According to Thomson's model, an atom consists of a positively charged sphere with electrons embedded in it. The total positive charge of the sphere is equal to the total negative charge of the electrons, making the atom electrically neutral as a whole.
According to Rutherford's model, the nucleus of an atom contains protons, which are positively charged sub-atomic particles.
In Bohr's model with three shells, the nucleus is at the center, surrounded by three concentric circular paths (shells) representing K, L, and M shells. Electrons revolve in these fixed orbits around the nucleus.
If a metal foil with lighter atoms than gold is used, most α-particles would pass straight through with less deflection, and fewer α-particles would be deflected back. This is because lighter atoms have smaller nuclei with less positive charge, resulting in weaker repulsive forces on the α-particles.
Canal rays are positively charged radiations that consist of positively charged particles. They were discovered by E. Goldstein and are also known as anode rays.
No, an atom containing one electron and one proton will be electrically neutral because the positive charge of the proton is exactly balanced by the negative charge of the electron.
Carbon (atomic number 6): K-shell = 2 electrons, L-shell = 4 electrons
Sodium (atomic number 11): K-shell = 2 electrons, L-shell = 8 electrons, M-shell = 1 electron
K-shell can hold maximum 2 electrons and L-shell can hold maximum 8 electrons. If both are full, the total number of electrons would be 2 + 8 = 10 electrons.
The three sub-atomic particles of an atom are: electrons, protons, and neutrons.
Atomic mass = Number of protons + Number of neutrons
4 = 2 + Number of neutrons
Number of neutrons = 4 - 2 = 2
Therefore, helium atom has 2 neutrons.
Valency is determined by the number of electrons an atom can lose, gain, or share to complete its octet.
- Chlorine (atomic number 17): Electronic configuration 2,8,7. It needs 1 electron to complete octet, so valency = 1
- Sulphur (atomic number 16): Electronic configuration 2,8,6. It needs 2 electrons to complete octet, so valency = 2
- Magnesium (atomic number 12): Electronic configuration 2,8,2. It can lose 2 electrons to complete octet, so valency = 2
(i) Atomic number = Number of protons = 8
(ii) Since number of electrons = number of protons, the atom is electrically neutral, so charge = 0
From standard periodic table:
- Oxygen: Atomic number = 8, Mass number = 16
- Sulphur: Atomic number = 16, Mass number = 32
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| H (Protium) | 1 | 0 | 1 |
| D (Deuterium) | 1 | 1 | 1 |
| T (Tritium) | 1 | 2 | 1 |
Isotopes (same atomic number, different mass number):
Chlorine-35 and Chlorine-37: Both have atomic number 17, electronic configuration: 2,8,7
Isobars (same mass number, different atomic number):
Argon-40 (atomic number 18): 2,8,8 and Calcium-40 (atomic number 20): 2,8,8,2
| Property | Electron | Proton | Neutron |
|---|---|---|---|
| Charge | -1 (negative) | +1 (positive) | 0 (neutral) |
| Mass | 1/1836 u (negligible) | 1 u | 1 u |
| Location | Outside nucleus | Inside nucleus | Inside nucleus |
| Discoverer | J.J. Thomson | E. Goldstein | James Chadwick |
Limitations of Thomson's model:
1. It could not explain the results of Rutherford's α-particle scattering experiment
2. It failed to explain how the positive charge holds the electrons in the atom
3. It could not explain the stability of atom
4. It did not account for the existence of nucleus
Limitations of Rutherford's model:
1. It could not explain the stability of atom (according to Maxwell's theory, revolving electron should radiate energy and fall into nucleus)
2. It could not explain the discrete line spectrum of atoms
3. It did not explain the distribution of electrons in different energy levels
Bohr's model of atom:
1. Electrons revolve around the nucleus in specific circular paths called orbits or shells
2. Each orbit has a fixed energy level
3. Electrons do not radiate energy while moving in stationary orbits
4. Energy is emitted or absorbed only when an electron jumps from one orbit to another
5. The angular momentum of electron is quantized (mvr = nh/2π)
| Model | Proposer | Key Features | Limitations |
|---|---|---|---|
| Thomson's Model | J.J. Thomson | Atom as a positively charged sphere with embedded electrons | Could not explain α-scattering; no nucleus concept |
| Rutherford's Model | Ernest Rutherford | Nucleus at center; electrons revolve around it | Could not explain stability; electron should spiral into nucleus |
| Bohr's Model | Niels Bohr | Electrons in fixed energy orbits; quantized angular momentum | Could not explain fine structure of spectra; violated uncertainty principle |
Rules for electron distribution:
1. Maximum number of electrons in K-shell (n=1) = 2 × 1² = 2
2. Maximum number of electrons in L-shell (n=2) = 2 × 2² = 8
3. Maximum number of electrons in M-shell (n=3) = 2 × 3² = 18
4. Electrons are filled from inner to outer shells
5. Outermost shell cannot have more than 8 electrons
Valency is the combining capacity of an atom, determined by the number of electrons it can lose, gain or share to attain stable configuration.
- Silicon (atomic number 14): Electronic configuration 2,8,4. It can either lose 4 electrons or share 4 electrons, so valency = 4
- Oxygen (atomic number 8): Electronic configuration 2,6. It needs 2 electrons to complete octet, so valency = 2
(i) Atomic number: Number of protons in an atom. Example: Carbon has atomic number 6
(ii) Mass number: Sum of protons and neutrons in an atom. Example: Carbon-12 has mass number 12
(iii) Isotopes: Atoms of same element with same atomic number but different mass numbers. Example: Hydrogen-1, Hydrogen-2, Hydrogen-3
(iv) Isobars: Atoms of different elements with same mass number but different atomic numbers. Example: Calcium-40 and Argon-40
Uses of isotopes:
1. Medical: Cobalt-60 for cancer treatment
2. Archaeological: Carbon-14 for dating ancient objects
Sodium atom (Na) has atomic number 11 with electronic configuration: K=2, L=8, M=1
Sodium ion (Na⁺) is formed when sodium atom loses one electron from M-shell
Electronic configuration of Na⁺ becomes: K=2, L=8
K-shell is completely filled with 2 electrons and L-shell is completely filled with 8 electrons
Average atomic mass = (Mass of isotope1 × % abundance1 + Mass of isotope2 × % abundance2) / 100
= (79 × 49.7 + 81 × 50.3) / 100
= (3926.3 + 4074.3) / 100
= 8000.6 / 100 = 80.006 u ≈ 80 u
Let percentage of 16X be x%, then percentage of 18X = (100 - x)%
Average atomic mass = (16 × x + 18 × (100 - x)) / 100 = 16.2
16x + 1800 - 18x = 1620
-2x + 1800 = 1620
-2x = 1620 - 1800 = -180
x = 90%
Therefore, 16X = 90% and 18X = 10%
Atomic number Z = 3 corresponds to Lithium (Li)
Electronic configuration of Lithium: K=2, L=1
It can lose 1 electron to achieve stable configuration, so valency = 1
Mass number of X = Protons + Neutrons = 6 + 6 = 12
Mass number of Y = Protons + Neutrons = 6 + 8 = 14
Both have same atomic number (6) but different mass numbers, so they are isotopes of carbon: Carbon-12 and Carbon-14
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. F (Rutherford proposed nucleus)
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. F (Neutron is elementary particle)
(c) The mass of an electron is about 1/2000 times that of proton. T
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. T
(a) Atomic Nucleus ✓
(c) different number of neutrons ✓
Chlorine atom (atomic number 17): Electronic configuration 2,8,7 → 7 valence electrons
Cl⁻ ion gains 1 electron → Electronic configuration 2,8,8 → 8 valence electrons
Answer: (b) 8 ✓
Sodium has atomic number 11, so electronic configuration: 2,8,1
Answer: (d) 2,8,1 ✓
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Name of the 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 | Deuterium |
| 1 | 1 | 0 | 1 | 1 | Protium |