Pick out the isoelectronic structures from the following
(i) `CH_(3)^(+)` , (ii) `H_(3)O^(+)`
(iii) `NH_(3)` , (iv) `CH_(3)^(-)`

Options

A. Option 1. (i) and (ii)
B. Option 2. (iii) and (iv)
C. Option 3. (i) and (iii)
D. Option 4. (ii) , (iii) and (iv)

Answer

To determine the isoelectronic structures from the given options, we need to calculate the total number of electrons in each species. Isoelectronic species are those that have the same number of electrons.

### Step-by-Step Solution:

1. **Calculate the number of electrons in CH₃⁺ (Methyl cation)**:
- Carbon (C) has an atomic number of 6, which means it has 6 electrons.
- Each hydrogen (H) has 1 electron, and there are 3 hydrogens, contributing a total of 3 electrons.
- The positive charge indicates a loss of 1 electron.
- Total electrons = 6 (C) + 3 (H) - 1 (charge) = 8 electrons.

2. **Calculate the number of electrons in H₃O⁺ (Hydronium ion)**:
- Oxygen (O) has an atomic number of 8, which means it has 8 electrons.
- Each hydrogen contributes 1 electron, and there are 3 hydrogens, contributing a total of 3 electrons.
- The positive charge indicates a loss of 1 electron.
- Total electrons = 8 (O) + 3 (H) - 1 (charge) = 10 electrons.

3. **Calculate the number of electrons in NH₃ (Ammonia)**:
- Nitrogen (N) has an atomic number of 7, which means it has 7 electrons.
- Each hydrogen contributes 1 electron, and there are 3 hydrogens, contributing a total of 3 electrons.
- Total electrons = 7 (N) + 3 (H) = 10 electrons.

4. **Calculate the number of electrons in CH₃⁻ (Methyl anion)**:
- Carbon (C) has 6 electrons.
- Each hydrogen contributes 1 electron, and there are 3 hydrogens, contributing a total of 3 electrons.
- The negative charge indicates a gain of 1 electron.
- Total electrons = 6 (C) + 3 (H) + 1 (charge) = 10 electrons.

5. **Compare the total number of electrons**:
- CH₃⁺ has 8 electrons.
- H₃O⁺ has 10 electrons.
- NH₃ has 10 electrons.
- CH₃⁻ has 10 electrons.

From the calculations, we can see that H₃O⁺, NH₃, and CH₃⁻ all have 10 electrons, making them isoelectronic. Therefore, the isoelectronic structures are H₃O⁺, NH₃, and CH₃⁻.

### Conclusion:
The correct answer is that the isoelectronic structures are the 2nd, 3rd, and 4th options.