2MendeleevDmitri Mendeleev first organized elements according to atomic mass.He noticed similarities in their chemical properties appeared at regular intervalsSuch a pattern is called periodicThing to remember: its just a general trend
4Periodic Law Mendeleev’s table has gaps! Two questions he posed: 1. Why could most elements be arranged in order of increasing atomic mass, but some couldn’t?2. What is the reason for chemical periodicity?
5Periodic LawMoseley, who worked with Rutherford, discovered that atomic number, not atomic mass, is the basis of organization. This led to…Periodic Law: The physical and chemical properties of the elements are periodic functions of their atomic numbers.
6Periodic Table Organization Vertical columns in the periodic table are known as groups or families
7Periodic Table Organization Horizontal Rows in the periodic table are known as Periods