2.2.8 Atomic Structure & the Periodic Table

The Early Periodic Table

• Before the discovery of the subatomic particles, scientists arranged the elements in order of their mass number 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 and physical properties, as well as the properties of their compounds
• He then started to arrange them horizontally in order of increasing mass number 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

• The group number of an element which is given on the Periodic Table indicates the number of electrons in the outer shell (valence electrons)
  
  • This rule holds true for all elements except helium; although is in Group 0, it has only one shell, the first and innermost shell, which holds only 2 electrons
• We can use the group number to predict how elements will react as the number of outer shell electrons in an element influences how the element reacts.
• Therefore, elements in the same group have similar chemical reactions
  
  • By observing the reaction of one element from a group, you can predict how the other elements in that group will react
  • By reacting two or more elements from the same group and observing what happens in those reactions you can make predictions about reactivity and establish trends in reactivity in that group
• For example, lithium, sodium and potassium are in Group 1 and can all react with elements in Group 7 to form an ionic compound
• The Group 1 metals become more reactive as you move down the group while the Group 7 metals show a decrease in reactivity moving down the group. It is common for trends to be the opposite for metals and non-metals