Early Versions of the Periodic Table
In the early 1800s, German scientist Johann Dobereiner observed patterns in the atomic masses of elements. He used these patterns to sort elements into groups that he called triads because each group contained three elements. In each triad, the middle element had an atomic mass that was the average of the other two—one with a smaller atomic mass and one with a larger atomic mass. The elements in the triads also had similar properties, which helped support Dobereiner’s classification method. An example of one of Dobereiner’s triads is the one made up of lithium, sodium, and potassium. All of these were known to be very reactive metals that exist as soft solids at room temperature.
Unfortunately, Dobereiner’s classification scheme was not successful in incorporating all of the elements known at that time, but the idea of grouping elements based on mass and similarities in properties inspired other scientists to try categorizing the elements.
In 1864, English scientist John Newlands listed the elements in order of increasing atomic mass. He constructed a table to list the first seven elements in one column, the next seven in the next column, and so on. In doing this, Newlands grouped elements with similar properties in rows. However, despite his interesting idea, Newlands was not successful in creating a useful classification of elements. One of the main flaws was that the table did not allow places for elements that scientists did not know about.
Figure 18 John Newlands’ element classification scheme is shown.
The elements are listed in order of atomic mass in columns of seven elements. Newlands observed that elements in the same row had similar properties.
The elements are listed in order of atomic mass in columns of seven elements. Newlands observed that elements in the same row had similar properties.
Finally, in 1872, Russian scientist Dmitri Mendeleev published his version of a classification scheme that was successful not only in taking into account all of the known elements at the time, but also in predicting the existence of elements that had not yet been discovered. Mendeleev’s classification scheme took the form of a table with the elements arranged in a series of horizontal rows and vertical columns.
Mendeleev’s creation is considered to be the first useful periodic table of the elements, a table that organizes elements according to their properties. Mendeleev’s table is based on atomic mass and on physical and chemical properties of the elements. Like Newlands, Mendeleev arranged elements in order of increasing atomic mass. When properties began to repeat, he started a new row. Mendeleev was careful to leave blank spaces for elements that he thought should belong to those spaces but had not yet been discovered. In this way, Mendeleev built a table that reflected the periodicity of properties of the elements, and that formed the basis for the modern periodic table we use today.
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