Atom

The atom is defined as the smallest subdivision of an element.
Atoms of the same element have the same size, mass and chemical properties. [1]
An atom is composed of three different particles: protons( p+ ), neutrons( n0 ) and electrons( e- ) .

Protons and neutrons are located in the nucleus of the atom, whereas the electrons spin around this nucleus in specific orbits. [1] [4]

Both the number of protons and neutrons are equal with the atomic mass of an element, whereas the number of protons equals with the number of the electrons of the same element and are equal with the atomic number of this element. [2]

How the atom really is structured had been a serious matter for many centuries and different theories raised from Democritus (c.300 BC), Dalton (c.1800), J.J. Thomson (c.1850), Rutherford (c. 1905) ,Neils Bohr , Bohr- Rutherford (c. 1920) to “Modern theory” which describes the orbits of the spinning electrons around the nucleus as a “cloud of energy levels”. [2] [3]