The bromide ion is stable because it has a full outer electron shell, while the iodine atom is unstable because it has an incomplete outer electron shell. This means that the bromide ion is not likely to react with other atoms or molecules, while the iodine atom is likely to react with other atoms or molecules in order to complete its outer electron shell.
The stability of potassium bromide is also due to the fact that the potassium ion and the bromide ion are held together by a strong ionic bond. This means that the potassium ion and the bromide ion are not likely to separate from each other, which would make the molecule unstable.
In contrast, iodine is an unstable molecule because the iodine atoms are held together by a weak covalent bond. This means that the iodine atoms are likely to separate from each other, which would make the molecule unstable.
In addition, iodine is a reactive free radical, which means that it is likely to react with other atoms or molecules in order to complete its outer electron shell. This makes iodine even more unstable than it would be if it were not a free radical.
