The Periodic Law History of the Periodic Table Mendeleev and Chemical Periodicity Elements arranged in order of increasing atomic mass, similarities in chemical properties appeared regularly (periodic). Mendeleev created a table where elements of similar properties are grouped together. Predicted properties of missing elements. Mendeleev’s 1st Periodic Table Mendeleev’s Periodic Table Moseley and the Periodic Law Moseley, working with Rutherford, discovered that elements fit into patterns better when arranged in inc. order of nuclear charge or number of protons. Led to modern definition of atomic number. Periodic Law. The Modern Periodic Table Since Mendeleev, chemists have discovered new elements and created new ones in the lab. Periodic Table: An arrangement of the elements in order of their atomic numbers so that the elements with similar properties fall in the same group. The Noble Gases Rayleigh and Ramsay credited with discovery of argon (Ar) first, then Ramsay proposed the new group (18). The final gas, radon (Rn) was discovered in 1900 by Friedrich Ernst Dorn. The Lanthanides The lanthanides are the 14 elements with atomic numbers from 58 (cerium) to 71 (lutetium). The elements are so similar in chemical and physical properties, separating and identifying them took the work of many chemists. The Actinides The actinides are the 14 elements with atomic numbers from 90 (thorium) to 103 (lawrencium). They fall into the 7th period. Periodicity Periodicity with respect to atomic number can be observed in any group of elements in the periodic table. In each of the Groups 1 and 18, the differences between the atomic numbers of successive elements are 8,8,18, and 32. Groups 2 and 13-17 follow similar patterns.