Atom refers to the basic particles which can not be separated in chemical reaction. But it can be divided in the physical state. An atom consists of a nucleus and electrons moving around it. An atom is the smallest unit of general matter, called an element. There are 118 known elements. So it has nuclear structure.

Atom is the earliest abstract concept with ontological significance in philosophy. With the progress of human understanding, atom has gradually changed from an abstract concept to a scientific theory. Nuclei and electrons are microscopic particles that make up atoms. And atoms can form molecules.

Source: @luis.herlopz