Test: Structure Of The Atom- 2 - UPSC 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 (ii): The positive charge is uniformly distributed over the space. Correct. In Thomson's model, the positive charge is spread across the entire atom.
• 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."

The mass number of an atom is calculated by adding the number of protons and neutrons together. In this case:

• Number of protons: 7
• Number of neutrons: 8

To find the mass number:

• Mass Number = Protons + Neutrons
• Mass Number = 7 + 8 = 15

Therefore, the mass number of the atom is 15.

The electron distribution in an atom with atomic number 14 is as follows:

• K: 2 electrons
• L: 8 electrons
• M: 4 electrons

This distribution is typical for silicon, which has 14 electrons. The arrangement follows the rules of electron configuration, filling the lowest energy levels first.

The valency of an element with an electron configuration of 2, 8, 3 is determined by the number of electrons in its outer shell. In this case:

• The outer shell has 3 electrons.
• Elements typically aim to have a full outer shell, which requires 8 electrons for stability.
• To achieve this, the element can either gain 5 electrons or lose 3 electrons.

Thus, the valency of the element is 3.

- 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.

An isotope of an element has 11 protons and 13 neutrons. The mass number is the total of protons and neutrons.

• Mass number = Protons + Neutrons
• For this isotope: 11 + 13 = 24

Now, for another isotope of the same element with 14 neutrons:

• Mass number = 11 + 14 = 25

The correct answer is 25.

Solution (Short Version):

• Electron: Discovered by J.J. Thomson in 1897 using a cathode ray tube.

• Proton: Discovered by E. Goldstein in 1886 through canal rays.

• Neutron: Discovered by James Chadwick in 1932.

All statements are correct, so the answer is: c) All of the above.

Electron distribution for an atom with 13 electrons can be described as follows:

• K shell: 2 electrons
• L shell: 8 electrons
• M shell: 3 electrons

According to Bohr's model, this arrangement is stable. However, Rutherford's model suggests potential instability for the following reasons:

• Electrons may lose energy and spiral into the nucleus.
• There is a risk of electrons gaining energy and escaping.
• Electrons in a circular orbit can become unstable over time.

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.