Which of the following correctly describes an atom?

An atom is defined as the smallest unit of an element that retains the properties of that element. Each atom consists of a nucleus, made up of protons and neutrons, surrounded by electrons in various energy levels. This foundational characteristic is crucial because it means that an atom embodies the essential chemical properties of its corresponding element, such as reactivity and atomic mass.

In contrast, a blend of two or more elements refers to a compound or a mixture, which consists of multiple atoms or molecules. The concept of a basic unit of a compound pertains to molecules formed from two or more atoms bonded together, rather than individual atoms themselves. Lastly, a mixture of different atoms implies the presence of multiple distinct atom types, which is not an accurate description of an atom, as each atom represents one specific type of element. Hence, identifying the atom as the smallest unit of an element is fundamental in understanding the structure of matter and the building blocks of all substances.