The structure of matter
Matter primarily consists of protons, neutrons and electrons. There are also a number of other building blocks however these are not stable. All of these particles are characterized by four properties: their electrical charge, their rest mass, their mechanical momentum and their magnetic momentum.
The number of protons in the nucleus is equal to the atom's atomic number. The total number of protons and the number of neutrons are approximately equal to the atom's total mass. This information is a part of the data that can be read off from the periodic system. The electron shell contains the same number of electrons as there are protons in the nucleus. This means the atom is electrically neutral.
The Danish physicist, Niels Bohr, produced a theory as early as 1913 that proved to correspond with reality where he, among others, demonstrated that atoms can only occur in a so-called, stationary state with a determined energy. If the atom transforms from one energy state to another a radiation quantum is emitted, a photon. It is these different transitions that make themselves known in the form of light with different wavelengths. In a spectrograph they appear as lines in the atom's spectrum of lines.