Out of `CH_(3)^(o+),H_(3)O^(o+),NH_(3),CH_(3)^(Theta)` the species which is not isoelectronic is________.

Explanation

To determine which of the given species is not isoelectronic, we need to calculate the total number of electrons in each species. Isoelectronic species are those that have the same number of electrons. Let's analyze each species step by step.

### Step 1: Calculate the number of electrons in CH₃⁺
- Carbon (C) has 6 electrons.
- Each hydrogen (H) has 1 electron, and there are 3 hydrogen atoms, contributing 3 electrons.
- Since CH₃⁺ has a positive charge, it means it has lost 1 electron.

**Calculation:**
\[ \text{Total electrons in CH}_3^+ = 6 \, (\text{C}) + 3 \, (\text{H}) - 1 \, (\text{charge}) = 8 \, \text{electrons} \]

### Step 2: Calculate the number of electrons in H₃O⁺
- Oxygen (O) has 8 electrons.
- There are 3 hydrogen atoms, contributing 3 electrons.
- H₃O⁺ has a positive charge, so it has lost 1 electron.

**Calculation:**
\[ \text{Total electrons in H}_3\text{O}^+ = 8 \, (\text{O}) + 3 \, (\text{H}) - 1 \, (\text{charge}) = 10 \, \text{electrons} \]

### Step 3: Calculate the number of electrons in NH₃
- Nitrogen (N) has 7 electrons.
- There are 3 hydrogen atoms, contributing 3 electrons.
- NH₃ is neutral, so there is no charge to consider.

**Calculation:**
\[ \text{Total electrons in NH}_3 = 7 \, (\text{N}) + 3 \, (\text{H}) = 10 \, \text{electrons} \]

### Step 4: Calculate the number of electrons in CH₃⁻
- Carbon (C) has 6 electrons.
- There are 3 hydrogen atoms, contributing 3 electrons.
- CH₃⁻ has a negative charge, meaning it has gained 1 electron.

**Calculation:**
\[ \text{Total electrons in CH}_3^- = 6 \, (\text{C}) + 3 \, (\text{H}) + 1 \, (\text{charge}) = 10 \, \text{electrons} \]

### Conclusion
Now we summarize the total number of electrons for each species:
- CH₃⁺: 8 electrons
- H₃O⁺: 10 electrons
- NH₃: 10 electrons
- CH₃⁻: 10 electrons

The species that is not isoelectronic is **CH₃⁺**, as it has 8 electrons, while the others have 10 electrons.

### Final Answer
The species which is not isoelectronic is **CH₃⁺**.