Pick out the isobar pair.

Options

A. `""_(1)^(1)H, ""_(1)^(2)H`
B. `""_(6)^(13)Cl, ""_(7)^(14)N`
C. `""_(17)^(35)Cl, ""_(17)^(37)Cl`
D. `""_(18)^(40)Ar, ""_(20)^(40)Ca`

Explanation

To identify the isobar pair from the given options, we need to follow these steps:

### Step 1: Understand the Definition of Isobars
Isobars are atoms that have the same mass number (A) but different atomic numbers (Z). The mass number is the total number of protons and neutrons in the nucleus, while the atomic number is the number of protons.

### Step 2: Analyze the Given Atoms
We are given pairs of atoms to analyze. Let's denote them as follows:
1. Hydrogen (1H1 and 1H2)
2. Argon (18Ar and 40Ar)
3. Calcium (20Ca and 40Ca)

### Step 3: Check the Mass Numbers and Atomic Numbers
- For **Hydrogen (1H1 and 1H2)**:
- Atomic number (Z) for both is 1 (same).
- Mass numbers (A) are 1 and 2 (different).
- Conclusion: These are isotopes, not isobars.

- For **Argon (18Ar and 40Ar)**:
- Atomic number (Z) for both is 18 (same).
- Mass numbers (A) are 18 and 40 (different).
- Conclusion: These are isotopes, not isobars.

- For **Calcium (20Ca and 40Ca)**:
- Atomic number (Z) for both is 20 (same).
- Mass numbers (A) are 20 and 40 (different).
- Conclusion: These are isotopes, not isobars.

### Step 4: Identify the Correct Isobar Pair
Now, let’s consider pairs that have the same mass number but different atomic numbers:
- **Argon (40Ar) and Calcium (40Ca)**:
- Atomic number of Argon (Z) = 18
- Atomic number of Calcium (Z) = 20
- Both have the same mass number (40).
- Conclusion: This is the isobar pair.

### Final Answer
The isobar pair is **Argon (40Ar) and Calcium (40Ca)**.

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