Chemistry Q53 – JEE-MAIN 2026

Home Chemistry Structure of AtomQuestion #605

Identify the correct statements from the following : A. Heisenberg uncertainty principle is applicable to electrons. B. The size of

2 p_{x}

orbital is less than the size of

3 p_{x}

orbital. C. The energy of

2 s

orbital of H atom is equal to the energy of

2 s

orbital of Li. D. The electronic configuration of Cr is

[A r] 3 d^{5} 4 s^{1}

Choose the correct answer from the options given below:

Clue 1 of 3: Fundamental Principles

Recall the Heisenberg Uncertainty Principle, factors affecting orbital size, and the dependence of orbital energy on nuclear charge and electron-electron interactions.

🧠 Full Solution Path

Step 1: Evaluate Statements A and B✦ Active

Statement A: The Heisenberg Uncertainty Principle is a fundamental concept in quantum mechanics, applicable to microscopic particles like electrons, stating that it's impossible to simultaneously know both the exact position and momentum of a particle. Thus, statement A is correct.

Statement B: The size of an atomic orbital increases with the principal quantum number ( $n$ ). A $3 p_{x}$ orbital has $n = 3$ , while a $2 p_{x}$ orbital has $n = 2$ . Therefore, the $3 p_{x}$ orbital is larger than the $2 p_{x}$ orbital. Thus, statement B is correct.

Step 2: Evaluate Statement C○ Expand

Statement C: For a hydrogen atom (single-electron system), the energy of an orbital depends only on the principal quantum number ( $n$ ). For multi-electron atoms like Lithium (Li), the energy of an orbital depends on both $n$ and the azimuthal quantum number ( $l$ ) due to electron-electron repulsion and shielding effects. Additionally, the nuclear charge ( $Z$ ) is different (H has $Z = 1$ , Li has $Z = 3$ ). Therefore, the energy of the $2 s$ orbital of H atom is not equal to the energy of the $2 s$ orbital of Li. Thus, statement C is incorrect.

💡 Teacher's Secret Hint

Remember that orbital energies are degenerate for a given 'n' in hydrogenic atoms, but not in multi-electron atoms.

Step 3: Evaluate Statement D and Conclude○ Expand

Statement D: Chromium (Cr, $Z = 24$ ) exhibits an anomalous electronic configuration. Instead of the expected $[A r] 3 d^{4} 4 s^{2}$ , it adopts $[A r] 3 d^{5} 4 s^{1}$ to achieve the extra stability associated with a half-filled $3 d$ subshell. Thus, statement D is correct.

Based on the evaluation, statements A, B, and D are correct. Therefore, the correct option is 'A, B and D Only'.

💡 Teacher's Secret Hint

Anomalous configurations for Cr and Cu are common exceptions to the Aufbau principle due to stability considerations.

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Chemistry Question 53 – JEE-MAIN 2026

Clue 1 of 3: Fundamental Principles

🧠 Full Solution Path