All atoms are composed of neutrons, protons, and electrons. The protons and neutrons are present in the atom's nucleus which is surrounded by electrons . The number of protons in an atom is unique for each element and represents the element's atomic number . For example, oxygen has eight protons, silicon has fourteen. Protons have a positive charge that is balanced by the negatively charged electrons.
Neutrons are neutral, they have no charge.
Different shells hold different numbers of electrons. The two innermost shells hold 2 and 8 electrons respectively. Outer shells can hold more electrons but are considered stable when they contain eight electrons. The number of electrons should ideally equal the number of protons. However, atoms may loose or gain electrons to reach a configuration with a stable outer shell.