History of the Periodic Table (AQA GCSE Chemistry: Combined Science)

• Before the discovery of the subatomic particles, scientists arranged the elements in order of their atomic weight and not their atomic number
• 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 atomic weight 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 weight 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

Mendeleev's Periodic Table showed gaps to allow for undiscovered elements

Why are there gaps in Mendeleev's Periodic Table?

• There were exceptions though as some elements didn't fit the pattern when arranged by atomic weight
• 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
• He 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
• When these elements were later discovered and found to fit the pattern developed by Mendeleev, it served to confirm his theories
• The existence and properties of “eka-silicon” for example, which we now know as germanium, was predicted by Mendeleev

• 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 element's chemical properties as well as their atomic mass when sorting them
• As soon as the subatomic particles were discovered, the atomic number was calculated for each element
• This number is used to arrange the elements in the modern-day Periodic Table which fits with Mendeleev's patterns