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The atom is a basic unit of matter that 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-1, which is the only stable nuclide with no neutrons ).

The electrons of an atom are bound to the nucleus by the electromagnetic force.

Likewise, a group of atoms can remain bound to each other, forming a molecule.

An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive charge if there are fewer electrons ( electron deficiency ) or negative charge if there are more electrons ( electron excess ).

A positively or negatively charged atom is known as an ion.

An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.