The atom is the smallest unit of any element. The atom contains all the properties of the element and is made of a nucleus surrounded by a cloud of negatively charged electrons.
The atom is the essence of an element. An atom contains a nucleus made up of positively charged protons and neutrally charged neutrons, and they are surrounded by a cloud of electrons that are bound to the nucleus by an electromagnetic field. The bulk of the mass of an atom is contained in the nucleus. All atoms are close to the same size. The most important characteristic of an atom is the number of protons housed in its nucleus; this is known as its atomic number.