
Q3. What are the limitations of Rutherford’s model of the atom?
Answer - Rutherford's model proposed a nucleus with electrons orbiting around it, but it had these limitations: (1) It could not explain why electrons do not spiral into the nucleus due to attraction. (2) It did not explain the stability of atoms. (3) It could not explain the discrete lines observed in atomic spectra.
Q4. Describe Bohr’s model of the atom.
Answer - Bohr's model states that electrons move in fixed circular orbits or shells around the nucleus without radiating energy. Each orbit corresponds to a specific energy level. Electrons can jump between orbits by absorbing or emitting specific amounts of energy (quanta). This explained atomic stability and spectral lines.
Q5. Compare all the proposed models of an atom given in this chapter.
Answer -
| Model | Key Features | Limitations |
|---|---|---|
| Thomson's Model | Electrons embedded in a positive sphere (plum pudding) | No nucleus; could not explain atom stability |
| Rutherford's Model | Small dense positively charged nucleus; electrons orbit nucleus | Could not explain electron stability; why electrons don't fall into nucleus |
| Bohr's Model | Electrons in fixed energy orbits; explained atomic spectra | Only explained hydrogen-like atoms; did not fully explain electron behavior |
Q6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Answer - The rules for electron distribution in shells (K, L, M...) are:
1. The maximum number of electrons in a shell is given by the formula 2n², where 'n' is the shell number.
- K shell (n=1): 2 electrons
- L shell (n=2): 8 electrons
- M shell (n=3): 18 electrons
2. Electrons are filled in shells in order of increasing energy (starting from K).
3. The outermost shell can have a maximum of 8 electrons, even if the next shell has space.
4. The next shell is not started until the previous one is completely filled (except for the outermost shell, which can have fewer than 8 electrons in some cases).
Q7. Define valency by taking examples of silicon and oxygen.
Answer - Valency is the combining capacity of an element. It is determined by the number of electrons in the outermost shell or the number required to complete the octet.
- Silicon (Atomic number 14): Electron configuration is 2, 8, 4. It has 4 valence electrons, so its valency is 4.
- Oxygen (Atomic number 8): Electron configuration is 2, 6. It needs 2 more electrons to complete its octet, so its valency is 2.
Q8. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
Answer - (i) Atomic number: Number of protons in the nucleus of an atom. E.g., Carbon has atomic number 6.
(ii) Mass number: Sum of protons and neutrons in the nucleus. E.g., Mass number of carbon-12 is 12 (6 protons + 6 neutrons).
(iii) Isotopes: Atoms of the same element with the same atomic number but different mass numbers. E.g., 12C and 14C.
(iv) Isobars: Atoms of different elements with the same mass number but different atomic numbers. E.g., 40Ar and 40Ca.
Uses of isotopes:
1. An isotope of iodine is used to treat goitre.
2. Isotopes are used as tracers in medical and industrial applications.
(ii) Mass number: Sum of protons and neutrons in the nucleus. E.g., Mass number of carbon-12 is 12 (6 protons + 6 neutrons).
(iii) Isotopes: Atoms of the same element with the same atomic number but different mass numbers. E.g., 12C and 14C.
(iv) Isobars: Atoms of different elements with the same mass number but different atomic numbers. E.g., 40Ar and 40Ca.
Uses of isotopes:
1. An isotope of iodine is used to treat goitre.
2. Isotopes are used as tracers in medical and industrial applications.
Q9. Na⁺ has completely filled K and L shells. Explain.
Answer - Sodium (Na) has atomic number 11 and electronic configuration 2, 8, 1. When it loses one electron to form Na⁺, the configuration becomes 2, 8. Thus, both K and L shells are completely filled in Na⁺, making it stable.
Q10. If bromine atom is available in the form of, say, two isotopes ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of bromine atom.
Answer - Average atomic mass of bromine = (79 × 49.7/100) + (81 × 50.3/100) = 39.263 + 40.743 = 80.006 u
Q11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ₈¹⁶X and ₈¹⁸X in the sample?
Answer - Let the percentage of ₈¹⁶X be x and that of ₈¹⁸X be (100 – x).
According to the question:
(16 × x + 18 × (100 – x)) / 100 = 16.2
⇒ (16x + 1800 – 18x) / 100 = 16.2
⇒ (–2x + 1800) = 1620
⇒ –2x = –180
⇒ x = 90
So, percentage of ₈¹⁶X = 90% and ₈¹⁸X = 10%
According to the question:
(16 × x + 18 × (100 – x)) / 100 = 16.2
⇒ (16x + 1800 – 18x) / 100 = 16.2
⇒ (–2x + 1800) = 1620
⇒ –2x = –180
⇒ x = 90
So, percentage of ₈¹⁶X = 90% and ₈¹⁸X = 10%
Q12. If Z = 3, what would be the valency of the element? Also, name the element.
Answer - Z = 3 means atomic number is 3, so the element is Lithium. Its electronic configuration is 2, 1. It has 1 electron in the outermost shell, so its valency is 1.
Q13. Composition of the nuclei of two atomic species X and Y are given as under: X → Protons = 6, Neutrons = 6; Y → Protons = 6, Neutrons = 8. Give the mass numbers of X and Y. What is the relation between the two species?
Answer - Mass number of X = 6 + 6 = 12
Mass number of Y = 6 + 8 = 14
Since both have same atomic number (protons) but different mass numbers, X and Y are isotopes.
Mass number of Y = 6 + 8 = 14
Since both have same atomic number (protons) but different mass numbers, X and Y are isotopes.
Q14. For the following statements, write T for True and F for False:
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer - (a) F
(b) F
(c) T
(d) F
(b) F
(c) T
(d) F
Q15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of:
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
Answer - ✔️ (a) Atomic Nucleus
✖️ (b) Electron
✖️ (c) Proton
✖️ (d) Neutron
✖️ (b) Electron
✖️ (c) Proton
✖️ (d) Neutron
Q16. Isotopes of an element have:
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.
Answer - ✖️ (a) the same physical properties
✖️ (b) different chemical properties
✔️ (c) different number of neutrons
✖️ (d) different atomic numbers
✖️ (b) different chemical properties
✔️ (c) different number of neutrons
✖️ (d) different atomic numbers
Q17. Number of valence electrons in Cl⁻ ion are:
(a) 16
(b) 8
(c) 17
(d) 18
(a) 16
(b) 8
(c) 17
(d) 18
Answer - ✔️ (b) 8
✖️ (a) 16
✖️ (c) 17
✖️ (d) 18
✖️ (a) 16
✖️ (c) 17
✖️ (d) 18
Q18. Which one of the following is a correct electronic configuration of sodium?
(a) 2,8
(b) 8,2,1
(c) 2,1,8
(d) 2,8,1
(a) 2,8
(b) 8,2,1
(c) 2,1,8
(d) 2,8,1
Answer - ✔️ (d) 2,8,1
✖️ (a) 2,8
✖️ (b) 8,2,1
✖️ (c) 2,1,8
✖️ (a) 2,8
✖️ (b) 8,2,1
✖️ (c) 2,1,8
Q19. Complete the following table:
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Name of Atomic Species |
|---|---|---|---|---|---|
| 9 | - | 10 | - | - | - |
| 16 | 32 | - | - | - | Sulphur |
| - | 24 | - | 12 | - | - |
| - | 2 | - | 1 | - | - |
| - | 1 | 0 | 1 | 0 | - |
Answer -
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Name of Atomic Species |
|---|---|---|---|---|---|
| 9 | 19 | 10 | 9 | 9 | Fluorine |
| 16 | 32 | 16 | 16 | 16 | Sulphur |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 1 | 2 | 1 | 1 | 1 | Hydrogen (Deuterium) |
| 1 | 1 | 0 | 1 | 0 | Hydrogen (Protium Ion) |