Mendeleev's Periodic Table
The Early Periodic Table
- Before the discovery of the subatomic particles, scientists arranged the elements in order of their atomic mass and instead of what we now use, their atomic number.
- Whilst the general trend is the same, with both increasing left to right and top to bottom, there are some elements which would not be in the same order with mass number as with atomic number
- When the elements that were known at that time were sorted by mass into a table, some patterns emerged at regular periods along the table, giving rise to the term "periodic"
- The early tables were incomplete as not all elements had been discovered. This meant that some elements were forced into a position to fill gaps which appeared during the sorting process, ruining the patterns in places
- Other elements were placed in the wrong group as they were sorted strictly on their mass and had their chemical properties ignored, so groups did not always share chemical behaviour as they do now
- There were many early versions of the tables as scientists in different countries grappled with the ordering of the elements
- These were the Periodic Tables that Mendeleev was working to improve
Mendeleev's Periodic Table
- 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 chemical properties, as well as 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
- 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
- This was unique as other chemists assumed that there were no gaps.
- He also switched the order of the elements to maintain consistency down the columns
Mendeleev's Periodic Table showing gaps
- 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
- 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
Examiner Tip
Remember that Mendeleev did not know about protons, electrons or neutrons at all. Do not mention these when talking about the development of the Periodic Table!