Smallest particle still characterizing a chemical element. It consists of a small nucleus charged positively, carrying almost all of the atom's mass, with electrons surrounding it. This tag should be applied to questions that specifically concern atoms or their properties. For the charged particles, please use [ions] instead. If your question is specifically about [protons], [neutrons], or [electrons], use those tags instead.
From the IUPAC GoldBook, an atom is the:
Smallest particle still characterizing a chemical element. It consists of a nucleus of a positive charge (Z is the atomic number and e the elementary charge) carrying almost all its mass (more than 99.9%) and Z electrons determining its size.
Note that questions about nuclear physics are most suited on Physics.SE instead.