An atom is the smallest unit of a chemical element that retains all of its physical and chemical properties.
At the core of an atom is a nucleus that contains protons which are positively charged sub-atomic particles. The number of protons defines the atomic number and thus the chemical element of the atom. For instance, a hydrogen atom has one proton.
In addition to protons, the nucleus of an atom also houses neutrons, sub-atomic particles with a mass slightly larger than protons but with no electrical charge.
Circling the nucleus are negatively charged particles called electrons, which are kept in place by their attraction to the positively charged protons in the nucleus.