Periodic table

• Early attempts to arrange the elements into a periodic table were made by Johan Dober, who noted that elements with similar chemical properties often occurred in threes, and called these Triads.
• John Newland arranged the elements in order of increasing atomic weight and observed that every eighth element reacts in a similar way, a phenomenon he called his law of octaves.
• Dimitri Mendeleev developed the first modern periodic table by arranging all the elements in order of increasing atomic weight, and then switching the order of specific elements to fit the patterns of other elements in the same group.
• Dimitri Mendeleev predicted the properties of the Undiscovered elements based on other elements in the same groups, and because of that, other scientists accepted that Mendeleev's table was correct.
• The modern periodic table is almost identical to Mendeleev's table, except for a few important differences: the elements are arranged in order of atomic number, and the table includes group zero, which consists of the noble gases.
• The problem with ordering by atomic weight is that elements can appear in the wrong order due to the presence of Isotopes, as seen in the case of thorium and iodine.
• The modern periodic table addresses this problem by ordering the elements by atomic number.