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.