A Historic Overview: Mendeleev And The Periodic Table

A A A A H H H H i i i i s s s s t t t t o o o o r r r r i i i i c c c c O O O O v v v v e e e e r r r r v v v v i i i i e e e e w w w w : : : :

C C C C o o o o s s s s m m m m i i i i c c c c C C C C h h h h e e e e m m m m i i i i s s s s t t t t r r r r y y y y : : : : A A A A n n n n E E E E l l l l e e e e m m m m e e e e n n n n t t t t a a a a l l l l Q Q Q Q u u u u e e e e s s s s t t t t i i i i o o o o n n n n

M M M M e e e e n n n n d d d d e e e e l l l l e e e e e e e e v v v v a a a a n n n n d d d d t t t t h h h h e e e e P P P P e e e e r r r r i i i i o o o o d d d d i i i i c c c c T T T T a a a a b b b b l l l l e e e e

S S S S T T T T U U U U D D D D E E E E N N N N T T T T T T T T E E E E X X X X T T T T

It's All in the Family

Think about your family today. It may consist of you, one or two adults who are your parents or guardians, and one or

more siblings. You may have an extended family including one or more grandparents, aunts, uncles, and cousins. All of

you share a family relationship. You may share certain characteristics. Has anyone ever told you that you look, walk, or

talk like your mother or father, grandmother or grandfather? You might even have a family member who has an interest

in genealogy and who has mapped out your family history, including a chart known as a

family tree. If you have seen such a tree, you know that each name represents a person, and

that some individuals on the tree are more closely related than others.

Now, think of the periodic table of elements. It too is a chart that shows relationships.

Like many families, certain individual elements in this chart share characteristics, some more

closely than others. However, all are related.

The Triad Model

A German scientist, Johan Dobereiner (1780-1849), tried to classify elements into

smaller and simpler subgroups. In 1829, he observed that elements with similar

physical and chemical properties fall into groups of three. He called these related

groups of three elements triads.

One of these triads included chlorine, bromine, and iodine; another consisted of

calcium, strontium, and barium. In each of these triads, the atomic weight of the

intermediate element is approximately the average of the atomic weights of the

other two elements. The density of that element is approximately the average of

the densities of the other two elements.

The problem with this arrangement was that Dobereiner’s model became outdated

as new elements were identified. A good model is able to incorporate newly

understood information. Dobereiner’s Triad Model was not useful, since several

newly discovered elements did not “fit” into it.

The Law of Octaves

In 1864, an English chemist, John Newlands, arranged the known elements in increasing order of their atomic weights.

He noted that chemically similar elements occurred every eight elements. Lighter sodium was like potassium, and so on

through pairs of elements until fluorine and chlorine, the seventh pair. Since potassium followed fluorine (the noble

gases had not yet been discovered), Newlands called the repeating pattern the Law of Octaves since the eighth element

resembled the first. His Law of Octaves was based on this observation.

However, there were some deficiencies in Newland’s proposed arrangement. Several known elements did not “fit” his

pattern. Newlands did not allow for the possibility of the discovery of additional elements at a later date. Further, he did

not question whether all the atomic masses known to that date were correct. Newlands’ Law of Octaves was not a good

model for explaining the relationship among the elements.

G E N E S I S

A Historic Overview: Mendeleev And The Periodic Table - Genesis

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