The nucleus of an atom is surrounded by an electron cloud that is held in place by an electromagnetic force. The nucleus accounts for significantly more of the atom's total mass than the cloud, although the cloud accounts for a greater volume of the atom.
An atom has three building blocks: protons, neutrons and electrons. The centrally located nucleus is composed of the positively charged protons and the neutral neutrons. These items are bound together by a nuclear force. The number of protons in the nucleus determines what element the atom belongs, commonly referred to as the atomic number. The surrounding electron cloud is entirely composed of negatively charged electrons. Despite the nucleus and surrounding cloud, most of an atom is empty space.
The cloud model was not the first concept. Several models were considered before the cloud was accepted in the 1920s. The earlier models included the Billiard Ball and Plum Pudding models. The Bohr Model was adopted in the twentieth century. In this model, the atom is similar to a solar system.
An electron has a charge of 1.0671 x 10^-19 coulomb. One important property of electrons is the spin. The spin on an electron is a specific rate of 1/2 times a unit of rotation, which may be assigned by the individual doing the measurement.