Which of the following best defines an atom?

An atom is indeed defined as the basic unit of a chemical element. It consists of a nucleus, which contains protons and neutrons, and is surrounded by electrons that orbit the nucleus in distinct energy levels. This structure is fundamental because it forms the building blocks for all matter and determines the chemical properties of each element based on the number and arrangement of these subatomic particles.

Understanding the atom is crucial for studying chemistry, as it provides insight into how elements interact, bond, and react with one another. For instance, the arrangement of electrons influences chemical behavior, while the nucleus defines the identity of the element itself. In essence, atoms are the simplest units of matter that retain the properties of an element, making option B the correct definition.