Here's why that happens.
As you know, the strength of an acid is determined by the amount of protons it realeases in aqueous solution.
This means that an acid's degree of dissociation will determine its strength
- Complete or significant dissociation
- Partial dissociation
So, hydrofluoric acid is indeed weaker than hydroiodic acid because it does not dissociate to the same extent that
There are actually two points of view to take into consideration here
- how tight is the proton being held
- how stable the resulting anion is
Let's take the first one. The difference in electronegativity between fluorine, which is the most electronegative element in the periodic table, and hydrogen causes the bond that forms between the two atoms to be very polar.
This means that the bonding electrons will spend most of their time around fluorine, leaving fluorine with a partial negative charge and hydrogen with a partial positive charge.
By comparison, the bond between hydrogen and iodine is much less polar.
This is important because the more polar a covalent bond is, the easier it is for the hydrogen to be plucked off by the water molecules.
If you go by this factor alone,
Fluorine is a very small atom. That means that the fluoride anion,
That happens because the negative charge must be distributed on a very small surface, i.e. it will be much more concentrated.
By comparison, iodine is a much larger atom than fluorine is. This means that the negaative charge that the iodide anion,
This in turn increases the stability of the anion considerably.
As it turns out, these two factors are competing against each other. When it comes to the ease with which the proton can be lost,
But the significant stability that the iodide anion has makes
In fact, this is why hydrofluoric acid is the only hydrohalic acid (compounds that have hydrogen bonded to a halogen) that's not considered a strong acid.