Final answer:
An atom is the smallest unit of an element that retains the element's chemical properties, including protons in the nucleus and electrons in the surrounding cloud.
Step-by-step explanation:
An atom is the smallest particle of an element that still retains all the properties of that element. Atoms are the basic building blocks of matter and are so small that they are beyond the direct observation of conventional microscopes. Within the atom, there is a nucleus containing protons and neutrons, and surrounding the nucleus is an electron cloud consisting of electrons. The number of protons in an atom defines the chemical element. For instance, an atom of gold contains the properties of gold, such as being a solid metal at room temperature, and cannot be broken down into anything smaller while still retaining the properties of gold.