An atom is classified according to its number of protons and neutrons: the number of protons determines the chemical element and the number of neutrons determines the isotope of that element.
The name atom comes from the Greek átomos, which means indivisible, something that cannot be divided further. The Greeks concluded that matter could be broken down into particles too small to be seen. These particles were called atoms. It wasn't until the early 20th century, that physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was not indivisible.
Atoms are minuscule objects with proportionately tiny masses. It is now known that the atom nucleus makes up more than 99.9% of the its mass but only about 1/100,000 of its volume, with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus. Electrons occupy a set of stable energy levels, or orbitals, and can transition between these states by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.