How can I write the formula for aluminum oxide?

The formula for aluminum oxide is ##Al_2O_3##.

The correct answer is ##Al_2O_3##.

Let us see how we got the answer; Look at the electronic arrangement of Al and O atoms.

Al ( Z= 13) has 13 electrons with following electronic configuration. 1##s^2##2##s^2##2##p^6##3##s^2##3##p^1##

It loses three electron in its 3s and 3p subshell to achieve stability and forms ion ##Al^(3+)##.

##Al^(3+)## = 1##s^2##2##s^2##2##p^6##

O ( Z=8) on the other hand has eight electrons and wants to gain two electrons to achieve stable noble gas configuration.Oxygen atom on gaining two electrons forms negative oxide ion, ##O^(2-)## ion.

O ( Z=7) = 1##s^2##2##s^2##2##p^4##

##O^(2-)## = 1##s^2##2##s^2##2##p^6##

Three Oxygen atoms gains two electrons each ( total of six) from two Al atoms, each Al atom loses three electrons (total of six) to three oxygen atom , in this process each Al atom becomes ##Al^(3+)## ion and Oxygen atom after gaining two electrons becomes ##O^(2-)## ion.

2##Al^(3+)## and 3 ##O^(2-)##

or the formula is ##Al_2O_3##.