What is the smallest unit of a chemical element?

The smallest unit of a chemical element is an atom. Atoms are the fundamental building blocks of matter and represent the simplest form of an element that retains its properties. Each element on the periodic table is made up of its own type of atom, characterized by a specific number of protons in its nucleus, which determines the element's identity.

Atoms consist of a nucleus containing protons and neutrons, surrounded by a cloud of electrons. This structure allows atoms to combine with one another to form molecules and compounds, which are larger and more complex structures that can be made up of two or more atoms bonded together.

In contrast, molecules are formed when two or more atoms (which can be of the same or different elements) bond together. Compounds, on the other hand, are a specific type of molecule that consists of two or more different elements chemically combined. Ions are charged species that result from atoms gaining or losing electrons, but they are not the basic unit of elements themselves. Therefore, the atom is the correct answer as it represents the smallest unit of a chemical element that still retains the properties of that element.