Matter is build by atoms that are bound by electric forces to molecules or solids. The name "atom" is derived from Greek and means indivisible. The physicist of earlier times had considered that you cannot divide matter in arbitrarily small pieces. But it exists a smallest piece called atom. Today we know that an atom has a inner structure with a positively charged nucleus surrounded by a negatively charged electron shell. Until now we know 118 different sorts of atoms, called elements, that are ordered in the periodic table of elements.
The periodic table of the elements
In the periodic table the elements are sorted in 18 groups. The physical properties of the atoms are determined by the charge of the nucleus that consists of positively charged protons and neutral neutrons. The order parameter Z describes the number of the protons each carrying a charge of one elementary electric charge e. The mass of the atoms is mainly determined by the number of protons and electrons in the nucleus. Atoms of one element with the same number of protons can have a different number of neutrons. These Atoms are called isotopes.
The sorting of the elements in the groups of the periodic table is based on the electrons in the electron shell that determines the chemical properties. Due to quantum physics rules the electrons in the shell can only occupy certain energy levels. Within the energy levels the electrons arrange in shells each with only a fixed number of electrons. Thus, in the first shell (s-shell) only two electrons can be placed, in the second shell (p-shell) 6 electrons, in the third shell (d-shell) 10 electrons and in the fourth shell (f-shell) 14 electrons. For the first energy level only the s-shell can be filled that's why we have only the two elements hydrogen and helium in the first group. Starting from the second energy level also the p-shells can be filled; from the fourth level the d-shells and from the sixth level the f-shells.
In principle there is the following sequence:
|2. period||2s 2p|
|3. period||3s 3p|
|4. period||4s 3d 4p|
|5. period||5s 4d 5p|
|6. period||6s 4f 5d 6p|
|7. period||7s 5f 6d 7p|
The elements technetium and promethium and all elements with a order parameter bigger than 87 are radioactive, so they decay to elements with lower order parameter. The elements with a order parameter bigger than 94 cannot be found natural on earth but they were produced as artificial elements in nuclear experiments.References:
Pure Appl. Chem., Vol. 83, No. 2, pp. 359 - 396,2011