A Historic Overview: Mendeleev And The Periodic Table

Mendeleev

In the mid-1800s, most chemists worldwide were convinced that the elements existed in families that had similar physical

and chemical properties. However, there was no widely accepted chart that explained relationships in chemical

properties among chemical groups. The periodic table, an information organizing tool that we take for granted today,

began as a simple question in the mind of a Russian scientist, Dmitrii I. Mendeleev (1843-1907). What is the relationship

of the elements to one another and to the chemical families to which they belong?

Mendeleev's passion for understanding the families of elements took him into previously

uncharted territory. He felt that the newly understood atomic mass measurements

would have greater significance once scientists clearly understood the relationships

among the elements. Mendeleev wrote his ideas into the chemistry textbooks from

which he taught. In Principles of Chemistry , published in 1869, Mendeleev introduced a

concept he called the Periodic Law that stated:

The properties of the elements are a periodic function of their atomic weights.

He subsequently published several versions of a periodic table of the elements, including all elements known at that

time. How was Mendeleev able to chart the relationships among the 63 known elements? It all started in a game of

cards.

A Game of Cards

In order to understand the properties of the known elements and their relationships to one another, Mendeleev

developed a card game. He wrote out the properties of each element on a different card and spent a great deal of time

arranging and rearranging them. He was looking for patterns or trends in the data on the cards. His friends called this

game “Patience.”

Mendeleev first arranged all the cards from lowest to highest atomic mass. The lightest element known in Mendeleev’s

time was hydrogen. Its properties were not like any other known element. So Mendeleev decided to leave it out of his

game.

Scientists who are initially struggling to understand a large mass of data commonly ignore, at least for a time, those data

points that seem too different from the others. These unusual instances are termed outliers. Whether or not outliers

can eventually be explained by a model often makes or breaks the scientific theory from which the model derives.

The second lightest element known to Mendeleev was lithium.

We now know that the second lightest element, between

hydrogen and lithium, is helium. But helium was not

discovered on earth until 1895.

So Mendeleev started his game with the element lithium.

G E N E S I S

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