The Search for Order

1. In a video store, movies are in categories such as Action or Comedy. When scientists organized the elements, they had to decide what categories to use and where to place each element. An organized table of the elements is one of the most useful tools in chemistry.

2. The Search for Order Until 1750, scientists had identified only 17 elements, mainly metals, such as copper and iron. As the number of known elements grew, so did the need to organize them into groups based on their properties. In 1789, Antoine Lavoisier grouped the known elements into categories he called metals, nonmetals, gases, and earths. For the next 80 years, scientists looked for different ways to classify the elements. But no system worked for all the known elements.

3. Mendeleev’s Periodic Table How did Mendeleev organize the elements in his periodic table?

4. Mendeleev’s Periodic Table Mendeleev arranged the elements into rows in order of increasing mass so that elements with similar properties were in the same column.

5. Mendeleev’s Periodic Table Mendeleev’s Proposal In the 1860s, Dmitri Mendeleev developed an approach for organizing the elements while playing the card game solitaire. In this game, the player sorts a deck of cards by suit and value.

6. Mendeleev’s Periodic Table Mendeleev made a “deck of cards” of the elements, listing an element’s name, mass, and properties on each card. When Mendeleev lined up the cards in order of increasing mass, a pattern emerged. The key was to break the elements into rows.

7. Mendeleev’s Periodic Table The final arrangement was similar to a winning arrangement in solitaire, except that the columns were organized by properties instead of suits. Within a column, the masses increased from top to bottom. Mendeleev’s chart was a periodic table. A periodic table is an arrangement of elements in columns, based on a set of properties that repeat from row to row.

8. Mendeleev’s Periodic Table Mendeleev’s Prediction At the time Mendeleev made his table, many elements had not yet been discovered. When he placed the elements where their properties fit, there were several gaps in the table. Mendeleev was able to offer the best explanation for how the properties of an element were related to its location in his table.

9. Mendeleev’s Periodic Table Mendeleev was confident that the gaps in his table would be filled by new elements. He used the properties of elements located near the blank spaces in his table to predict properties for undiscovered elements. Some scientists didn’t accept those predictions. Others used the predictions to help in their search for undiscovered elements.

10. Mendeleev’s Periodic Table Evidence Supporting Mendeleev’s Table What evidence helped verify the usefulness of Mendeleev’s table?

11. Mendeleev’s Periodic Table The close match between Mendeleev’s predictions and the actual properties of new elements showed how useful his periodic table could be.

12. Mendeleev’s Periodic Table Mendeleev named missing elements after elements in the same group. He gave the name eka-aluminum to the missing element one space below aluminum in the table. Mendeleev predicted that eka-aluminum would • be a soft metal, • have a low melting point, and • have a density of 5.9 g/cm3.

13. Mendeleev’s Periodic Table In 1875, a French chemist discovered a new element. He named the element gallium (Ga) in honor of France. (The Latin name for France is Gallia.) Gallium • is a soft metal, • has a melting point of 29.7°C, and • has a density of 5.91 g/cm3.

14. Mendeleev’s Periodic Table Heat from a person's hand can melt gallium. In some traffic signals, there are tiny light emitting diodes (LEDs) that contain a compound of gallium.

15. Mendeleev’s Periodic Table Scientists use the periodic table to explain the chemical behavior of different groups of elements. • The properties of gallium are remarkably similar to the predicted properties of eka-aluminum. Scientists concluded that gallium and eka-aluminum are the same element. • The discovery of scandium (Sc) in 1879 and the discovery of germanium (Ge) in 1886 provided more evidence.

16. Assessment Questions • In Mendeleev’s periodic table, elements with similar properties were grouped • in the same row. • in the same column. • in diagonal lines that run from top left to the bottom right. • in pairs of two.

17. Assessment Questions • In Mendeleev’s periodic table, elements with similar properties were grouped • in the same row. • in the same column. • in diagonal lines that run from top left to the bottom right. • in pairs of two.ANS: B

18. Assessment Questions • For which element did Mendeleev correctly predict the properties even before it had been discovered? • gallium • hydrogen • bromine • aluminum

19. Assessment Questions • For which element did Mendeleev correctly predict the properties even before it had been discovered? • gallium • hydrogen • bromine • aluminum ANS: A