The part of an atom that is actively exchanged or shared in a chemical bond is ELECTRON.
An atom is made up of three sub particles, which are electron, proton and neutron. The proton and the neutron are located in the nucleus of the atom and they make up the major mass of the atom. The electron is located outside of the nucleus and it orbit around the nucleus; it has negligible mass. The electron is negatively charged and because it is located outside of the nucleus, it is the one that is always involved in chemical reactions. There are different types of chemical bonds in chemical compounds and it is electrons that are normally used to form these bonds. During bond formation, electrons can either be donated or shared.