The 2 first electrons are in 3s 2 and the last electron is in a 2p orbital. Aluminium Z=13, meaning that the layer n=1 is complete with 2 electrons, the layer n=2 is complete with 8 electrons (2 in 2s 2 and 6 in 2p 6) and that 3 electrons are to be distributed in the n=3 layer. The number of electrons by layer can be determined from Z but is often indicated on the periodic table. The weight is the property that we seek most of the times on a Mendeleev table. The weight of an element is the average of the mass of its isotopes, taking in account their proportions. Its symbol is Al, its atomic number is 13 and it has a weight of 26.981538 units of atomic mass (which is equivalent to the molar mass, in g/mol). Some of them only show the symbol, name, atomic number (Z) and weight of each element but many other information can be displayed for each element. There is a large number of different models for the periodic tables. The table contains a gigantic number of information for each atom, but also shows some periodic trends. It may not look like, but the determination of this table was a huge improvement for the chemists. In the table showed above, the elements from 110 to 118 are theoretical. Scientists have been able to fulfil the periodic table by the artificial synthesis of those elements. Some holes were present in the table but it was assumed that the missing elements were not yet discovered. The atoms of same oxidation were sorted by weight to obtain the different lines. Mendeleev sorted the elements with regard to their oxidation/reduction by O/H More than a simple list of the existing atoms. ![]() The table of Mendeleev is also called the periodic table of the elements.
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