Which of the following describes an atom?

An atom is defined as the basic unit of matter that retains the properties of an element. It is composed of three main subatomic particles: protons, which are positively charged, neutrons, which have no charge, and electrons, which are negatively charged. Protons and neutrons reside in the nucleus at the center of the atom, while electrons orbit around the nucleus in various energy levels or shells.

The understanding that atoms consist of electrons, protons, and neutrons is fundamental in chemistry because it explains the structure of elements and how they interact with one another. Each of these particles plays a crucial role in the chemical behavior of the atom. For instance, the number of protons determines the atomic number and essentially defines the element, while the arrangement of electrons influences how an atom will bond with other atoms.

In contrast, other choices do not accurately describe an atom. For example, breaking down an atom into smaller particles (the first choice) refers to subatomic particles and is not characteristic of an atom itself. The notion that an atom contains only protons (the second choice) is incorrect because it ignores the existence of neutrons and electrons. Lastly, describing an atom as the largest unit of matter (the fourth choice) is misleading,