Which of the following best describes an atom?

The description of an atom as the smallest unit of an element that retains its properties is fundamentally accurate. Atoms serve as the basic building blocks of matter, representing the individual units that make up elements in the periodic table. Each type of atom corresponds to a specific element and possesses unique characteristics defined by its number of protons, neutrons, and electrons.

This characterization emphasizes that even when atoms bond together to form molecules, they maintain the distinct properties of their respective elements. For example, a single hydrogen atom exhibits different chemical behavior compared to a molecule of hydrogen gas (H2), which consists of two bonded hydrogen atoms, yet still reflects the properties of hydrogen as an element.

In contrast, the other descriptions fall short: defining an atom strictly as a neutral particle from two elements disregards that atoms can exist independently and be found in pure elemental form. Asserting that an atom must always have a positive charge is inaccurate, since atoms can be neutral, and those that lose or gain electrons become ions, with negative or positive charges, respectively. Lastly, describing an atom as an aggregation of molecules misrepresents the fundamental nature of atoms, as molecules themselves are collections of two or more atoms bonded together. These clarifications highlight the essential role of atoms in