Arranging the Elements

Specification Point 1.13:
  • Describe how Mendeleev arranged the elements known at that time, in a periodic table by using properties of these elements and their compounds.
  • In 1869 Dmitri Mendeleev created his first draft of the Periodic Table.
  • He started by writing down the names of the 50 elements known at that time on pieces of paper and placing them on a board.
  • He organised them into vertical columns based on the properties of the elements and 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 of 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.

Predicting Element Properties

Specification Point 1.14:
  • Describe how Mendeleev used his table to predict the existence and properties of some elements not then discovered.
  • Mendeleev realised quickly that elements with the same properties should be placed in the same column.
  • He surmised 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 as of yet undiscovered elements.
  • In this way he used his Periodic table to predict the physical and chemical properties of these 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.

Mendeleev's Early Periodic Table, Edexcel GCSE Chemistry

Mendeleev’s early Periodic Table showing the gaps

Isotopes & the Periodic Table

Specification Point 1.15:
  • Explain that Mendeleev thought he had arranged elements in order of increasing relative atomic mass but this was not always true because of the relative abundance of isotopes of some pairs of elements in the periodic table.
  • 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 poorly understood 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 as he used atomic mass to arrange the elements.
  • 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 also fits with Mendeleev ‘s patterns.

The modern Periodic Table shows elements in order of increasing atomic number from left to right

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.