Assertion: The average mass of one Mg atom is `24.305 amu`, which is not actual mass of one Mg atom.
Reason: Three isotopes, `24 Mg, 25 Mg and 26 Mg`, of Mg are found in nature.

Options

A. Statement-I is true, Statement-II is true , Statement-II is correct explanation for Statement-I
B. Statement-I is true, Statement-II is true , Statement-II is `NOT` a correct explanation for statement-I
C. Statement-I is true, Statement-II is false
D. Statement-I is false, Statement-II is true

Answer

To solve the given question, we need to analyze the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that the average mass of one magnesium (Mg) atom is 24.305 amu, which is not the actual mass of one Mg atom.

### Step 2: Understand the Reason
The reason provided states that there are three isotopes of magnesium: \(^{24}\text{Mg}\), \(^{25}\text{Mg}\), and \(^{26}\text{Mg}\).

### Step 3: Relate the Assertion and Reason
The average atomic mass of an element is calculated based on the natural abundance of its isotopes. In the case of magnesium, the average atomic mass (24.305 amu) is derived from the contributions of its isotopes weighted by their relative abundances.

### Step 4: Calculate Average Atomic Mass
To calculate the average atomic mass, we use the formula:
\[ \text{Average Atomic Mass} = \frac{(m_1 \cdot f_1) + (m_2 \cdot f_2) + (m_3 \cdot f_3)}{f_1 + f_2 + f_3} \]
Where:
- \(m_1\), \(m_2\), and \(m_3\) are the masses of the isotopes (24, 25, and 26 amu).
- \(f_1\), \(f_2\), and \(f_3\) are the fractional abundances of these isotopes.

### Step 5: Conclusion
Since the average atomic mass (24.305 amu) is calculated based on the isotopes and their abundances, it is indeed different from the actual mass of the most common isotope (\(^{24}\text{Mg}\)), which is 24 amu. Therefore, both the assertion and reason are correct.

### Final Answer
Both the assertion and reason are correct, and the reason correctly explains the assertion.

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