To draw the Lewis structure for the ion \( BF_4^- \), we will follow these steps:
### Step 1: Count the total number of valence electrons
- Boron (B) is in Group 13 and has 3 valence electrons.
- Fluorine (F) is in Group 17 and has 7 valence electrons. Since there are 4 fluorine atoms, we multiply 7 by 4, which gives us 28 valence electrons from fluorine.
- The ion \( BF_4^- \) has a negative charge, which means we need to add 1 extra electron to our total count.
**Calculation:**
\[
\text{Total valence electrons} = 3 \, (\text{from B}) + 28 \, (\text{from 4 F}) + 1 \, (\text{for the negative charge}) = 32 \, \text{electrons}
\]
### Step 2: Arrange the atoms
- Place the boron atom in the center because it is less electronegative than fluorine.
- Arrange the four fluorine atoms around the boron atom.
### Step 3: Form bonds between boron and fluorine
- Each fluorine atom will form a single bond with the boron atom. A single bond consists of 2 electrons.
- Since there are 4 fluorine atoms, the total number of electrons used in bonding is \( 4 \times 2 = 8 \) electrons.
### Step 4: Calculate remaining electrons
- We started with 32 valence electrons and used 8 for bonding.
- Remaining electrons = \( 32 - 8 = 24 \) electrons.
### Step 5: Distribute remaining electrons as lone pairs
- Each fluorine atom needs to complete its octet. Each fluorine already has 2 electrons from the bond with boron, so it needs 6 more electrons (3 lone pairs).
- Since there are 4 fluorine atoms, we will place 3 lone pairs on each fluorine atom:
\[
4 \, (\text{F}) \times 3 \, (\text{lone pairs}) = 12 \, \text{lone pairs} = 24 \, \text{electrons}
\]
### Step 6: Verify the structure
- Each fluorine atom has 8 electrons (2 from the bond and 6 from the lone pairs), satisfying the octet rule.
- The boron atom has 4 electrons (from the 4 bonds), which is acceptable as boron can be an exception to the octet rule.
### Step 7: Indicate the charge
- Since we have a negative charge on the ion, we will indicate this by placing a minus sign near the structure.
### Final Lewis Structure
The final Lewis structure for \( BF_4^- \) is represented as follows:
```
F
|
F--B--F
|
F
[ - ]
```
Each fluorine has three lone pairs of electrons around it.

Lewis -structure `BF_(4)^(-)`

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