The shape of `CH_(3)^(-)` is......................while that of `CH_(3)^(+)` is................. .

Answer

To determine the shapes of the species CH₃⁻ (methyl anion) and CH₃⁺ (methyl cation), we need to analyze their electron pair geometries and steric numbers.

### Step-by-Step Solution:

1. **Identify the Central Atom and Valence Electrons:**
- The central atom in both CH₃⁻ and CH₃⁺ is carbon (C).
- Carbon has 4 valence electrons.

2. **Analyze CH₃⁻ (Methyl Anion):**
- In CH₃⁻, carbon is bonded to three hydrogen atoms and has an extra electron, giving it a total of 5 electrons around it.
- The structure can be represented as:
- 3 bond pairs (C-H bonds)
- 1 lone pair (the extra electron)
- **Steric Number Calculation:**
- Steric number = Number of bond pairs + Number of lone pairs = 3 (bond pairs) + 1 (lone pair) = 4.
- **Electron Geometry:**
- With a steric number of 4, the electron geometry is tetrahedral.
- **Molecular Shape:**
- Since there is one lone pair, the molecular shape is affected. The presence of the lone pair leads to a pyramidal shape (specifically, trigonal pyramidal) due to lone pair-bond pair repulsion being stronger than bond pair-bond pair repulsion.

3. **Analyze CH₃⁺ (Methyl Cation):**
- In CH₃⁺, carbon is bonded to three hydrogen atoms but has lost one electron, resulting in a total of 3 electrons around it.
- The structure can be represented as:
- 3 bond pairs (C-H bonds)
- 0 lone pairs.
- **Steric Number Calculation:**
- Steric number = Number of bond pairs + Number of lone pairs = 3 (bond pairs) + 0 (lone pairs) = 3.
- **Electron Geometry:**
- With a steric number of 3, the electron geometry is trigonal planar.
- **Molecular Shape:**
- Since there are no lone pairs, the molecular shape is also trigonal planar.

### Final Answers:
- The shape of CH₃⁻ is **pyramidal** (specifically, trigonal pyramidal).
- The shape of CH₃⁺ is **trigonal planar**.