2. The Periodic Table  Currently about 118 known elements are known to scientists.

3. The Periodic Table  Currently about 118 known elements are known to scientists.  The number is not exact because a few elements that have been synthesized must be confirmed before scientists agree that these elements do exists.

4. The Periodic Table  Currently about 118 known elements are known to scientists.  The number is not exact because a few elements that have been synthesized must be confirmed before scientists agree that these elements do exists.  Other elements may be discovered by the time we finish this lesson.

5. The Periodic Table  Currently about 118 known elements are known to scientists.  The number is not exact because a few elements that have been synthesized must be confirmed before scientists agree that these elements do exists.  Other elements may be discovered by the time we finish this lesson.  About 90 of the elements occur in nature, and the rest have been made by physicists in laboratories.

6. The Periodic Table  Before the eighteenth century, only 13 elements were known.

7. The Periodic Table  Before the eighteenth century, only 13 elements were known.  These elements included copper, gold, and sulfur.

8. The Periodic Table  Before the eighteenth century, only 13 elements were known.  These elements included copper, gold, and sulfur.  As scientists studied matter and broke compounds into their components, they discovered many more elements.

9. The Periodic Table  Before the eighteenth century, only 13 elements were known.  These elements included copper, gold, and sulfur.  As scientists studied matter and broke compounds into their components, they discovered many more elements.  By the middle of the nineteenth century, more than 60 elements were known, including gases such as oxygen, nitrogen, and chlorine.

10. The Periodic Table: Using Atomic Mass  In the mid-1800s, a Russian scientist named Dmitri Mendeleev looked for away to organize the 63 elements that were known at that time.

11. The Periodic Table: Using Atomic Mass  In the mid-1800s, a Russian scientist named Dmitri Mendeleev looked for away to organize the 63 elements that were known at that time.  Mendeleev made up a set of cards, one for each known element, that included the chemical and physical properties of the element.

12. The Periodic Table: Using Atomic Mass  In the mid-1800s, a Russian scientist named Dmitri Mendeleev looked for away to organize the 63 elements that were known at that time.  Mendeleev made up a set of cards, one for each known element, that included the chemical and physical properties of the element.  Although protons and neutrons were not yet known, the atomic masses of the elements had been determined experimentally, so those values were included on the cards.

13. The Periodic Table: Using Atomic Mass  Mendeleev worked with different arrangements of the cards, sorting the cards into rows and columns.

14. The Periodic Table: Using Atomic Mass  Mendeleev worked with different arrangements of the cards, sorting the cards into rows and columns.  When he arranged the elements according to increasing atomic masses, he noticed that those that had similar properties occurred in a repeating pattern.

15. The Periodic Table: Using Atomic Mass  Mendeleev worked with different arrangements of the cards, sorting the cards into rows and columns.  When he arranged the elements according to increasing atomic masses, he noticed that those that had similar properties occurred in a repeating pattern.  The periodic table of elements is a chart in which all the elements are arranged in columns and rows so that the properties of the elements occur in a repeating pattern.

16. The Periodic Table: Using Atomic Mass  To keep the repeating pattern in his periodic table, Mendeleev had to leave some blank spaces in his arrangement.

17. The Periodic Table: Using Atomic Mass  To keep the repeating pattern in his periodic table, Mendeleev had to leave some blank spaces in his arrangement.  He made a rather bold prediction: new elements would be discovered that would fit into those blank spaces.

18. The Periodic Table: Using Atomic Mass  To keep the repeating pattern in his periodic table, Mendeleev had to leave some blank spaces in his arrangement.  He made a rather bold prediction: new elements would be discovered that would fit into those blank spaces.  He even predicted the properties of those unknown elements.

19. The Periodic Table: Using Atomic Mass  Many scientists were skeptical and questioned the ability to predict the properties of an unknown elements.

20. The Periodic Table: Using Atomic Mass  Many scientists were skeptical and questioned the ability to predict the properties of an unknown elements.  However, the value of the chart as an organizational tool was shown within a few years.

21. The Periodic Table: Using Atomic Mass  Many scientists were skeptical and questioned the ability to predict the properties of an unknown elements.  However, the value of the chart as an organizational tool was shown within a few years.  Mendeleev developed his chart between 1868 and 1870.

22. The Periodic Table: Using Atomic Mass  Many scientists were skeptical and questioned the ability to predict the properties of an unknown elements.  However, the value of the chart as an organizational tool was shown within a few years.  Mendeleev developed his chart between 1868 and 1870.  In 1875, the new element gallium was discovered, followed by germanium in 1886.

23. The Periodic Table: Using Atomic Mass  Many scientists were skeptical and questioned the ability to predict the properties of an unknown elements.  However, the value of the chart as an organizational tool was shown within a few years.  Mendeleev developed his chart between 1868 and 1870.  In 1875, the new element gallium was discovered, followed by germanium in 1886.  The properties of each of these elements matched Mendeleev’s predictions.

24. The Periodic Table: Using Atomic Numbers  Mendeleev did have one problem.

25. The Periodic Table: Using Atomic Numbers  Mendeleev did have one problem.  A few elements did not fit the repeating pattern that he had established in his periodic table.

26. The Periodic Table: Using Atomic Numbers  Mendeleev did have one problem.  A few elements did not fit the repeating pattern that he had established in his periodic table.  Recall that Mendeleev had organized his periodic table based on the atomic masses of the elements.

27. The Periodic Table: Using Atomic Numbers  Mendeleev did have one problem.  A few elements did not fit the repeating pattern that he had established in his periodic table.  Recall that Mendeleev had organized his periodic table based on the atomic masses of the elements.  He had to switch the order of certain elements so that that the one with the greater atomic mass appeared first on periodic table.

28. The Periodic Table: Using Atomic Numbers  Mendeleev did have one problem.  A few elements did not fit the repeating pattern that he had established in his periodic table.  Recall that Mendeleev had organized his periodic table based on the atomic masses of the elements.  He had to switch the order of certain elements so that that the one with the greater atomic mass appeared first on periodic table.  Only then did the properties of these elements fit the repeating pattern.

29. The Periodic Table: Using Atomic Numbers  Mendeleev did have one problem.  A few elements did not fit the repeating pattern that he had established in his periodic table.  Recall that Mendeleev had organized his periodic table based on the atomic masses of the elements.  He had to switch the order of certain elements so that that the one with the greater atomic mass appeared first on periodic table.  Only then did the properties of these elements fit the repeating pattern.

30. The Periodic Table: Using Atomic Numbers  Mendeleev thought that experiments would later reveal that the atomic masses of these elements had been wrongly calculated.

31. The Periodic Table: Using Atomic Numbers  Mendeleev thought that experiments would later reveal that the atomic masses of these elements had been wrongly calculated.  Because protons had not yet been discovered, he didn’t know about atomic numbers.

32. The Periodic Table: Using Atomic Numbers  Mendeleev thought that experiments would later reveal that the atomic masses of these elements had been wrongly calculated.  Because protons had not yet been discovered, he didn’t know about atomic numbers.  In 1914, a British scientists named Henry Moseley suggested that the elements be arranged in a periodic table based on increasing atomic numbers, not on increasing mass numbers.

33. The Periodic Table: Using Atomic Numbers  Once this was done, all the elements, including the ones identified since Mendeleev’s time, were arranged so that their repeating properties were evident.

34. The Periodic Table: Using Atomic Numbers  Once this was done, all the elements, including the ones identified since Mendeleev’s time, were arranged so that their repeating properties were evident.  The modern periodic table is a chart that displays all of the elements, arranged in order of atomic number.