The term " basic unit of matter" refers to an atom.
The atom consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons except in the case of hydrogen which is considered the only stable nuclide with no neutrons.
The electrons of an atom are bound to the nucleus by electromagnetic force. Two or more atoms can be bound to each to form a molecule.