Which of the following triads represents isotones?

Options

A. `""_(6)^(12)C, ""_(6)^(13)C, ""_(6)^(14)C`
B. `""_(18)^(40)Ar, ""_(20)^(42)Ca, ""_(21)^(43)Se`
C. `""_(18)^(40)Ar, ""_(20)^(40)Ca, ""_(21)^(41)Sc`
D. `""_(7)^(14)N, ""_(8)^(16)O, ""_(9)^(18)F`

Answer

To determine which of the given triads represents isotones, we need to follow these steps:

### Step 1: Understand the Definition of Isotones
Isotones are atoms that have the same number of neutrons but different numbers of protons (and thus different atomic numbers).

### Step 2: Identify the Triads
We have four triads to analyze:
1. Carbon-12 (C-12), Carbon-13 (C-13), Carbon-14 (C-14)
2. Argon-40 (Ar-40), Calcium-42 (Ca-42), Selenium-43 (Se-43)
3. Argon-40 (Ar-40), Calcium-40 (Ca-40), Scandium-41 (Sc-41)
4. Nitrogen-14 (N-14), Oxygen-16 (O-16), Fluorine-18 (F-18)

### Step 3: Calculate the Number of Neutrons for Each Triad
1. **Triad 1:**
- C-12: Mass number = 12, Atomic number = 6 → Neutrons = 12 - 6 = 6
- C-13: Mass number = 13, Atomic number = 6 → Neutrons = 13 - 6 = 7
- C-14: Mass number = 14, Atomic number = 6 → Neutrons = 14 - 6 = 8
- **Result:** Different number of neutrons (6, 7, 8) → Not isotones.

2. **Triad 2:**
- Ar-40: Mass number = 40, Atomic number = 18 → Neutrons = 40 - 18 = 22
- Ca-42: Mass number = 42, Atomic number = 20 → Neutrons = 42 - 20 = 22
- Se-43: Mass number = 43, Atomic number = 34 → Neutrons = 43 - 34 = 9
- **Result:** Different number of neutrons (22, 22, 9) → Not isotones.

3. **Triad 3:**
- Ar-40: Mass number = 40, Atomic number = 18 → Neutrons = 40 - 18 = 22
- Ca-40: Mass number = 40, Atomic number = 20 → Neutrons = 40 - 20 = 20
- Sc-41: Mass number = 41, Atomic number = 21 → Neutrons = 41 - 21 = 20
- **Result:** Different number of neutrons (22, 20, 20) → Not isotones.

4. **Triad 4:**
- N-14: Mass number = 14, Atomic number = 7 → Neutrons = 14 - 7 = 7
- O-16: Mass number = 16, Atomic number = 8 → Neutrons = 16 - 8 = 8
- F-18: Mass number = 18, Atomic number = 9 → Neutrons = 18 - 9 = 9
- **Result:** Different number of neutrons (7, 8, 9) → Not isotones.

### Step 4: Conclusion
After analyzing all the triads, we find that only the second triad, which is Argon-40, Calcium-42, and Selenium-43, has the same number of neutrons (22). Therefore, this triad represents isotones.

### Final Answer
The triad that represents isotones is **Argon-40, Calcium-42, and Selenium-43**.

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