An atom is the smallest particle of an element and chemically not separable. An atom consists of a very dense nucleus and an atomic shell. The main mass of the atom is concentrated in the nucleus in the form of protons (electrically positively charged particles) and neutrons (electrically neutral elementary particles). The atomic shell consists of negatively charged electrons orbiting the nucleus. Atoms are electrically neutral as the number of protons in the nucleus and the number of electrons in the shell is identical.