Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
I. Choose the Best Answer (MCQs)
Q1. The subatomic particle that has no charge is
- (a) Proton (b) Neutron (c) Electron (d) Positron
Q2. The number of protons in an atom is called its
- (a) Mass number (b) Atomic number (c) Atomic mass (d) Neutron number
Q3. The outermost shell of an atom can hold a maximum of
- (a) 2 electrons (b) 8 electrons (c) 18 electrons (d) 32 electrons
Q4. The first shell (K shell) can hold a maximum of
- (a) 8 electrons (b) 2 electrons (c) 18 electrons (d) 32 electrons
Q5. Who proposed the planetary model of the atom?
- (a) J.J. Thomson (b) Ernest Rutherford (c) Niels Bohr (d) John Dalton
Q6. The mass of an electron is approximately
- (a) Equal to a proton (b) 1/1837 of a proton (c) Equal to a neutron (d) 1837 times a proton
Q7. Isotopes are atoms of the same element with different numbers of
- (a) Protons (b) Electrons (c) Neutrons (d) Nuclei
Q8. The valency of an element with 7 electrons in its outermost shell is
- (a) 7 (b) 1 (c) 3 (d) 5
II. Fill in the Blanks
1. The positively charged particle in the nucleus is the proton. 2. The neutral particle in the nucleus is the neutron. 3. The negatively charged particle revolving around the nucleus is the electron. 4. The number of protons in an atom is called its atomic number. 5. The sum of protons and neutrons in the nucleus is the mass number. 6. The paths along which electrons revolve around the nucleus are called shells or orbits. 7. The maximum number of electrons in a shell is given by the formula 2n². 8. Atoms of the same element with different mass numbers are called isotopes. 9. Atoms of different elements with the same mass number are called isobars. 10. The outermost shell of an atom is called the valence shell.
III. True or False
IV. Match the Following
| Column I | Column II | Answer |
|---|---|---|
| Proton | Positive charge | Proton — Positive charge |
| Neutron | No charge | Neutron — No charge |
| Electron | Negative charge | Electron — Negative charge |
| Atomic number | Number of protons | Atomic number — Number of protons |
| Mass number | Protons + Neutrons | Mass number — Protons + Neutrons |
| Isotopes | Same proton number, different neutron number | Isotopes — Same proton number, different neutron number |
| Isobars | Different proton number, same mass number | Isobars — Different proton number, same mass number |
V. Short Answer Questions
Q1. What are the subatomic particles of an atom? State their charges and masses. Answer:
| Particle | Charge | Mass |
|---|---|---|
| Proton | +1.6 × 10⁻¹⁹ C (positive) | 1.0073 u ≈ 1 u |
| Neutron | 0 (neutral) | 1.0087 u ≈ 1 u |
| Electron | −1.6 × 10⁻¹⁹ C (negative) | 0.00055 u ≈ 1/1837 u |
Protons and neutrons are in the nucleus; electrons revolve in orbits around it.
Q2. What is the difference between atomic number and mass number? Answer:
| Property | Atomic Number (Z) | Mass Number (A) |
|---|---|---|
| Definition | Number of protons in the nucleus | Sum of protons and neutrons |
| Formula | Z = number of protons | A = number of protons + number of neutrons |
| Determines | Identity of the element | Approximate mass of the atom |
| Example (Carbon) | Z = 6 | A = 12 (6 protons + 6 neutrons) |
Q3. 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 number of neutrons).
Examples: 1. Hydrogen isotopes: Protium (¹H, 1 proton, 0 neutrons), Deuterium (²H, 1 proton, 1 neutron), Tritium (³H, 1 proton, 2 neutrons). 2. Carbon isotopes: Carbon-12 (6 protons, 6 neutrons), Carbon-14 (6 protons, 8 neutrons).
Q4. What are isobars? Give an example. Answer: Isobars are atoms of different elements that have the same mass number but different atomic numbers.
Example: Calcium-40 (20 protons, 20 neutrons) and Argon-40 (18 protons, 22 neutrons) both have mass number 40 but different atomic numbers.
Q5. State the rules for filling electrons in shells. Answer: 1. Electrons fill shells starting from the innermost shell (K) outward. 2. The maximum number of electrons in a shell is given by the formula 2n², where n is the shell number.
3. The outermost shell can hold a maximum of 8 electrons (octet rule). 4. A new shell starts filling only when the previous shell is completely filled.
- K shell (n = 1): maximum 2 electrons
- L shell (n = 2): maximum 8 electrons
- M shell (n = 3): maximum 18 electrons
- N shell (n = 4): maximum 32 electrons
Q6. Define valency. How is it related to valence electrons? Answer: Valency is the combining capacity of an atom — the number of electrons an atom can lose, gain, or share to achieve a stable octet.
Relationship with valence electrons:
- If an atom has 1-4 valence electrons, valency = number of valence electrons (it loses them).
- If an atom has 5-7 valence electrons, valency = 8 minus number of valence electrons (it gains electrons to complete octet).
- If an atom has 8 valence electrons (or 2 in case of helium), valency = 0 (already stable).
Example: Sodium (2, 8, 1) has valency 1. Chlorine (2, 8, 7) has valency 1 (8 − 7 = 1).
VI. Detailed Answer Questions
Q1. Describe J.J. Thomson's model of the atom and its limitations. Answer: Thomson's Model (Plum Pudding Model, 1897): Thomson proposed that an atom is a sphere of positive charge in which electrons are embedded like plums in a pudding. The total positive charge is distributed uniformly throughout the atom, and the electrons are scattered within this positive sphere.
Key features: 1. An atom is electrically neutral (positive charge = total negative charge of electrons). 2. Electrons are embedded in a positively charged sphere. 3. The atom has no empty space.
Limitations: 1. It could not explain the results of Rutherford's gold foil experiment (most alpha particles passed straight through, which meant most of the atom is empty space). 2. It could not explain the stability of the atom — electrons embedded in a positive sphere should radiate energy and spiral inward. 3. It could not explain the large deflection of some alpha particles in the gold foil experiment.
Q2. Describe Rutherford's gold foil experiment and the nuclear model of the atom. Answer: Gold Foil Experiment (1911): Rutherford bombarded a thin gold foil with fast-moving alpha particles (positively charged helium nuclei). A fluorescent screen around the foil detected the scattered alpha particles.
Observations: 1. Most alpha particles passed straight through the foil without deflection. 2. Some alpha particles were deflected at small angles. 3. Very few alpha particles (about 1 in 12,000) bounced back (large deflection).
Conclusions: 1. Since most particles passed through, most of the atom is empty space. 2. Some particles were deflected, indicating the presence of a positively charged centre. 3. Very few bounced back, showing the positive centre (nucleus) is very small compared to the atom.
Rutherford's Nuclear Model: 1. An atom has a tiny, dense, positively charged nucleus at its centre. 2. The nucleus contains protons (and later discovered, neutrons). 3. Electrons revolve around the nucleus in circular paths, like planets around the Sun. 4. The size of the nucleus is very small compared to the size of the atom. 5. Most of the atom is empty space.
Limitations: Rutherford's model could not explain the stability of the atom. According to classical physics, a revolving electron should continuously radiate energy, lose speed, and spiral into the nucleus, which does not happen.
Q3. Describe Bohr's model of the atom. How did it improve upon Rutherford's model? Answer: Bohr's Model (1913): Niels Bohr modified Rutherford's model by adding the concept of energy levels (shells/orbits).
Postulates: 1. Electrons revolve around the nucleus in certain fixed circular paths called orbits or energy levels (K, L, M, N). 2. Each orbit has a fixed energy — electrons in a particular orbit do not radiate energy. 3. An electron can jump from a lower energy orbit to a higher energy orbit by absorbing energy, and fall back by emitting energy. 4. The maximum number of electrons in an orbit is given by 2n² (n = orbit number).
How it improved Rutherford's model:
- Rutherford's model could not explain why electrons don't spiral into the nucleus. Bohr explained this by proposing that electrons in fixed orbits do not radiate energy.
- Bohr's model explained the stability of atoms and the emission spectra of hydrogen.
Limitations:
- It could not explain the spectra of atoms with more than one electron.
- It did not account for the wave nature of electrons.
- It could not explain the splitting of spectral lines in a magnetic field (Zeeman effect).
Q4. Write the electronic configuration of the first 20 elements and state their valency. Answer:
| Element | Symbol | Atomic Number | Electronic Configuration | Valency |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Helium | He | 2 | 2 | 0 |
| Lithium | Li | 3 | 2, 1 | 1 |
| Beryllium | Be | 4 | 2, 2 | 2 |
| Boron | B | 5 | 2, 3 | 3 |
| Carbon | C | 6 | 2, 4 | 4 |
| Nitrogen | N | 7 | 2, 5 | 3 |
| Oxygen | O | 8 | 2, 6 | 2 |
| Fluorine | F | 9 | 2, 7 | 1 |
| Neon | Ne | 10 | 2, 8 | 0 |
| Sodium | Na | 11 | 2, 8, 1 | 1 |
| Magnesium | Mg | 12 | 2, 8, 2 | 2 |
| Aluminium | Al | 13 | 2, 8, 3 | 3 |
| Silicon | Si | 14 | 2, 8, 4 | 4 |
| Phosphorus | P | 15 | 2, 8, 5 | 3 |
| Sulphur | S | 16 | 2, 8, 6 | 2 |
| Chlorine | Cl | 17 | 2, 8, 7 | 1 |
| Argon | Ar | 18 | 2, 8, 8 | 0 |
| Potassium | K | 19 | 2, 8, 8, 1 | 1 |
| Calcium | Ca | 20 | 2, 8, 8, 2 | 2 |
Common Mistakes
- Confusing atomic number with mass number: Atomic number = number of protons only. Mass number = protons + neutrons.
- Saying electrons have significant mass: Electrons have negligible mass (1/1837 of a proton). The mass of an atom is almost entirely in the nucleus.
- Confusing isotopes and isobars: Isotopes have the same atomic number but different mass numbers (same element). Isobars have the same mass number but different atomic numbers (different elements).
- Applying 2n² to the outermost shell: The 2n² rule gives the maximum electrons per shell, but the outermost shell never exceeds 8 electrons (octet rule).
- Confusing valency with valence electrons: Valency is the combining capacity. For elements with 5-7 valence electrons, valency = 8 − valence electrons, not equal to valence electrons.
- Saying the nucleus contains only protons: The nucleus contains both protons and neutrons (collectively called nucleons).
Exam Checklist
- [ ] Can you describe the subatomic particles and their properties?
- [ ] Can you distinguish between atomic number and mass number?
- [ ] Can you explain Thomson's, Rutherford's, and Bohr's atomic models?
- [ ] Can you write the electronic configuration of elements (up to atomic number 20)?
- [ ] Can you define and give examples of isotopes and isobars?
- [ ] Can you calculate valency from electronic configuration?
- [ ] Can you apply the formula 2n² for maximum electrons in a shell?
- [ ] Can you explain the gold foil experiment and its conclusions?
- [ ] Can you state the limitations of each atomic model?
- [ ] Can you determine the number of protons, neutrons, and electrons given the atomic number and mass number?