* Gold (Au) is an element. It's a pure substance that cannot be broken down into simpler substances.

* Brass is an alloy, which is a mixture. It's a combination of two or more metals, typically copper (Cu) and zinc (Zn).

Key Differences:

* Elements are found on the periodic table and represent the simplest building blocks of matter.

* Mixtures can be separated by physical means (like magnetism, filtration, evaporation). The components retain their individual properties.

* Compounds are formed when two or more elements chemically combine in a fixed ratio. They have properties different from the original elements.

So, in summary:

* Gold is an element.

* Brass is a mixture (an alloy).