1. J OURNAL #43 Who is Niels Bohr? What was his contribution to chemistry? What do you already know about the periodic table of elements?

2. T HE P ERIODIC L AW

3. M ENDELEEV Russian Chemist He observed that when elements are arranged in order of increasing atomic mass, certain similarities in their chemical properties appeared at regular intervals. Such repeating pattern is referred to as periodic. Mendeleev’s principle was not widely accepted (as there were only 60 known elements, leaving gaps in the table)

4. M OSELEY The elements in the periodic table fit into patterns better when arranged in increasing order according to nuclear charge, or number of protons in the nucleus. Now, Mendeleev’s principle is correctly stated in what is known as the periodic law: The physical and chemical properties of the elements are periodic functions of their atomic numbers.

5. T HE MODERN PERIODIC TABLE The periodic table has undergone extensive changes since Mendeleev’s time. Chemists have discovered new elements, and synthesized new ones in the lab. The periodic table is an arrangement of the elements in order of their atomic numbers so that elements with similar properties fall in the same column or group.

6. T HE MODERN PERIODIC TABLE The noble gases, also known as group 18 elements, are all rather unreactive. The lanthanides- are the 14 elements with atomic numbers from 58 Ce to 71 Lu. The actinides- are the 14 elements with atomic numbers from 90 Th to 103 Lr

7. L ET ’ S R EVIEW : Who discovered the Periodic Law? Answer: Mendeleev Who established atomic numbers as the basis for organizing the periodic table? Answer: Moseley

8. D ESIGN YOUR OWN PERIODIC TABLE Can you design your own periodic table using information similar to that available to Mendeleev? Organize the cards for the elements in a logical pattern as you think Mendeleev might have done. Answer the following: Why are atomic masses given instead of atomic numbers? Can you identify each element by name? How many groups are in your periodic table? How many periods are in your table? Predict the characteristics of missing elements.