Test: Structure Of The Atom- 2 - Class 9 MCQ

• Statement (i): The mass of the atom is assumed to be uniformly distributed over the atom.
  Correct. According to Thomson, the atom is a uniformly distributed mass.
• Statement (iii): The electrons are uniformly distributed in the positively charged sphere.
  Correct. Electrons are embedded in the positively charged sphere like "plums in a pudding."

- Rutherford's Model: Proposed a small, dense, positively charged nucleus with electrons orbiting it, similar to a solar system.
- Statement (i): Correct. The nucleus is positively charged.
- Statement (ii): Incorrect. It doesn't specify \(\alpha\)-particles' mass.
- Statement (iii): Correct. The atomic model is compared to a solar system.
- Statement (iv): Incorrect. Contradicts Thomson's "plum pudding" model.
- Conclusion: Correct statements are (i) and (iii). The answer is A.

• Electron: Electrons have a negative charge: So, Electron → (iii)
• Proton: Protons have a positive charge: So, Proton → (i)
• Neutron: Neutrons have no charge.: So, Neutron → (ii)

- An atom with 3 protons is a Lithium atom, as the atomic number represents the number of protons.
- Lithium's electron configuration is 2, 1, meaning it has one electron in its outer shell.
- Valency is determined by the number of electrons an atom needs to lose, gain, or share to achieve a full outer shell.
- Lithium has one electron to lose to achieve a stable configuration like Helium.
- Therefore, the valency of lithium, and hence the atom, is 1.

- Atoms and Charge:
- An atom is electrically neutral, meaning it has no overall charge.
- This neutrality is achieved when the number of protons (positively charged) equals the number of electrons (negatively charged).
- Electrons:
- Electrons orbit the nucleus and balance the positive charge of protons.
- Protons:
- Protons are found in the nucleus and determine the atomic number.
- Neutrons:
- Neutrons are also in the nucleus and usually differ in number from protons and electrons.
- Conclusion:
- Statement A is always correct because an atom must have equal numbers of protons and electrons to remain neutral.

Rutherford's a-scattering experiment and its conclusion:
Background:

• Rutherford's a-scattering experiment was conducted by physicist Ernest Rutherford in 1911.
• The experiment involved bombarding a thin gold foil with alpha particles (helium nuclei) and observing their scattering pattern.

Experiment:

• Rutherford expected that the alpha particles would pass through the gold foil without significant deflection because the prevailing atomic model at the time suggested that atoms were composed of a uniform positive charge with electrons embedded in it.
• However, the experimental results surprised Rutherford.

Observations:

• Most of the alpha particles passed straight through the gold foil, indicating that the atoms were mostly empty space.
• - Some alpha particles were deflected at small angles, suggesting the presence of a concentrated positive charge within the atom.
• A few alpha particles were deflected at very large angles, and some even bounced back, indicating the presence of an extremely dense and positively charged center within the atom.

Conclusion:
Based on the observations from the a-scattering experiment, Rutherford concluded that:

• The mass and the positive charge of an atom are concentrated in a tiny, dense, and positively charged region called the nucleus.
• The nucleus is surrounded by mostly empty space where electrons are located.
• The nucleus is very small compared to the overall size of the atom.
• The existing atomic model, known as the "Plum Pudding" model, was incorrect.

Answer: The conclusion reached from Rutherford's a-scattering experiment is that the mass and the positive charge of an atom are concentrated in the nucleus (Option B).

Figures (ii) and (iv) not correctly represent the Bohr's model of an atom.
It is because maximum number of electrons in K (I) shell is 2, not 4, so (ii) is wrong and maximum capacity of L (II) shell is 8 electrons, not 9. So, (iv) is also wrong.

The K, L, M, N shells of an atom can hold a maximum number of electrons as follows:
K shell: 2 electrons
L shell: 8 electrons
M shell: 18 electrons
N shell: 32 electrons
If these shells are full, the total number of electrons is calculated by adding the maximum capacities:
= 2 + 8 + 18 + 32 = 60
Therefore, the correct answer is option A: 60.

Thomson's Atomic Model:

• Proposed by J.J. Thomson in 1904.
• Known as the "plum pudding model," it described the atom as a positively charged sphere with negatively charged electrons embedded within it.
• This model was the first to incorporate the existence of electrons.

Rutherford's Atomic Model:

• Developed by Ernest Rutherford in 1911 following his gold foil experiment.
• It introduced the concept of a small, dense, positively charged nucleus at the center of the atom, with electrons orbiting around it.
• This model discarded the "plum pudding" concept and introduced the idea of a nuclear atom.

Bohr's Atomic Model:

• Proposed by Niels Bohr in 1913.
• It refined Rutherford's model by introducing quantized electron orbits, which explained atomic emission spectra.
• Bohr's model was a significant step toward modern quantum theory.

The correct chronological order is: Thomson, Rutherford, and then Bohr, corresponding to option C: (ii), (i), and (iii).

• Atomic Number: The atomic number of an element is the number of protons in its nucleus. For sodium (Na), the atomic number is 11. This is a defining property of the element and does not change.
• Electrons: Sodium is typically neutral with 11 electrons, equal to its atomic number. However, in this case, Na has 10 electrons, indicating it has lost one electron to become a positively charged ion Na⁺.
• Neutrons and Mass Number: The mass number is the sum of protons and neutrons in the nucleus. Given that sodium has 11 protons and 12 neutrons, the mass number is 11 + 12 = 23.

Correct Option: Option B states that Na has an atomic number of 11 and a mass number of 23, which correctly corresponds to the composition of sodium in this scenario.