2) Development of the periodic table

• When John Newlands tried to put together a periodic table in 1864, only 63 elements were known. Many were still undiscovered.
• Newlands arranged the known elements in order of atomic weight.
• He noticed periodicity (repeated patterns), although the missing elements caused problems.
• However, strictly following the order of atomic weight created issues - it meant some of the elements were in the wrong place
• Dimitri Mendeleev realised that some elements had yet to be discovered. When he created his table, in 1869, he left gaps to allow for their discovery. He also reordered some elements
• Each element was placed in a vertical column or 'group' with elements that had similar properties.
• Mendeleev used his periodic table to predict the existence and properties of new elements.
• When the subatomic particles were later discovered, it revealed that Mendeleev had organised the elements in order of increasing atomic number (number of protons).