Mendeleyev’s Table

Selected Points from Mendeleyev’s Paper (as recalled in 1889)

• The elements, if arranged according to their atomic weights, exhibit an evident periodicity of properties

• Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (e.g., platinum, iridium, osmium) or which increase regularly (e.g., potassium, rubidium, caesium)

• The magnitude of the atomic weight determines the character of the element just as the magnitude of the molecule determines the character of a compound body

• We must expect the discovery of many yet unknown elements, for example, elements analogous to aluminium and silicon, whose atomic weight would be between 65 and 75

• The atomic weight of an element may sometimes be amended by a knowledge of those of the contiguous elements. Thus, the atomic weight of tellurium must lie between 123 and 126, and cannot be 128

• Certain characteristic properties of the elements can be foretold from their atomic weights

• Clearly illustrates the remarkable predictions made by Mendeleyev’s arrangment

• What’s missing? – noble gases

– lanthanides and actinides

Atomic Weight vs. Equivalent Weight

• Many chemists of the day (1860 that is) interchanged “atomic weight” with what we would now call “equivalent weight” (remember, atoms were still controversial)

• Equivalent weight – defined as the quantity of an element will react with a fixed amount of another– example: 39 grams of potassium (K) is “equivalent” to 20 grams

of calcium (Ca)

– but the atomic weights of K and Ca are 39 and 40, respectively

• This is because potassium and calcium have different “valencies” – they react with oxygen to form K2O and CaO

Chemical Periodicity (hydrides)

In Mendeleyev’s Words…

“The solution of the problem advanced but slowly, because the facts, and not the law, stood foremost in all attempts; and the law could not awaken a general interest so long as elements, having no apparent connection with each other, were included in the same octave…”

Atomic Weight vs. Atomic Number

• Atoms are made up of protons, neutrons and electrons

• The atomic weight of an isotope is determined by the number of protons + the number of neutrons

• Different isotopes have different atomic weights

• The atomic weight of an element is an average of the weights of the naturally occurring isotopes

• Arranging the elements strictly by atomic weight puts some elements out of order

• It was only after the structure of the nucleus was understood that chemists realized that the best way to arrange the elements by atomic number = # of protons

Atomic Weight vs. Atomic Number

Versions of the Periodic Table

• Since Dmitrii Ivanovich Mendeleev first published it in 1869, more than 700 different graphical representations of this cornerstone of modern inorganic chemistry have been published. Each has advantages and disadvantages, and the quest for an "ideal" or "perfect" periodic table continues unabated.

Janet / Tarantola

• Beautiful in it’s simplicity and symmetry, but lacking in several ways

• The position of helium doesn’t really work

• The metals are split up (some of the practical aspects of the chemistry are lost in this arrangement)

Origins of Periodicity

• The energies of the electrons within atoms ultimately determines the chemical properties of all the elements

• The laws of quantum mechanics define these energies (analogous to the harmonic series for the musically inclined)

• Creates a pattern of 2, 6, 10, 14 that appears in the table

Benfey’s Periodic Table

The Astronomer’s Table

Triangular Table

A Physicist’s Periodic Table

More Cool Tables

• http://www.webelements.com/

• http://umbbd.ahc.umn.edu/periodic/periodic.html

• http://www.wsu.edu/~wherland/pt3d2fm.htm