What is the definition of an atom?

An atom is defined as the smallest unit of an element that retains the properties of that element. This definition is significant because it emphasizes that although atoms are indeed tiny and numerous, they still possess the unique characteristics of the element from which they derive. These characteristics can include the element's reactivity, phase, and atomic structure, which are still evident even at the atomic level.

The concept of the atom encompasses its structure, which includes protons, neutrons, and electrons. However, just classifying it as a particle of matter, as mentioned in the first option, doesn't capture its role in the identity of chemical substances. The third option incorrectly describes an atom as indivisible; while this is historically how atoms were viewed, modern science recognizes that atoms can be divided into subatomic particles. Lastly, a charged particle formed from ions describes ions rather than atoms, as ions are atoms or molecules that have gained or lost one or more electrons, resulting in a net charge. This distinction clarifies that an atom is the foundational unit of elements, which is best conveyed through the chosen definition.