Which of the following are isotones ?

Options

A. `._(18)Ar^(40)`
B. `._(20)Ca^(42)`
C. `._(21)Se^(43)`
D. `._(21)^(Se)^(41)`

Explanation

To determine which of the given elements are isotones, we need to follow these steps:

### Step 1: Understand the Definition of Isotones
Isotones are nuclides (elements) that have the same number of neutrons but different numbers of protons.

### Step 2: Identify the Elements and Their Atomic Numbers
We will consider the following elements:
1. Argon (Ar) with atomic number 18 and atomic mass 40
2. Calcium (Ca) with atomic number 20 and atomic mass 42
3. Scandium (Sc) with atomic number 21 and atomic mass 43
4. Cerium (Ce) with atomic number 58 and atomic mass 141 (not relevant here as it was mentioned incorrectly in the transcript)

### Step 3: Calculate the Number of Neutrons for Each Element
The number of neutrons (N) can be calculated using the formula:
\[ N = A - Z \]
where A is the atomic mass number and Z is the atomic number.

1. For Argon (Ar):
- A = 40, Z = 18
- N = 40 - 18 = 22

2. For Calcium (Ca):
- A = 42, Z = 20
- N = 42 - 20 = 22

3. For Scandium (Sc):
- A = 43, Z = 21
- N = 43 - 21 = 22

### Step 4: Compare the Number of Neutrons
Now we compare the number of neutrons calculated:
- Argon: 22 neutrons
- Calcium: 22 neutrons
- Scandium: 22 neutrons

Since all three elements have the same number of neutrons (22), they are isotones.

### Conclusion
The isotones among the given options are Argon (Ar), Calcium (Ca), and Scandium (Sc).

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