How many interhalogen compounds are not possible ?
`IC l,IB r,Brl,ClBr_(3),ClF_(3),BrCl_(5),BrI_(5),Icl_(3),IF_(7)`

Answer

To determine how many interhalogen compounds are not possible from the given list, we need to apply the criteria for the formation of interhalogen compounds. The criteria state that an interhalogen compound (X-X') can only exist if:

1. X is less electronegative than X'.
2. X has a larger atomic size than X'.

Now, let's analyze each compound in the list:

1. **ICl**:
- Iodine (I) is less electronegative than Chlorine (Cl) and has a larger size.
- **Exists**.

2. **IBr**:
- Iodine (I) is less electronegative than Bromine (Br) and has a larger size.
- **Exists**.

3. **BrI**:
- Iodine (I) is less electronegative than Bromine (Br), but the size of Br is smaller than I.
- **Does not exist**.

4. **ClBr3**:
- Chlorine (Cl) is more electronegative than Bromine (Br) and has a smaller size.
- **Does not exist**.

5. **ClF3**:
- Chlorine (Cl) is less electronegative than Fluorine (F) and has a larger size.
- **Exists**.

6. **BrCl5**:
- Bromine (Br) is less electronegative than Chlorine (Cl) and has a larger size.
- **Exists**.

7. **BrI5**:
- Bromine (Br) is more electronegative than Iodine (I) and has a smaller size.
- **Does not exist**.

8. **ICl3**:
- Iodine (I) is less electronegative than Chlorine (Cl) and has a larger size.
- **Exists**.

9. **IF7**:
- Iodine (I) is less electronegative than Fluorine (F) and has a larger size.
- **Exists**.

Now, let's summarize the findings:

- Compounds that **exist**: ICl, IBr, ClF3, BrCl5, ICl3, IF7
- Compounds that **do not exist**: BrI, ClBr3, BrI5

Thus, the total number of interhalogen compounds that are not possible is **3**.

### Final Answer:
**3 interhalogen compounds are not possible.**