The atomic mass is the number of protons and neutrons in the nucleus of an atom.
An atom is the smallest unit of matter. It is composed of three subatomic particles - positively charged protons, neutrons that are neutral (have neither positive nor negative charge), and negatively charged electrons. The positive protons and neutral neutrons make up the central nucleus of the atoms. Therefore, the overall charge of the nucleus is positive. The negative electrons circle the nucleus in either orbits or electrons clouds. Electrons travel at speeds so fast that it is impossible to determine an electron's exact position at any given moment. A theme throughout all sciences is that opposite charges attract. The positve and negative electromagnetic attraction between the nucleus and the electrons is what keeps the electrons in orbit around the nucleus.