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Elementary Ideas about the Structure of Atom

Chapter: Atomic Structure & Laws

Question 1 of 5 NDA MCQ

The mass number of argon is 40. Which one ofthe following statements is correct?

Detailed Explanation

Correct Answer

✓ Option (b) is correct. The number of neutrons in argon is 18.

📖 Governing Law / Core Concept

The atomic number of an element indicates the number of protons, while the mass number is the total number of protons and neutrons in the nucleus. For argon (Ar), we have the following definitions:

Atomic number (Z) = Number of protons

Mass number (A) = Number of protons (p) + Number of neutrons (n)

For argon:

Mass number (\(A\)) = 40

Atomic number (\(Z\)) = 18

🧪 Step-by-Step Breakdown

\[ \begin{aligned} & A = p + n \\ & 40 = 18 + n \\ & n = 40 - 18 \\ & n = 22 \end{aligned} \]

Thus, argon has 22 neutrons. Since argon is neutral, the number of electrons also equals the number of protons, confirming there are 18 electrons.

🔍 Option Analysis

  • Option A: Incorrect. The number of protons in argon is 18, not 22.
  • Option B: Correct. The number of neutrons in argon is 22, calculated as shown.
  • Option C: Incorrect. The number of electrons is 18, equal to the number of protons.
  • Option D: Incorrect. The sum of protons and electrons is 36, not 40.

⚡ Mnemonic / Speed-Run

Remember: "Argon is neutral, argon is great, with 18 protons (as a plate)!" This helps recall that with 18 protons, neutrons add to 22 to make 40.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class 11 Chemistry, Chapter 2 (Structure of Atom).

Question 2 of 5 NDA MCQ

Which one of the following findings is NOT observed in Rutherford’s Nuclear Model of Atom?

Detailed Explanation
✓

1. Correct Answer

Option A is correct. Rutherford’s Nuclear Model of Atom did not propose the presence of a neutral center in the atom, such as a nucleus.

📖 Short Explanation

Rutherford’s Nuclear Model revolutionized atomic theory by introducing the existence of a small, dense nucleus, but it did not suggest the nucleus was neutral.

💡 3. Why Correct Answer is Correct

Rutherford’s model showed that the atom is mostly empty space with electrons orbiting a positively charged nucleus. The nucleus, which is very small and dense, contains most of the atom's mass. Rutherford concluded this through his famous gold foil experiment. While the nucleus was thought to consist of protons (positive charge), the notion of a neutral component within it (neutrons) was not introduced until later by James Chadwick in 1932.

❌ 4. Why Other Options are Wrong

  • •Option B: Correct as Rutherford’s model does indicate that nearly all the mass of the atom resides in the nucleus.
  • •Option C: Correct because the model depicts electrons revolving around the nucleus in circular paths.
  • •Option D: Correct as it reflects the fact that the size of the nucleus is very small compared to the rest of the atom.
⚡

NDA Speed-Run / Mnemonic Shortcut

Remember that Rutherford's atom model introduced the nucleus and its positive charge without detailing the presence of neutrons which are neutral.

🎯

5. Key Learning Point

Rutherford's experiment identified a nucleus filled with protons; it did not account for neutrons.

6. Visual Suggestion

A diagram showing a compact nucleus at the center with electrons orbiting in elliptical paths and differential sizes between the nucleus and the atom as a whole.

Question 3 of 5 NDA MCQ

Which one of the following conclusions could not be derived from Rutherfords -particle scattering experiment?

Detailed Explanation

Correct Answer

✓ Option (c) is correct. Electrons do not move in fixed energy orbits according to Rutherford's model.

📖 Governing Law / Core Concept

The foundational concept from Rutherford's experiment illustrates that atoms consist primarily of empty space, with most mass concentrated in a dense nucleus.

🧪 Step-by-Step Breakdown

Rutherford's gold foil experiment revealed several key points:

  • Most of the atom's volume is empty.
  • The nucleus is significantly smaller than the atom (radius ratio: ~105).
  • Rutherford did not propose a model for electron orbits, which was later introduced by Bohr.

Thus, the statement regarding fixed energy orbits for electrons could not be derived from his findings.

🔍 Option Analysis

  • A: Correctly states the atom's empty space.
  • B: Accurate ratio derived from the nucleus's size.
  • D: Correctly identifies that most atomic mass is nuclear.
  • C: Incorrect inference as stationary orbits were not a part of Rutherford's conclusions.

⚡ Mnemonic / Speed-Run

Remember: "Empty Nucleus Relates (E.N.R.)" to recall key findings of Rutherford's model (empty space, nucleus mass concentration).

6. Visual Suggestion

6. Visual Suggestion

A diagram illustrating the Rutherford experiment setup, indicating the alpha particle beam, gold foil, and deflection angles, would clarify the conclusions drawn from the experiment.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XI Chemistry, Chapter 2 (Structure of Atom).

Question 4 of 5 NDA MCQ

Rutherfords alpha-particle  scattering experiment was responsible for the discovery of which one of the following?

Detailed Explanation

Correct Answer

✓ Option C is correct. Rutherford's experiment led to the discovery of the atomic nucleus.

📖 Governing Law / Core Concept

The atomic nucleus is the central component of an atom that contains protons and neutrons, comprising most of an atom's mass.

🧪 Step-by-Step Breakdown

Rutherford's alpha-particle scattering experiment involved directing alpha particles onto a thin gold foil. The key observations were:

  • A majority of the alpha particles passed through the foil, indicating that atoms are mostly empty space.
  • A small fraction were deflected at large angles, suggesting the presence of a dense, positively charged nucleus.

🔍 Option Analysis

A: Electron - Discovered by J.J. Thomson in 1897, these are negatively charged subatomic particles.

B: Proton - Identified by Ernest Rutherford in 1919, protons are positively charged particles found in the nucleus.

D: Neutron - Discovered by James Chadwick in 1932, neutrons are neutral particles also located in the nucleus.

⚡ Mnemonic / Speed-Run

Remember "Rutherford Rebuilt Atom" to recall that Rutherford's experiment redefined atomic structure with the nucleus at its core.

6. Visual Suggestion

6. Visual Suggestion

A diagram of Rutherford's gold foil experiment showing the paths of alpha particles, illustrating how most pass through while a few are deflected at sharp angles, highlighting the nucleus's presence.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XI Chemistry, Chapter 2 (Structure of Atom).

Question 5 of 5 NDA MCQ

What is the maximum number of electrons in the M-Shell?

Detailed Explanation

Correct Answer

✓ Option C is correct. The M-Shell can accommodate a maximum of 18 electrons.

📖 Governing Law / Core Concept

The maximum number of electrons in a shell is given by the formula \(2n^2\), where \(n\) is the principal quantum number.

🧪 Step-by-Step Breakdown

In this case, for the M-Shell:

\[ n = 3 \quad (\text{M-Shell}) \]

Substituting into the formula:

\[ \text{Maximum Electrons} = 2n^2 = 2(3^2) = 2 \times 9 = 18 \]

Thus, the M-Shell can hold a maximum of 18 electrons.

🔍 Option Analysis

  • A: 6 - Incorrect; this number corresponds to the maximum capacity of the L-Shell (n=2).
  • B: 8 - Incorrect; this is the maximum capacity for the K-Shell (n=1), and the L-Shell (n=2) can hold 8 electrons.
  • C: 18 - Correct; as derived above, represents M-Shell capacity.
  • D: 32 - Incorrect; refers to the maximum capacity of the N-Shell (n=4).

⚡ Mnemonic / Speed-Run

💡 Mnemonic for Shell Capacities:

Remember the formula \(2n^2\) and the capacities:

  • K-Shell (n=1): 2
  • L-Shell (n=2): 8
  • M-Shell (n=3): 18
  • N-Shell (n=4): 32

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class 11 Chemistry, Chapter: Structure of Atom.

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