the periodic table

• Mendeleev ordered his table in order of atomic mass, but not always strictly – i.e. in some places he changed the order based on atomic weights, he also left gaps for elements that he thought had not been discovered yet
• Mendeleev realised elements with similar properties belonged in the same groups in the periodic table so was able to leave gaps and place the discovered elements where they fit best elements with properties predicted by Mendeleev were later discovered and filled the gaps
• 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, knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct, because some elements have a higher mass than others when isotopes are taken into account, but a lower one if you only look at one specific isotope
• elements are arranged in order of atomic (proton) number (bottom number) and so that elements with similar properties are in columns, known as groups, elements in the same group have the same amount of electrons in their outer shell, which gives them similar chemical properties.
• how are elements arranged in the periodic table?

  • elements are arranged in order of increasing atomic number, in rows called periods
  • elements with similar properties are placed in the same vertical columns called groups
• metals are found to the left and bottom of the periodic table, because they lose electron(s) in order to form these positive ions, forming an electronic structure that is stable, like that of a noble gas
• non-metals are found towards the right and top of the periodic table, because they gain electron(s) in order to form these negative ions, forming an electronic structure that is stable, like that of a noble gas
• the electronic configuration of an element tells you how many electrons are in each shell around an electron’s nucleus
• the group an element is in tells you the number of electrons in the outer shell, the row the element is in tells you how many shells the element has