Quantum Mechanical Model Of Atom

To solve the problem, we need to determine the valency of Calcium (Ca).

1. Understanding Valency:
The valency of an element is the number of electrons an atom of the element can donate or accept to form a bond.

2. Determining the Valency of Calcium:
Calcium (Ca) is an alkaline earth metal with an atomic number of 20. Its electron configuration is:
Calcium has 2 electrons in its outermost shell, and it readily loses these 2 electrons to achieve a stable electron configuration, similar to the nearest noble gas (Argon). Therefore, the valency of Calcium is 2.

Final Answer:
The correct answer is option (B): 2.

To solve this question, we need to find the correct quantum numbers for the valence electron of a sodium atom.

1. Electronic Configuration of Sodium (Na):
Atomic number of sodium = 11
Electronic configuration = 1s² 2s² 2p⁶ 3s¹
So, the valence electron is in the 3s orbital.

2. Quantum Numbers: For the 3s¹ electron:

• Principal quantum number (third shell)
• Azimuthal quantum number (s orbital)
• Magnetic quantum number (only one value for s orbital)
• Spin quantum number (can be either +½ or -½, usually +½ is taken)

3. Evaluating the Options:

• (1) – Incorrect; l = 1 corresponds to p-orbital
• (2) – Correct ✔
• (3) – Incorrect; m must be 0 for l = 0
• (4) – Incorrect; same reason as (1)

Final Answer:
The correct answer is (B) .

To solve the problem, we need to identify the molecule in which the central atom possesses exactly one lone pair of electrons in its valence shell.

1. Analyzing the Molecules:
- CH₄ (Methane): Carbon forms 4 sigma bonds with hydrogen atoms. It has no lone pairs.
- BF₃ (Boron trifluoride): Boron forms 3 bonds with fluorine atoms and has an incomplete octet with no lone pairs.
- BeCl₂ (Beryllium chloride): Beryllium forms 2 bonds and also has an incomplete octet with no lone pairs.
- NH₃ (Ammonia): Nitrogen forms 3 bonds with hydrogen atoms and has 1 lone pair of electrons in its valence shell.

2. Conclusion:
Among the given molecules, only NH₃ has a lone pair on the central atom (nitrogen).

Final Answer:
The molecule is