Structure of the Atom
Detailed NCERT Solutions & Atomic Models
In-Text Questions (Page 47)
Q1. What are canal rays?
Answer: Canal rays are positively charged radiations produced in a discharge tube containing gas at low pressure and high voltage. These rays led to the discovery of protons.
Q2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer: The atom will be electrically neutral. This is because the negative charge of one electron (-1) cancels out the positive charge of one proton (+1).
In-Text Questions (Page 49)
Q1. On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
According to Thomson's model:
- An atom consists of a positively charged sphere and electrons are embedded in it.
- The negative and positive charges are equal in magnitude.
- Therefore, the atom as a whole is electrically neutral.
Q2. On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?
Proton (positively charged particle) is present in the nucleus.
In-Text Questions (Page 50 & 52)
Q1. Write the distribution of electrons in carbon and sodium atoms.
Carbon (Atomic No. 6): K=2, L=4. (2, 4)
Sodium (Atomic No. 11): K=2, L=8, M=1. (2, 8, 1)
Q2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Max electrons in K shell = 2
Max electrons in L shell = 8
Total electrons = 2 + 8 = 10 electrons.
In-Text Questions (Page 53)
Q1. How will you find the valency of chlorine, sulphur and magnesium?
Valency is determined by the number of valence electrons.
- Chlorine (2, 8, 7): Need 1 electron to complete octet. Valency = 8 - 7 = 1.
- Sulphur (2, 8, 6): Need 2 electrons. Valency = 8 - 6 = 2.
- Magnesium (2, 8, 2): Lose 2 electrons. Valency = 2.
Main Textbook Exercises
Q1. Compare the properties of electrons, protons and neutrons.
| Property | Electron | Proton | Neutron |
|---|---|---|---|
| Charge | Negative (-1) | Positive (+1) | Neutral (0) |
| Mass | Negligible (1/1840 u) | 1 u | 1 u |
| Location | Outside nucleus (in orbits) | Nucleus | Nucleus |
Q2. J.J. Thomson's model of the atom limitations.
1. It failed to explain how the positive charge holds on the electrons inside the atom.
2. It failed to explain the results of Rutherford's alpha-particle scattering experiment (deflection
of alpha particles).
Q3. Rutherford's model of the atom limitations.
According to electromagnetic theory, any charged particle in circular motion radiates energy. So, the electron would lose energy and spiral into the nucleus, making the atom unstable. Rutherford could not explain the stability of the atom.
Q10. Calculate average atomic mass of Bromine (isotopes 79Br-49.7% and 81Br-50.3%).
Average Mass = (79 × 49.7/100) + (81 × 50.3/100)
= 39.263 + 40.743
= 80.006 u
Structure of the Atom
Detailed Chapter Analysis & Atomic Models
1. Discovery of Sub-atomic Particles
Dalton's atomic theory suggested that the atom was indivisible and indestructible. However, the discovery of two fundamental particles (electrons and protons) inside the atom led to the failure of this aspect of Dalton’s atomic theory.
- Discovered by J.J. Thomson (1897).
- Cathode Ray Experiment.
- Rest mass = 9.1 × 10-31 kg.
- Charge = -1.6 × 10-19 C.
- Observed by E. Goldstein (1886) as Canal Rays.
- Anode Ray Experiment.
- Mass is approx 2000 times that of electron.
- Charge = +1.6 × 10-19 C.
2. Evolution of Atomic Models
An atom is a positively charged sphere, and electrons are embedded in it like seeds in a watermelon. The negative and positive charges are equal in magnitude, so the atom is electrically neutral.
Based on Alpha-particle scattering experiment.
- Most space inside atom is empty.
- Positive charge is concentrated in a small volume called Nucleus.
- Electrons revolve around the nucleus in circular paths.
- Drawback: Could not explain stability (charged particle in acceleration radiates energy).
Overcame Rutherford's limitations.
- Electrons revolve in discrete orbits called shells or energy levels.
- While revolving in these discrete orbits, electrons do not radiate energy.
- Shells are represented by K, L, M, N... or n = 1, 2, 3, 4...
3. Electronic Configuration Rules
Rule 1: The maximum number of electrons in a shell is given by 2n2, where 'n' is the orbit number.
- K shell (n=1): 2(1)2 = 2
- L shell (n=2): 2(2)2 = 8
- M shell (n=3): 2(3)2 = 18
Rule 2: The maximum number of electrons that can be accommodated in the outermost orbit is 8.
Rule 3: Electrons are not accommodated in a given shell unless the inner shells are filled (step-wise filling).
4. Neutron and Valency
Discovered by J. Chadwick (1932). It has no charge and mass nearly equal to that of a proton. Present in the nucleus of all atoms except Hydrogen.
Combinging capacity of the atom. It is the number of electrons gained, lost
or shared to make the octet of electrons in the outermost shell.
If valence electrons <= 4, Valency=Valence Electrons.
If valence electrons > 4, Valency = 8 - Valence Electrons.
5. Isotopes and Isobars
Total number of protons in the nucleus. Z = p = e (in neutral atom).
Sum of total number of protons and neutrons. A = p + n.
Atoms of the same element, having the same atomic number but different mass numbers.
Example: Protium (1H), Deuterium (2H), Tritium (3H).Applications:
- Uranium isotope: Fuel in nuclear reactors.
- Cobalt isotope: Treatment of cancer.
- Iodine isotope: Treatment of goitre.
Atoms of different elements with different atomic numbers, which have the same mass number.
Example: Calcium (40) and Argon (40).Key Facts & Definitions
50+ Important Points to Remember
Positively charged radiations discovered by E. Goldstein in 1886.
Negatively charged subatomic particle discovered by J.J. Thomson.
Positively charged subatomic particle present in the nucleus.
Neutral subatomic particle discovered by J. Chadwick in 1932.
Small, heavy, positively charged center of the atom discovered by Rutherford.
Number of protons in the nucleus of an atom.
Sum of protons and neutrons in the nucleus.
Number of Protons = Number of Electrons.
Protons and neutrons residing in the nucleus.
Plum pudding model; electrons embedded in positive sphere.
Alpha-particle scattering experiment using gold foil.
Doubly charged helium ions (He2+).
Used by Rutherford because he wanted a layer as thin as possible (1000 atoms thick).
Most alpha particles passed straight; very few deflected.
The radius of nucleus is about 105 times smaller than the radius of atom.
Discrete orbits or shells where electrons revolve without radiating energy.
Shells K, L, M, N correspond to energy levels n=1, 2, 3, 4.
Formula to find maximum number of electrons in a shell.
The outermost shell can accommodate a maximum of 8 electrons.
Combining capacity of an atom determined by valence electrons.
Electrons present in the outermost shell of an atom.
Helium, Neon, Argon have completely filled outermost shells. Valency = 0.
Atoms of same element with different mass numbers (Different Neutrons).
Isotope of Hydrogen (1H) with 0 neutrons.
Isotope of Hydrogen (2H) with 1 neutron.
Isotope of Hydrogen (3H) with 2 neutrons.
Atoms of different elements with same mass number (e.g., Ca and Ar).
Isotope used as fuel in nuclear reactors.
Isotope used in treatment of cancer.
Isotope used in treatment of goitre.
Approximately 1 u (1.67 × 10-27 kg).
Approximately 1 u, slightly higher than proton.
1/1840 u (Negligible).
2, 8, 1 (Total 11 electrons).
2, 8, 7 (Total 17 electrons).
Bond formed by transfer of electrons (e.g., NaCl).
Bond formed by sharing of electrons (e.g., H2).
35.5 u (Average of 35 and 37 isotopes).
2 (Rule 2n²).
8 (Rule 2n²).
18 (Rule 2n²).
32 (Rule 2n²).
Group 18 elements with stable configuration.
2 (Duplet). Stable.
2, 8 (Octet). Stable.
2, 8, 8 (Octet). Stable.
+2 (Loses 2 electrons).
+3 (Loses 3 electrons).
-2 (Gains 2 electrons).
Determine by the number of valence electrons (which are same for isotopes).