1. History of the Periodic Table The scientists behind the organization

2. Dmitri Mendeleev 1834 – 1907, Russian Arranged elements in order of increasing atomic mass Noticed patterns in chemical and physical properties Arranged elements with the most similar properties side by side

3. Mendeleev continued There were blank spots in the table because not all elements had been discovered Periodic Law: Elements arranged according to their atomic masses present a clear periodicity of properties

4. Henry Moseley 1887 – 1915, British 1913 – determined atomic number of elements Rearranged periodic table according to atomic number Went to WWI, killed at age 28

5. British response to Moseley’s death British government no longer assigns scientists to combat duty in times of war We will never know what his brilliant mind could have accomplished had he lived longer

6. Terms explained Atomic Mass: mass relative to carbon, approximately the number of protons + neutrons Atomic Number: the number of protons in a single atom of an element. In a neutral atom, the number of electrons is the same as the number of protons.