If the Lord's supper was at passover, why in the Greek bible do they use the word Artos,which means leavened or raised bread?
I am not a scholar of Greek, but those that are do not appear to make the distinction you do. Generally the word artos means bread of any sort, not specifically raised bread. Some say it is from the root “Aro” which means a mixture, but does not imply whether or not leaven is included. Others say it is from the root “Ar” which means earth, and thus a plant. It is sometimes used for food in general (not just bread). The only time the Greek New Testament uses the word unleavened (azumos) is in specific reference to the Feast of Unleavened (Passover). Yet when speaking of the bread at that ceremony it uses artos.
To one who knew about the Passover, saying bread automatically implies unleavened bread. This is especially true when just a few verses before the reference to bread, the word unleavened is used.