Final answer:
An atom is the smallest unit of an element that retains the element's distinct chemical properties. Atoms can be subdivided into protons, neutrons, and electrons, but only as a collective do they exhibit elemental characteristics. The size of an atom is incredibly small, necessitating quantum mechanics for accurate predictions of behavior.
Step-by-step explanation:
The smallest particle of an element that still retains its distinctive chemical properties is an atom. While an atom can be further divided into subatomic particles - protons, neutrons, and electrons - it is the atom as a whole that exhibits the properties of an element. Each element is characterized by the number of protons in its nucleus, which determines its chemical behavior. For example, a hydrogen atom, which is the smallest and most basic atom, can combine with an oxygen atom to form water, reflecting its unique reactiveness.
Atoms are extremely tiny, with sizes estimated around 100 picometers. This is so small that traditional physics fails to accurately predict their behavior, necessitating the use of quantum mechanics. It's fascinating that something as minute as an atom is the fundamental building block of all matter. In fact, it would take about fifty million hydrogen atoms lined up in a row to make a line that is 1 cm long.
Atoms of some elements can vary in their mass and density due to isotopes. For instance, chlorine has isotopes with mass numbers 35 and 37, but both retain the chemical properties of chlorine. The indivisible nature of atoms as it was once thought has been proven incorrect, but despite being composed of smaller particles, the atom is the smallest unit that participates in chemical reactions, maintaining the identity of an element.