The atom is the smallest particle of an element that can take part in a chemical reaction. The subatomic particles are Cloze (1): JXUwMDM2JXUwMDBiJXUwMDEwJXUwMDAxJXUwMDA2JXUwMDFkJXUwMDAxJXUwMDFk , Cloze (2): JXUwMDI4JXUwMDAyJXUwMDFkJXUwMDFiJXUwMDFiJXUwMDAxJXUwMDFkJXUwMDVm and Cloze (3): JXUwMDNkJXUwMDA5JXUwMDA5JXUwMDA2JXUwMDE3JXUwMDA2JXUwMDFkJXUwMDAxJXUwMDFkJXUw MDVk The atom is made up two regions, the central part called the Cloze (4): JXUwMDM2JXUwMDFiJXUwMDE2JXUwMDBmJXUwMDA5JXUwMDEwJXUwMDA2 . The electrons occupy the Cloze (5): JXUwMDNkJXUwMDBiJXUwMDBiJXUwMDE3JXUwMDE1JXUwMDFlJXUwMDU5JXUwMDRjJXUwMDA5JXUw MDEzJXUwMDEzJXUwMDA5JXUwMDFm . Atomic number is the number of  Cloze (6): JXUwMDI4JXUwMDAyJXUwMDFkJXUwMDFiJXUwMDFiJXUwMDAx Cloze (7): JXUwMDJiJXUwMDUzJXUwMDQ5 n the nucleus of an atom. Mass number is the sum  of the Cloze (8): JXUwMDI4JXUwMDAyJXUwMDFkJXUwMDFiJXUwMDFiJXUwMDAxJXUwMDFk and neutrons in an atom of an element.