* Compound: A substance formed when two or more different elements chemically combine in a fixed ratio.

* Element: A pure substance that cannot be broken down into simpler substances by chemical means.

* Mixture: A combination of two or more substances that are not chemically bonded.

In the case of aluminum bromide, aluminum (Al) and bromine (Br) are chemically bonded together in a 1:3 ratio, forming a new substance with properties different from either of the original elements.