* Definition of an Element: An element is a pure substance that cannot be broken down into simpler substances by chemical means.

* Atoms as Building Blocks: Atoms are the smallest unit of an element that retains the chemical properties of that element.

* Unique Identity: Each element has a unique atomic number, which represents the number of protons in its nucleus. This number defines the element's identity.

Example: Gold (Au) is an element. Every atom in a piece of gold will have 79 protons. If an atom doesn't have 79 protons, it's not gold.

Key Takeaway: The defining characteristic of an element is that it consists of only one type of atom. This atom is characterized by its unique atomic number.