An element such as copper cannot be broken into simpler substances because it consists of:

1. Breaking a substance down means separating it into things made of fewer types of atoms. 2. If an element such as copper consists of only one kind of atom, then it cannot be broken down into simpler substances. 3. Simpler substances would mean substances composed of fewer types of atoms, and one is already the minimum. 4. A compound can be broken down precisely because it holds more than one kind. 5. Isotopes of copper still count as one kind of atom chemically. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 2 'Atoms, Molecules, and Ions', sections 2.1-2.6_