Mendeleev´s Periodic Table (Edexcel GCSE Chemistry: Combined Science)

• Before the discovery of the subatomic particles, scientists struggled to find ways of ordering the growing knowledge of elements and their chemistry
• When the elements that were known at that time were sorted by mass into a table, patterns emerged at regular periods along the table, giving rise to the term periodic
• The earlier tables were incomplete as some elements were forced into a position to fill gaps which appeared during the sorting process
• Other elements were placed in the wrong group as they were sorted strictly on their mass and had their chemical properties ignored
• There were many early versions of the tables as scientists in different countries grappled with the ordering of the elements
• In 1869 the Russian chemist Dmitri Mendeleev created his first draft of the periodic table
• He organised the elements into vertical columns based on their properties and the properties of their compounds
• He then started to arrange them horizontally in order of increasing atomic mass and as he worked, he found that a pattern began to appear in which chemically similar elements fell naturally into the same columns

Mendeleev's Periodic Table showing gaps

• There were exceptions though as some elements didn't fit the pattern when arranged by atomic mass
• Mendeleev worked to include all the elements, but he didn't force an element to fit the pattern, rather he left gaps in the table that he thought would best be filled by elements that had not yet been discovered
• He also switched the order of the elements to maintain consistency down the columns

• Mendeleev quickly realised that elements with the same properties should be placed in the same column
• He realised that gaps in the table must correspond to elements that had not yet been discovered or isolated
• He used the properties and trends of other elements in the group with the gap to predict the properties of these undiscovered elements
• Mendeleev left a gap between silicon and tin and used his knowledge of the properties of those two elements to make predictions about the physical and chemical properties of the undiscovered element
• He called this element 'eka-silicon' which comes from the Greek 'like silicon' and when the element germanium was discovered in 1887 it was found to almost exactly match the properties Mendeleev had predicted
• No one doubted that Mendeleev had got the right idea about ordering the elements
• Strangely enough, Mendeleev always denied the existence of an eighth group of elements, even after the discovery of the noble gases in Mendeleev's final years

• Once he was finished, Mendeleev thought he had organised the elements systematically but there were still some elements which didn't quite fit in as neatly as he wanted.
• This is because isotopes were not known in Mendeleev's time, and he made no provisions for them in his table
• This meant that there was always going to be some level of inaccuracy in Mendeleev ́s work even though he did also consider the elements chemical properties as well as their atomic mass when sorting them
• Mendeleev switched the order of tellurium and iodine is his table, because even though tellurium was heavier than iodine, the chemistry of iodine fitted better with the other halogen elements; it was a nagging problem that was not solved in his lifetime
• The discovery of the proton lead to the determination of atomic number for each element
• This number is used to arrange the elements in the modern-day periodic table which fits with Mendeleev 's patterns