Getting to know the periodic table

This exercise is a part of Educator Guide: The periodic table turns 150 / View Guide

Directions: Ask students to answer the following questions based on the Science News article “The periodic table turns 150.”

1. Before reading the article and based on your prior knowledge, what characteristics does the periodic table have? What is the periodic table’s purpose?

The periodic table includes all the known elements and organizes the elements into rows and columns. It is ordered, concise and clear. The periodic table typically shows the elements’ chemical symbols, atomic numbers and atomic weights. The table’s purpose is to convey what the elements are, how they are related to one another and to provide some clues to how they behave. It is a useful guide for chemistry students and researchers.

2. Why, according to the article, is the anniversary of the periodic table a cause for celebration?

The periodic table is familiar to everyone. It concisely captures the elements that make up all earthly substances and their relationships. The periodic table reveals deep truths about chemistry and has played an important role in our understanding of the atom and in quantum theory. The birth and evolution of the periodic table is also an interesting story to tell.

3. What patterns did 19th century scientists — including John Dalton, Johann Wolfgang Döbereiner, John Newlands and Dmitrii Mendeleev — identify in the elements?

John Dalton suggested that elements were distinguished from each other by the weight of their atoms. German chemist Johann Wolfgang Döbereiner noticed that certain “triads” exist, in which three elements of increasing weight (such that one is the average of the other two) had similar chemical behaviors. John Newlands saw that arranging the elements in order of increasing atomic weight led to a recurring pattern in their chemical properties every eight elements. Dmitrii Mendeleev also noticed the relationship between atomic weight and chemical properties, but found that the pattern was a bit more complicated than Newlands had suggested.

4. Why did Dmitrii Mendeleev begin organizing elements? What other facts do you find interesting about Mendeleev’s life?

Mendeleev began organizing the elements because he was writing a textbook for an inorganic chemistry class and he needed a way to organize the text. Among other interesting facts: He was the 17th child in his family. He nearly died from a serious illness during college. Along with a tutor and lecturer, he was a popular science writer and editor, translator and consultant to chemical industries. He won a cash prize by writing a handbook on organic chemistry.

5. Why does the author of the article call Dmitrii Mendeleev’s original periodic table an “oracle”? Give an example to support your answer.

The organization of Mendeleev’s table explained established chemical relationships so well that it was able to predict undiscovered elements, three of which were found in Mendeleev’s lifetime. Mendeleev’s table not only predicted the existence of the elements, but also their properties. For example, gallium was discovered in 1875, with an atomic weight of 69.9 and a density six times that of water. Mendeleev had predicted the element with the same density and an atomic weight of a very close 68.

6. Based on the text and the graphic on Page 15, name at least three differences between Mendeleev’s original table and the table we use today.

Mendeleev’s table was organized vertically and today’s is organized horizontally. Mendeleev’s table depended on ordering by atomic weight; today we order the elements by atomic number, or number of protons. Mendeleev’s table did not include the noble gases or the transuranics or other synthetic elements. (Many elements have been discovered since Mendeleev’s day.) Mendeleev’s table did not have an element named after Mendeleev; today’s table does.

7. What were the limitations of Mendeleev’s original periodic table? How did these limitations affect Mendeleev’s understanding of the patterns in his table?

Mendeleev’s table was limited by current scientific knowledge. It included only 63 known elements. Many of the atomic weights were not yet certain. Because scientists didn’t know what was in an atom, Mendeleev organized his table in general by atomic weight. But he had to add some exceptions to this ordering to get elements with similar chemical properties grouped together. Mendeleev also didn’t know why his table worked; the underlying rules or facts that gave rise to the periodic behaviors remained a mystery.

8. How have atomic discoveries since Mendeleev’s day informed our understanding the periodic table?

The discovery of electrons, the nucleus and protons revealed the subatomic structure of the atom and added clarity to the ordering of the elements, as well as explaining the recurring nature of chemical properties. Quantum theory deepened the understanding of electron arrangements and highlighted the importance of the outermost electrons in governing an element’s behavior. A fuller understanding of atomic structure has led to the creation of synthetic elements. Though not discussed directly in the article, the discovery and explanation of radioactivity led to an understanding of isotopes and of the transmutation between elements.

9. Why do you think this article is labeled an “essay,” and not a “feature” or “news” article?

Though the article depends on researched and reported facts, as do all of the articles in Science News, the author takes a specific position — that the periodic table’s creation is a cause for celebration and the periodic table has had an outsized role in scientific advancement. This is the thesis of the essay. The “essay” puts a finding in deeper context, including personal details, historical analysis, interpretation and commentary on existing knowledge. It also takes a less formal structure than a typical news article and offers novel insights by making connections to other areas of science.

10. What lessons does the history and study of the periodic table offer to other fields of science, and the pursuit science more generally?

Much of science depends on finding patterns, order and structure in seeming chaos. This ordering often leads to rules that govern the world, and those rules can help scientists predict new features of the world, new facts to be newly ordered. In this way, science progresses gradually. Science requires objective reasoning, of the kind Mendeleev applied when he sorted his note cards, and presupposes the existence of general principles, of causes and effects. The sciences are often built on classification — of elements, species, subatomic particles, cells — creating a shared language that yields a shared understanding. The periodic table was also built over time with contributions from many scientists, and is still being built today.