# The INTERNET Database of Periodic Tables

There are hundreds of periodic tables in web space, but there is only one comprehensive database of periodic tables & periodic system formulations. If you know of an interesting periodic table that is missing, please contact the database curator: Dr Mark R Leach.

1950

The Modern Periodic Table

The modern periodic table is based on quantum numbers and blocks, here.

A periodic table can be constructed by listing the elements by n and l quantum number:

The problem with this mapping is that the generated sequence is not continuous with respect to atomic number atomic number, Z: Check out the sequence Ar to K, 18 to 19.

Named after a French chemist who first published in the formulation in 1929, the Janet or Left-Step Periodic Table uses a slightly different mapping:

While the Janet periodic table is very logical and clear it does not separate metals from non-metals as well as the Mendeleev version, and helium is a problem chemically.

However, it is a simple mapping to go from the Janet or Left-Step periodic table to a modern formulation of Mendeleev's periodic table:

On this page web, "full" f-block included periodic tables are shown wherever possible, as above.

However, the periodic table is usually exhibited in book and on posters in a compressed form with the f-block "rare earths" separated away from the s-block, p-block and d-block elements:

However, the compression used introduces the well known problem known as a "fence post error".

The effect is that:

La and Ac: move from f-block to d-block
Lu and Lr: move from p-block to f-block

Chemically, the elements can be fitted in and classified either way. Many thanks to JD for pointing the situation with the periodic table is a fence post error.

Mark Winter's Web Elements project, here, uses the formulation shown below:

Interestingly, the IUPAC periodic table separates out 15 lanthanides, La-Lu, and 15 actinides, Ac-Lr by leaving gaps in period 3 under Sc & Y:

This corresponds to:

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 Periodic Table, What is it showing? Binary Compounds

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