The Periodic Table

• Mendeleev arranged elements in order of atomic mass and placed them in groups according to their chemical properties; he lined these elements up. If the properties of an element did not fit his pattern, he swapped elements around and left gaps, assuming that the atomic mass measurements were incorrect and that some elements were yet to be discovered. He also predicted the properties of missing elements from group trends. However, it was not until protons were discovered that the real reason for Mendeleev's table was revealed
• 114 elements
• The periodic table is organised into rows called periods (horizontal) and columns called groups (vertical)
• 7 Horizontal rows and 18 vertical groups
• Arranging elements

  The arrangement, pattern and shape of the periodic table reveals trends.

  The position of elements is linked to their physical and chemical properties of the elements and their compounds
• Atomic number

  From left to right elements are arranged in order of increasing atomic number. Each successive element has one extra proton
• Groups

  Each element in a group has atoms with the same number of outer-shell electrons.

  Elements in groups have similar properties
• Periods and Periodicity

  The horizontal rows of the periodic table are known as periods

  The number of a period gives the number of the highest energy electron shell in the element's atoms

  Across each period there is a repeating trend in the properties of elements - this is known as periodicity

  The most obvious example is moving from metals to non-metals
• Periodic trend in electron configuration

  The chemistry of each element is determines by its electron configuration, particularly the outer, highest energy electron shell
• Trend in electron configuration across a period

  Each period starts with a new highest energy shell

  Across period 2, the 2s sub-shell fills with two electrons, followed by the 2p sub-shell with 6 electrons (same pattern of filling for period 3 for 3s and 3p sub-shells)

  Across period 4, although 3d sub-shell is involved, highest shell number is n=4 - from n=4 sub-shell only the 4s and 4p sub-shells are occupied
• For each period, the s and p sub-shells are filled in the same way - a periodic trend
• Trend in electron configuration down a group

  Elements in each group have atoms with the same number of electrons in their outer shell

  Elements in each group also have atoms with the same number of electrons in each sub-shell

  This similarity in electron configuration gives elements in the same group their similar properties.
• Trend in electron configuration in blocks

  Elements can be divided into blocks corresponding to their highest energy sub-shell

  Gives 4 distinct blocks: 2,p,d,f
• Group 1 = Alkali metals, they are soft and have low melting and boiling points
• Group 2 = Alkaline Earth Metals (Al, Ba, Ce, La, Nd)
• Group 5 = pnictogens
• Group 6 = chalcogens
• Group 7 = Halogens (fluorine, chlorine, bromine, iodine, astatine)
• Group 0 = Noble gases, they are inert and non-reactive