1. What are canal rays?
Canal rays are streams of positively charged particles (also known as positive ions) discovered by E. Goldstein in 1886. These rays were observed in a gas discharge tube and led to the discovery of protons.
2. If an atom contains one electron and one proton, will it carry any charge or not?
No, the atom will not carry any charge. Since the electron has a negative charge (-1) and the proton has a positive charge (+1), their charges cancel each other out, making the atom electrically neutral.
1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Thomson’s model of an atom, also known as the "plum pudding model," suggests that an atom consists of negatively charged electrons embedded in a positively charged sphere, like plums in a pudding. Since the total positive charge is equal to the total negative charge, the atom as a whole remains electrically neutral.
2. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
According to Rutherford’s model, the nucleus of an atom contains positively charged protons. Later, it was discovered that neutrons, which are neutral particles, are also present in the nucleus.
3. Draw a sketch of Bohr’s model of an atom with three shells.
- A small central nucleus containing protons and neutrons.
- Three concentric circular orbits (shells) around the nucleus.
- Electrons placed in these orbits (first shell: 2 electrons, second shell: 8 electrons, third shell: up to 8 electrons).
4. 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?
If a metal foil other than gold, such as silver or copper, were used, the observations would remain similar, provided the metal is thin enough. However, if the metal is too thick, α-particles may not penetrate properly. The conclusions—most of the atom being empty space, a small dense nucleus, and positive charge concentrated in the nucleus—would still hold true.
1. Name the three sub-atomic particles of an atom.
The three sub-atomic particles of an atom are:
- Protons (positively charged)
- Neutrons (neutral charge)
- Electrons (negatively charged)
2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
The number of neutrons in an atom can be calculated using the formula:
For helium:
So, the helium atom has 2 neutrons in its nucleus.
1. Write the distribution of electrons in carbon and sodium atoms.
- Carbon (Atomic number = 6): Electron distribution → K = 2, L = 4
- Sodium (Atomic number = 11): Electron distribution → K = 2, L = 8, M = 1
2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
The maximum number of electrons in each shell:
- K-shell can hold 2 electrons.
- L-shell can hold 8 electrons.
So, if both K and L shells are full, the total number of electrons in the atom would be 2 + 8 = 10 electrons.
1. How will you find the valency of chlorine, sulphur, and magnesium?
The valency of an element is determined by the number of electrons an atom needs to gain, lose, or share to achieve a stable electronic configuration (usually a full outer shell like the noble gases).
-
Chlorine (Atomic number = 17)
- Electron distribution: K = 2, L = 8, M = 7
- It needs 1 more electron to complete its octet.
- Valency = 1 (gains 1 electron).
-
Sulphur (Atomic number = 16)
- Electron distribution: K = 2, L = 8, M = 6
- It needs 2 more electrons to complete its octet.
- Valency = 2 (gains 2 electrons).
-
Magnesium (Atomic number = 12)
- Electron distribution: K = 2, L = 8, M = 2
- It loses 2 electrons to achieve a stable configuration.
- Valency = 2 (loses 2 electrons).
1. If the number of electrons in an atom is 8 and the number of protons is also 8, then:
(i) What is the atomic number of the atom?
- The atomic number (Z) of an atom is equal to the number of protons.
- Since the atom has 8 protons, its atomic number is 8 (which corresponds to Oxygen).
(ii) What is the charge on the atom?
- A neutral atom has an equal number of protons (+ charge) and electrons (- charge).
- Since the number of protons = number of electrons (8 = 8), the atom has no overall charge (it is neutral).
- Charge on the atom = 0 (neutral atom).
2. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
-
Mass number (A) = Number of protons + Number of neutrons
-
From the periodic table:
-
Oxygen (O):
- Atomic number = 8
- Number of neutrons = 8
- Mass number = 8 + 8 = 16
-
Sulphur (S):
- Atomic number = 16
- Number of neutrons = 16
- Mass number = 16 + 16 = 32
-
Thus, the mass number of oxygen = 16 and the mass number of sulphur = 32.
Answers:
1. For the symbols H, D, and T, tabulate the three sub-atomic particles found in each of them.
Sub-Atomic Particles in Hydrogen Isotopes
| Symbol | Name | Protons | Neutrons | Electrons |
|---|---|---|---|---|
| H | Protium | 1 | 0 | 1 |
| D | Deuterium | 1 | 1 | 1 |
| T | Tritium | 1 | 2 | 1 |
- H (Protium): The most common hydrogen isotope, has no neutrons.
- D (Deuterium): A heavier isotope of hydrogen, has 1 neutron.
- T (Tritium): A radioactive isotope of hydrogen, has 2 neutrons.
2. Write the electronic configuration of any one pair of isotopes and isobars.
Isotopes (Same atomic number, different mass number)
Example: Carbon-12 (¹²C) and Carbon-14 (¹⁴C)
- Carbon-12: Atomic number = 6, Mass number = 12
- Electronic configuration: K = 2, L = 4
- Carbon-14: Atomic number = 6, Mass number = 14
- Electronic configuration: K = 2, L = 4
(They have the same number of protons and electrons but different neutrons.)
- Electronic configuration: K = 2, L = 4
Isobars (Same mass number, different atomic number)
Example: Calcium-40 (⁴⁰Ca) and Argon-40 (⁴⁰Ar)
- Calcium-40: Atomic number = 20, Mass number = 40
- Electronic configuration: K = 2, L = 8, M = 8, N = 2
- Argon-40: Atomic number = 18, Mass number = 40
- Electronic configuration: K = 2, L = 8, M = 8
(They have the same mass number but different atomic numbers and electron configurations.)
Answers:
1. Compare the properties of electrons, protons, and neutrons.
| Subatomic Particle | Charge | Mass | Location |
|---|---|---|---|
| Electron (e⁻) | -1 | ~1/2000 of proton | Outside the nucleus (in shells) |
| Proton (p⁺) | +1 | 1 atomic mass unit (u) | Inside the nucleus |
| Neutron (n⁰) | 0 (Neutral) | 1 atomic mass unit (u) | Inside the nucleus |
2. What are the limitations of J.J. Thomson’s model of the atom?
- It failed to explain the existence of a nucleus in the atom.
- It could not explain how electrons were arranged within the atom.
- It did not account for the presence of empty space inside the atom.
3. What are the limitations of Rutherford’s model of the atom?
- It could not explain why electrons do not lose energy and spiral into the nucleus.
- It did not explain the arrangement of electrons in shells around the nucleus.
4. Describe Bohr’s model of the atom.
- Electrons move in fixed orbits (shells) around the nucleus.
- Only certain discrete energy levels are allowed.
- Electrons do not lose energy while revolving in these fixed orbits.
- Energy is absorbed or emitted only when an electron moves between energy levels.
5. Compare all the proposed models of an atom given in this chapter.
| Model | Key Features | Limitations |
|---|---|---|
| J.J. Thomson's Model | Atom is a positively charged sphere with embedded electrons (Plum pudding model). | No nucleus, could not explain atomic structure properly. |
| Rutherford's Model | Atom has a small, dense nucleus with electrons moving around it. | Could not explain electron stability. |
| Bohr's Model | Electrons move in fixed orbits (shells) and do not lose energy. | Could not explain behavior of complex atoms. |
6. Summarise the rules for writing the distribution of electrons in various shells for the first eighteen elements.
- The maximum number of electrons in a shell = 2n² (where n is the shell number).
- The K shell (n=1) holds 2 electrons, L shell (n=2) holds 8 electrons, M shell (n=3) holds 18 electrons.
- The outermost shell cannot have more than 8 electrons.
- Electrons fill lower energy levels first (starting from K shell).
7. Define valency by taking examples of silicon and oxygen.
- Valency is the number of electrons an atom gains, loses, or shares to achieve a stable electronic configuration.
- Silicon (Atomic number = 14) → Electronic configuration: 2, 8, 4 → Needs 4 more electrons → Valency = 4.
- Oxygen (Atomic number = 8) → Electronic configuration: 2, 6 → Needs 2 more electrons → Valency = 2.
8. Explain with examples:
(i) Atomic number (Z): The number of protons in an atom. Example: Oxygen has Z = 8.
(ii) Mass number (A): The sum of protons and neutrons. Example: Carbon-12 has A = 12 (6 protons + 6 neutrons).
(iii) Isotopes: Atoms with same atomic number but different mass numbers. Example: ¹²C and ¹⁴C.
(iv) Isobars: Atoms with same mass number but different atomic numbers. Example: ⁴⁰Ca and ⁴⁰Ar.
Two uses of isotopes:
- Uranium-235 is used as nuclear fuel.
- Iodine-131 is used for thyroid treatment.
9. Na⁺ has completely filled K and L shells. Explain.
- Sodium (Na) has 11 electrons: K = 2, L = 8, M = 1.
- When it loses one electron (Na⁺), its configuration becomes K = 2, L = 8 (stable).
- Thus, Na⁺ has completely filled K and L shells.
10. Calculate the average atomic mass of bromine atom (Br-79 and Br-81).
Given:
- Br-79 (49.7%) → Mass = 79
- Br-81 (50.3%) → Mass = 81
Thus, the average atomic mass of bromine is 80.1 u.
11. The average atomic mass of an element X is 16.2 u. What are the percentages of isotopes ¹⁶X and ¹⁸X?
Let the percentage of ¹⁶X = x%, then ¹⁸X = (100 - x)%.
So, ¹⁶X = 90% and ¹⁸X = 10%.
12. If Z = 3, what would be the valency of the element? Also, name the element.
- Z = 3 (Atomic number) → Element is Lithium (Li).
- Electronic configuration: K = 2, L = 1.
- It loses 1 electron to become stable.
- Valency = 1.
13. Composition of the nuclei of two atomic species X and Y:
| Species | Protons | Neutrons | Mass Number (A) |
|---|---|---|---|
| X | 6 | 6 | 12 |
| Y | 6 | 8 | 14 |
- Since X and Y have the same number of protons (6), they are isotopes of Carbon (¹²C and ¹⁴C).
14. True (T) or False (F):
(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. → (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. → (T)
15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of:
✅ (a) Atomic Nucleus
16. Isotopes of an element have:
✅ (c) Different number of neutrons
17. Number of valence electrons in Cl⁻ ion are:
✅ (b) 8
Let's address both questions one by one:
18. Correct electronic configuration of sodium:
Sodium (Na) has an atomic number of 11. The electronic configuration of sodium is the arrangement of electrons in the energy levels.
The correct electronic configuration of sodium is:
2, 8, 1
This corresponds to 2 electrons in the first shell, 8 electrons in the second shell, and 1 electron in the third shell.
So, the correct answer is: (d) 2,8,1
19. Completing the table:
| Atomic Number | Atomic Mass Number | Number of Protons | Number of Neutrons | Number of Electrons | Name of the Atomic Species |
|---|---|---|---|---|---|
| 9 | 19 | 9 | 10 | 9 | Fluorine (F) |
| 16 | 32 | 16 | 16 | 16 | Sulphur (S) |
| 12 | 24 | 12 | 12 | 12 | Magnesium (Mg) |
| 1 | 2 | 1 | 1 | 1 | Hydrogen (H) |
| 0 | 1 | 0 | 1 | 1 | Neutron |
Please don not use wrong word