Assertion (A): Magnesium does not impart any characteristic colour to the flame.
Reason (R ): Due to small size and high effective nuclear charge, magnesium requires a large amount of energy for excitation of electrons.

Options

A. If both `(A)` and `(R )` are correct and `(R )` is the correct explanation of `(A)`.
B. If both `(A)` and `(R )` are correct, but `(R )` is not the correct explanation of `(A)`.
C. If `(A)` is correct, but `(R )` is incorrect.
D. If `(A)` is incrrect, but `(R )` is correct.

Answer

To analyze the assertion and reason provided in the question, we can break down the information step by step.

### Step 1: Understanding the Assertion
The assertion states that "Magnesium does not impart any characteristic colour to the flame." This means that when magnesium is heated in a flame, it does not produce a distinct color that is typically associated with other elements.

### Step 2: Understanding the Reason
The reason given is that "Due to small size and high effective nuclear charge, magnesium requires a large amount of energy for excitation of electrons." This implies that magnesium's atomic structure makes it difficult for its electrons to be excited to higher energy levels, which is necessary for producing a flame color.

### Step 3: Analyzing Magnesium's Electron Configuration
Magnesium has an atomic number of 12, which gives it the electron configuration of:
- 1s² 2s² 2p⁶ 3s²

In this configuration, the outermost shell (3s) is fully filled with 2 electrons.

### Step 4: Stability of the Configuration
A fully filled valence shell is a stable configuration. The stability means that it is less likely for the outer electrons to be excited to higher energy levels when energy is applied (like in a flame test).

### Step 5: Ionization Energy
The high ionization energy of magnesium indicates that it requires a significant amount of energy to remove an electron from this stable configuration. This high ionization energy is a result of the small atomic size and high effective nuclear charge.

### Step 6: Conclusion
Since magnesium does not easily allow its electrons to be excited due to its stable electron configuration and high ionization energy, it does not impart any characteristic color to the flame. Therefore, both the assertion and reason are correct, and the reason correctly explains the assertion.

### Final Answer
Both the assertion (A) and reason (R) are true, and R is the correct explanation for A.

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