Ok so let’s say aluminum bromide and chlorine gas react to form aluminum chloride and bromine has where does the 3 in bromide come from ???? And why does chlorine gas get a 2

Question

I meant to put gas my bad!

Answer 1

Let us write a correctly balanced equation for the reaction:

2AlBr₃ + 3Cl₂(g) ==> 2AlCl₃ + 3Br₂(g)

So, you ask where does the 3 in bromide come from. Are you asking about the 3 in front of Br₂(g) or the the 3 in AlBr₃? The 3 in front of bromine (3Br2) is based on the balancing of the equation since you must have the same number of Br atoms on each side. The 3 in AlBr₃ is there because Al has a valence of 3+ and Br has a valence of 1-, so it takes 3 Br to balance the 1 Al.

You ask why does chlorine gas get a 2? Chlorine is a diatomic gas, just like H₂, O₂, N₂, Br₂, etc. You just have to memorize which gases are diatomic.