Which of the following statements are correct?

Options

A. Nuclei having an odd number of protons and an odd number of neutrons are generally unstable.
B. The mas number of a nucleus is equal to its atomic number only for the nucleus `""_(1)^(1)H`.
C. A radioactive element of half life 1.5 years completely disintegrates in 4.5 years
D. The mass per nucleon in an oxygen atom is slightly less than ina hydrogen atom.

Answer

To determine which statements are correct, let's analyze each statement one by one:

### Step 1: Analyze Statement 1
**Statement 1:** Nuclei having an odd number of protons and an odd number of neutrons are generally unstable.

- **Explanation:** It is a known fact in nuclear physics that nuclei with odd numbers of both protons and neutrons tend to be less stable compared to those with even numbers. This is due to pairing effects in nuclear forces. Therefore, this statement is **correct**.

### Step 2: Analyze Statement 2
**Statement 2:** The mass number of the nucleus is equal to its atomic number only for the nucleus H-1 (Hydrogen-1).

- **Explanation:** The mass number (A) is the total number of protons and neutrons in the nucleus, while the atomic number (Z) is the number of protons. For hydrogen-1 (H-1), which has 1 proton and 0 neutrons, the mass number (1) equals the atomic number (1). For all other elements, the mass number is greater than the atomic number. Thus, this statement is **correct**.

### Step 3: Analyze Statement 3
**Statement 3:** A radioactive element with a half-life of 1.5 years completely disintegrates in 4.5 years.

- **Explanation:** The half-life of a radioactive substance is the time taken for half of the substance to decay. After 4.5 years (which is 3 half-lives), the remaining amount of the substance would be (1/2)^3 = 1/8 of the original amount. Therefore, it does not completely disintegrate in 4.5 years. This statement is **incorrect**.

### Step 4: Analyze Statement 4
**Statement 4:** The mass per nucleon in an oxygen atom is slightly less than in a hydrogen atom.

- **Explanation:** The mass per nucleon is calculated by dividing the total mass of the nucleus by the number of nucleons (protons + neutrons). Hydrogen has a lower mass per nucleon compared to heavier elements like oxygen due to binding energy differences. Therefore, this statement is **correct**.

### Conclusion
Based on the analysis:
- Statement 1: Correct
- Statement 2: Correct
- Statement 3: Incorrect
- Statement 4: Correct

Thus, the correct statements are **1, 2, and 4**.

### Final Answer
The correct statements are: **1, 2, and 4**.

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