The particle a is the proton and the particle b is the electrons.
Where are electrons and protons found?
Outside of the atomic nucleus, electrons are located in electron shells, which have distinct energy levels around the nucleus. The distribution of electrons, which are negatively charged subatomic particles, surrounding the nucleus establishes the chemical characteristics of the atom.
In contrast, protons are found in the atomic nucleus. Protons are subatomic particles with a positive charge that add to the nucleus's positive charge.