Questions and Answers
1. Compare the properties of electrons, protons, and neutrons.
| Property | Electron | Proton | Neutron |
|---|
| Charge | Negative (-1) | Positive (+1) | Neutral (0) |
| Mass | 1/1836 times the mass of proton | 1 atomic mass unit (amu) | 1 atomic mass unit (amu) |
| Location | Outside the nucleus | In the nucleus | In the nucleus |
| Symbol | e⁻ | p⁺ | n |
2. What are the limitations of J.J. Thomson’s model of the atom?
- It did not explain the results of Rutherford's gold foil experiment.
- It did not account for the existence of a dense nucleus in the atom.
- It did not explain atomic spectra and the behavior of electrons in different orbits.
3. What are the limitations of Rutherford’s model of the atom?
- It could not explain the stability of the atom. According to classical physics, electrons moving in circular orbits would lose energy and spiral into the nucleus.
- It could not explain the line spectra of elements.
- It did not account for the fixed orbits of electrons.
4. Describe Bohr’s model of the atom.
- Bohr proposed that electrons move in fixed, quantized orbits around the nucleus.
- Electrons in these orbits do not radiate energy, which prevents them from spiraling into the nucleus.
- The energy of an electron is quantized and each orbit corresponds to a specific energy level.
- This model successfully explained the spectral lines of hydrogen.
5. Compare all the proposed models of an atom given in this chapter.
| Aspect | Thomson’s Model | Rutherford’s Model | Bohr’s Model |
|---|
| Atom Structure | Positive sphere with electrons inside | Nucleus with orbiting electrons | Nucleus with electrons in discrete orbits |
| Electron Behavior | Random distribution | Electrons orbit nucleus but with no defined orbits | Electrons move in fixed orbits with quantized energy |
| Atomic Stability | Unstable, couldn't explain stability | Could not explain electron stability | Explained the stability of the atom and electron orbits |
| Key Contribution | Discovery of electrons | Discovery of atomic nucleus | Explanation of electron energy levels and atomic spectra |
6. Summarise the rules for writing the distribution of electrons in various shells for the first eighteen elements.
- The maximum number of electrons in each shell is given by the formula 2n², where n is the shell number.
- The first shell (K-shell) holds a maximum of 2 electrons, the second (L-shell) holds 8, the third (M-shell) holds 18, and the fourth (N-shell) holds 32.
- Electrons fill the shells in a stepwise manner: K-shell first, then L-shell, and so on.
7. Define valency by taking examples of silicon and oxygen.
- Valency is the combining capacity of an atom, or the number of electrons an atom can lose, gain, or share to achieve a full outer shell.
- Silicon (Si) has 4 valence electrons (in its outermost shell), so its valency is 4.
- Oxygen (O) has 6 valence electrons, so its valency is 2 (it needs 2 electrons to complete its outer shell).
8. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes, and (iv) Isobars. Give any two uses of isotopes.
- Atomic Number (Z): The number of protons in an atom’s nucleus. It defines the element. Example: For carbon, Z = 6.
- Mass Number (A): The total number of protons and neutrons in the nucleus. Example: Carbon-12 has 6 protons and 6 neutrons, so A = 12.
- Isotopes: Atoms of the same element with different mass numbers due to different numbers of neutrons. Example: Carbon-12 and Carbon-14 are isotopes of carbon.
- Isobars: Atoms of different elements that have the same mass number but different atomic numbers. Example: Calcium-40 and Argon-40 are isobars.
- Uses of isotopes:
- Cobalt-60 is used in cancer treatment.
- Uranium-235 is used as fuel in nuclear reactors.
9. Na⁺ has completely filled K and L shells. Explain.
- Na⁺ is a sodium ion. The neutral sodium atom has 11 electrons, with the electron configuration 2, 8, 1. When it loses one electron to form Na⁺, it has the electron configuration 2, 8. The K-shell is filled with 2 electrons, and the L-shell is filled with 8 electrons, giving Na⁺ a stable, full outer shell.
10. If bromine atom is available in the form of, say, two isotopes 79⁶⁴Br (49.7%) and 81⁶⁴Br (50.3%), calculate the average atomic mass of bromine atom.
The average atomic mass = (mass of isotope 1 × % of isotope 1) + (mass of isotope 2 × % of isotope 2)
Average atomic mass = (79 × 0.497) + (81 × 0.503)
= 39.243 + 40.683
= 79.926 u
Thus, the average atomic mass of bromine is 79.926 u.
11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ¹⁶₈X and ¹⁸₈X in the sample?
Let the percentage of isotope ¹⁶₈X be x%.
Then, the percentage of isotope ¹⁸₈X will be (100 - x)%.
The average atomic mass is calculated as follows:
16x + 18(100 - x) = 16.2 × 100
Solving:
16x + 1800 - 18x = 1620
-2x = -180
x = 90%
Thus, the percentage of ¹⁶₈X is 90%, and the percentage of ¹⁸₈X is 10%.
12. If Z = 3, what would be the valency of the element? Also, name the element.
- The element with Z = 3 is Lithium (Li).
- Lithium has 3 electrons, and its electron configuration is 2, 1. It can lose 1 electron to achieve a stable configuration, so its valency is 1.
13. Composition of the nuclei of two atomic species X and Y are given as under:
| Species | Protons | Neutrons | Mass Number |
|---|
| X | 6 | 6 | 12 |
| Y | 6 | 8 | 14 |
- Mass number of X = 6 protons + 6 neutrons = 12
- Mass number of Y = 6 protons + 8 neutrons = 14
- Relation between X and Y: They are isotopes of the element carbon (same atomic number but different mass numbers).
14. 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. F
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. F
(c) The mass of an electron is about 1/2000 times that of a proton. T
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. T
15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
- (a) Atomic Nucleus ✓
- (b) Electron ×
- (c) Proton ×
- (d) Neutron ×
16. Isotopes of an element have
- (a) the same physical properties ×
- (b) different chemical properties ×
- (c) different number of neutrons ✓
- (d) different atomic numbers ×
17. Number of valence electrons in Cl⁻ ion are:
- (a) 16 ×
- (b) 8 ✓
- (c) 17 ×
- (d) 18 ×
18. 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 ×
