the periodic table

• • Dmitri Mendeleev published his first periodic table in 1869
• • Mendeleev arranged elements in order of increasing relative atomic mass
  • the chemical properties of the elements and their compounds showed a trend
  • he arranged elements by putting those with similar properties in groups (columns)
• to make his classification work, Mendeleev:

  • switched the order of a few elements to keep groups consistent
  • left gaps for undiscovered elements
• • Mendeleev left gaps in his periodic table for elements that had not been discovered
  • he made predictions about the properties of undiscovered elements based on the properties of nearby elements

  example: he predicted the existence of ‘eka-aluminium’, which fit in a gap next to aluminium. Gallium was discovered and had very similar properties to those Mendeleev had predicted for ‘eka-aluminium’
• pair reversals:

  • Mendeleev reversed some positions of elements to group elements with similar properties together
  • this meant that his periodic table was not fully in order of increasing relative atomic mass
  • this led to some scientists rejecting his idea when it was first published

  example:

  • tellurium and iodine were switched since iodine had similar properties to fluorine, chlorine, bromine
  • iodine exists as Iodine-127
  • Tellurium has isotopes Te-128 and 130, which are more abundant than Te-126
  • therefore Tellurium has a higher relative atomic mass than iodine
• • the atomic number of an element used to be its position in the periodic table
  • Moseley fired high energy electrons at different elements, making them radiate X-rays
  • he discovered that for every step increase in atomic number there was a step increase in energy of X-rays
  • he realised that an atomic number of an element was equal to the number of positive charges in the nucleus of an atom
  • protons were then discovered and scientists realised that atomic number = number of protons
• in the modern periodic table:

  • elements are arranged in order of increasing atomic number
  • elements with similar properties were placed in columns called groups
  • rows are called periods
• metals are:

  • they are found to the left side of the periodic table
  • they lose electrons to form positive ions which forms a structure that is stable
  • they have less than 4 electrons in their outer shell
• non-metals are:

  • found to the right of the periodic table
  • gain electrons to form negative ions
  • have more than 4 electrons in their outer shell
• an electronic configuration is the way in which electrons are arranged in an atom or ion
• for elements with atomic numbers 1-20:

  • first shell holds maximum two electrons
  • second shell contains maximum 8 electrons
  • third shell contains maximum 8 electrons
• you always use atomic number to calculate electron configuration because its the same as the number of electrons in the element
• example:

  atomic number of sodium is 11 so:

  • 2 electrons in first shell
  • 8 electrons in second shell
  • 1 electron in third shell
  • configuration: 2.8.1
  • sum of numbers is 11
• how electronic configuration of an element is related to its position on periodic table:

  • number of shells = period (row) number
  • number of electrons in outer shell = group number
  • total number of electrons = atomic number