The valence electrons are the electrons that are present in the outer shell of an atom.
In this case, the "Al" works with 3 electrons of valence, while the "OH-" has only 1 electron of valence, however we have 4 atoms of "OH-", in this case, the OH- donates a lone pair of atoms to make the covalent bonds with the "Al" but kind of weak, that is why , usually this molecule has special conditions to be formed, like high pH, where the negative state remain longer.
There is only one valence electron in Al(OH)4- anion.
<h3>Explanation:</h3>
Valency is the number of electrons in the outer most shell of an atom.
In this compound, aluminium has the valency of +3. It means Al needs only 3 electrons to complete its outer most shell.
Hence the stable molecule Al(OH)3.
But the addition of one another OH- leads to the negative charge and there is one spare electron which can make bond to an atom to stable the compound.