Use lewis structures to explain why br3 - and i3 - are stable, while f3 - is not

If you sketch a Lewis dot structure of I3-, you'll perceive that the center I atom formulates two bonds, and also has three non-bonding pairs of electrons. That's an over-all of five electron pairs - beyond an octet. Lots of compounds disrupt the octet rule that means, but the first long-row elements like F can't because the lower elements like iodine can use an obtainable d-orbital to house the additional pair of electrons (sp3d hybridization), but F just doesn't have a d-orbital that it can use.