Theory Periodic Table of the Elements

Devised by Mendeleiev, a Russian, around 1870, the periodic table of elements has always the purpose to distribute elements in an orderly fashion, which allows to sort possible elements to come. Its main current use however is to easily know the properties of an element and what elements it will yield when combined with others. Elements are basic chemical elements which no chemical reaction may broke further into other elements. Since 1914 the basis of the sorting is the atomic number of the atom. The atomic number is the number of protons in the atom's nucleus (which contains these positively charged particles and the neutrons, neutral particles). Before elements were sorted by their atomic weight, the weight of one atom (nowadays, atomic weight is given as atomic mass, which is the average mass of the atom in atomic mass units -"amu"); average number of neutrons is obtained by subtracting the number of protons (i.e. the atomic number) from the atomic mass). At the beginning of the 20th century, the progress of science led too to understand that the natural sorting of elements into groups -or families- which the table showed, was due to the fact that elements of a same group had the same number of outer-shell electrons (electrons were discovered to be distributed into shells around the nucleus). Hence elements groups (also called "families"), share too similar properties. Group 1 of the periodic table, e.g. contains soft metals which react with water to form hydrogen. Elements, at last, sort according to periods, i.e. according to the maximum number of electrons that can be found in the farthest atom's electron shell. All elements of a same row have the same maximum number allowed of electrons in its largest electron shell. This maximum number unfolds from 2 for the first row, to 32 for the last, passing through 8, 8, 18, 18, 32. This maximum number moreover matches the number of elements in the row

a more practical view of the elements

A usual periodic table shows the elements in a frame corresponding to the values above: the table is defined by the groups (top; columns) and the periods (left; rows). Elements are distributed from left to right and top to bottom in order of their atomic number. Groups 1 and 2 are metals and alkaline earth metals respectively. The following ten columns are a unique group (columns 3 to 12): the group of the transition metals; it is ditributed into 8 sub-groups, one of them containing three sub-sub-groups. The six remaining groups contain both nonmetals, in the upper-right part of the block, and metal in the lower-left part. At last, between alkaline earth metals and transition metals should take place in the table the rare earth elements (lanthanides, actinides), which are strongly similar in characteristics. This whole is placed under the main table for convenience. In the table it would be a vast intermediate group between colums 2 and 3. A much inordinary way to use the table is too to see it as a way to know about the atomic architecture of the elements: in each group, size of atoms grows going down the table. This is due to the fact that first -top- elements of each group has only one or two electron shells, as the further elements has more of them, yielding a larger atom. In each period at the opposite, size of atoms decreases, from left to right: the farthest shell of the atom reaches its maximum number of electrons indeed but now new shell however appears; moreover, the more right the atom, the greater the number of protons in its nucleus, thus the stronger the link to the electrons, the closer the electrons to the nucleus. Hence these atoms are smaller in size. At last, the table of the elements draws too four main groups which each file for a specific atomic orbital of the electrons of their atom; orbitals does not here relate to electrons' orbits but are taken here in their quantum significance and mean zones of density where there is a 99.5 per cent chance to find the electron: group at the left have type s orbitals (spherical); group at the right have type p orbitals (two lobes); group at the center have type d orbitals (four lobes); group of lanthanides and actinides have type f orbitals

Here is a periodic table. Althoug not strictly part of the usual amateur astronomy tools, it is always useful when it comes to understand symbols of elements regarding atmospheres, or planetary or stellar components. A good clickable periodic table is at Los Alamos National Laboratory's Chemistry Division

click to a periodic table of the elements

(since late 2010, the International Union of Pure and Applied Chemistry (IUPAC) decided that weights of 10 elements have now to be expressed as ranges of a starting, and ending mass, instead of a single value. This is true for hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulphur, chlorine and thallium. Atomic weight of zinc was modified by 2006 )