‘A quick overview of the modern periodic table’ explains the ordering of the periodic table, and introduces the concept of periodic law. Elements were originally ordered by their equivalent weight, but this was superseded by atomic weight and then atomic number. They are arranged into vertical groups of elements with similar properties. There are many versions of the periodic table, but all obey periodic law, which states that elemental characteristics repeat themselves as one goes down the table. Developments in physics, especially quantum theory and relativity, have changed the way we think about elements and periodicity.