What is an atom?

An atom is defined as the smallest unit of an element that retains its chemical properties. This means that even when an atom is isolated from a substance, it still exhibits the characteristics and behaviors associated with that element. For instance, a hydrogen atom will always have properties that define hydrogen, such as its reactivity and the types of bonds it can form.

In terms of its structure, an atom consists of a nucleus made up of protons and neutrons, surrounded by a cloud of electrons. This intricate structure is fundamental in determining how atoms interact and bond with each other, forming molecules and compounds, which are the building blocks of matter. Understanding that atoms are the smallest unit of an element important because it lays the foundation for further studies, including chemical reactions and the formation of various materials.

The other options presented do not accurately describe what an atom is: atoms are far from being the largest unit of an element; they are single units rather than collections of molecules; and while energy plays a critical role in the behavior of atoms and their interactions, atoms themselves are tangible entities composed of matter rather than just energy.