This post is by Michael Apfeldorf of the Library of Congress.

The year 2019 marks the 150^th anniversary of Dmitry Mendeleyev’s ground-breaking Periodic Table of the Elements, and provides an opportunity both to celebrate Mendeleyev’s historic accomplishment and to reflect on important lessons that students can learn about the nature of science from his experiences.

Mendeleyev was not the first to consider the periodicity of the chemical elements. Before 1869, others had suggested that when the known elements were arranged in order of their atomic weights, a predictable recurrence of properties could be seen. However, Mendeleyev took this work to new levels. Inspired by the card game solitaire, Mendeleyev wrote the name of each known element on a note card, along with its atomic weight and properties. He then arranged the elements in a table – horizontally by atomic weight and vertically by properties. One of the key breakthroughs of Mendeleyev’s technique was that he was willing to adjust this table to keep elements together based on properties – not simply atomic weight – even if meant contradicting current understandings of an element’s weight, or leaving blanks in the table where an undiscovered element might belong. Later, scientists would be able to use this information to test Mendeleyev’s hypotheses and build on our understanding of chemistry.

Students can gain a deeper appreciation of Mendeleyev’s accomplishment – and the nature of science generally – by comparing and contrasting Mendeleyev’s 1869 table with the modern one used today. Ask students to identify as many similarities and differences as they can, speculating about why these exist or posing questions that could be researched further.

First Periodic Table of Chemical Elements demonstrating the periodic law. Dmitry Ivanovich Mendeleyev, 1869

Observations and reflections might include:

• There are fewer elements in Mendeleyev’s table than our modern one. (Fewer elements were known to scientists at that time.)
• There are question marks and blanks in Mendeleyev’s notations, which appear to correlate with one another. (Once Mendeleyev identified a general pattern, he allowed himself to speculate about the existence of new, undiscovered elements.)
• The numbers associated with specific elements on Mendeleyev’s table are often greater than their counterparts on our modern one; for example: Oxygen = 16 or 8. (Our modern table uses atomic numbers – the number of protons within an element – as opposed to atomic weight, which includes the weight of both protons and neutrons. In the 19^th century, scientists did not yet understand this subatomic structure. Only later would they realize that this structure – and particularly outer shell electrons – often explained why certain elements shared common properties).

As an extension, put students in the role of Mendeleyev by giving them an incomplete set of cards, on which you have listed the names of elements with atomic weights and properties, and challenge them to arrange the cards in a pattern that makes sense to them. Along with examining Mendeleyev’s original periodic table, this activity can help students reflect on what it is like to develop new scientific understandings.