SSC CGL Science – Matter & Atomic Structure – Tier II Level · Marathon

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SSC CGL Science – Matter & Atomic Structure – Tier II Level · Marathon – 10 questions, subject: General Science. Every question has the correct answer with a one-line explanation; tap "Show Answer". Free practice for TNPSC, UPSC, SSC, Bank and RRB exams – Shanmugam IAS Academy, Coimbatore.

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1.  
The ground-state electronic configuration of chromium (Z = 24) is:

Correct Answer: A [1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹]

Explanation:

Half-filled 3d⁵ with 4s¹ is more stable – a known exception.

2.  
The number of unpaired electrons in an iron atom (Z = 26) is:

Correct Answer: B [4]

Explanation:

3d⁶: five d orbitals, one doubly filled → 4 unpaired.

3.  
The maximum number of electrons in a d subshell is:

Correct Answer: C [10]

Explanation:

Answer: 10. d has 5 orbitals × 2 = 10.

4.  
“An orbital can hold at most two electrons, and they must have opposite spins.” This is:

Correct Answer: D [Pauli's exclusion principle]

Explanation:

Pauli: no two electrons in an atom have all four quantum numbers the same.

5.  
The total number of orbitals in the shell with n = 3 is:

Correct Answer: A [9]

Explanation:

n² = 9 (one 3s + three 3p + five 3d).

6.  
Assertion–Reason
A: A phosphorus atom (Z = 15) has three unpaired electrons.
R: By Hund's rule, electrons occupy orbitals of equal energy singly before pairing.

Correct Answer: D [Both A and R are true and R explains A]

Explanation:

3p³ – one electron in each 3p orbital, as Hund's rule requires.

7.  
According to the Aufbau order, the subshell that fills just after 4s is:

Correct Answer: C [3d]

Explanation:

Order: … 3p, 4s, 3d, 4p … (4s is lower in energy than 3d for these atoms).

8.  
In a neutral chlorine atom (Z = 17), the number of electrons with principal quantum number n = 2 is:

Correct Answer: A [8]

Explanation:

2s² 2p⁶ → 8 electrons in the L shell.

9.  
The number of water molecules in 9 g of water is:

Correct Answer: B [3.011 × 10²³]

Explanation:

9 ÷ 18 = 0.5 mol → 0.5 × 6.022 × 10²³.

10.  
Two-quantity comparison
Quantity I: number of 3d electrons in copper (Z = 29)
Quantity II: number of 3d electrons in zinc (Z = 30)

Correct Answer: C [Quantity I = Quantity II]

Explanation:

Cu is 3d¹⁰ 4s¹ (exception) and Zn is 3d¹⁰ 4s² – both have 10 d electrons.

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