1. H ISTORY OF THE P ERIODIC T ABLE (PT) Dmitri Mendeleev arranged the first periodic table in 1872. At this time there were only about 70 known elements. He arranged them in order of increasing atomic mass and according to similar properties of elements.

2. S KETCH OF F IRST D RAFT OF P ERIODIC T ABLE

3. H ENRY M OSELEY In 1913, Henry Moseley discovered atomic number and arranged the PT accordingly. Based on this discovery, the re-arranged the table according to increasing atomic number. This arrangement of the periodic table had fewer inconsistencies and therefore is still used today.

4. T HE M ODERN P ERIODIC T ABLE Periodic Law – “ Physical & chemical properties of the elements are periodic functions of their atomic number.” Element properties show a repeated pattern when they are arranged in order of increasing atomic number (number of protons). This allows us to identify trends in the periodic table.

5. GROUPS & PERIODS