Find the oxidation number of the following elements.
Al =
Br =
F=
Ca =
Answers
Aluminium doesn't have one definite oxidation number. There is only one element on the table that I can say has only one oxidation state and that is Fluorine. (Note I am not including the oxidation state of 0, it is an oxidation state all elements can have). However Aluminium has a number of common oxidation states. The most common ones are +3,+2,+1. However what determines these oxidation states is a set of rules which help give the number.
The oxidation state of any uncombined or unimolecular species will always be 0. E.g. H2, O2 and S8.
The net oxidation state of a neutral molecle will also be 0. This is determined by the oxidation states of the individual atoms in the molecule. E.g. In H2O, the overall oxidation state (O/S) is 0, but oxygen has an O/S of -2 and the H an O/S of +1 each.
If the molecule is charged then the net oxidation state will equal the charge in the same way as the neutral molecule above. E.g. OH^-1, the oxygen again is -2 and the hydrogen +1. The over all charge on the molecule is -1 and -2+1=-1.
In a molecule the more electronegative molecule will always be given the negative O/S. Which is why Fluorine is always -1, there's nothing more electronegative than it. The least electronegative will take the positive charge O/S
There are some elements that pretty much always have the same O/S.
Group 1 metals are normally the least electronegative so will always have a positive charge, they also only have one valence electron to donate so will almost always be +1
Group 2 metals is pretty much the same as group 1 but will have an O/S of +2 instead
Oxygen is almost always -2 O/S. The exceptions are when it is bonded to Fluorine (F2O) (+2) or in a peroxide (H2O2) (-1)
Hydrogen is nearly always +1 because it only has one electron it can donate. However when in a Hydride like LiH then it has an O/S of -1
Fluorine as I said is always -1
And similar Chlorine is the same as Fluorine as it is usually -1 except when bonded to Fluorine or oxygen.
Now let's look at an aluminium compound and determine is O/S.
AlCl3 -- neutral molecule so overall 0 oxidation state, the Chlorine are going to be -1 here and there are three so that makes a total of -3. To make this neutral Aluminium must be +3 O/S
Al2O -- again neutral so overall it'll be 0. Oxygen will be -2. So aluminium will be +2 but there is two there so each aluminium is +1
AlO -- as always it is neutral so 0. Similar to the last example but the aluminium will be +2 as it is split over 1 aluminium instead of 2.
I hope this answers your question :)
Answer:
Al = +3
Be = -1
F = -1
Ca = +2