Main Points

Periodic Classification

  • Dobereiner proposed that when three elements of a triad are arranged in increasing order of their atomic masses then the atomic mass of the middle element would be roughly equal to the average of atomic masses of the remaining two elements.
  • Newlands’ Law of Octaves says that if we begin to count from a particular element then every eighth element shows similar properties as the first element.
  • Mendeleev’s Periodic Law: “The properties of elements are periodic function of their atomic masses.”
  • Mendeleev left some gaps in his periodic table and predicted that they would be occupied by new elements to be discovered in future.
  • When noble gases were discovered, they could be placed in a new group without disturbing the original order of Mendeleev’s Periodic Table.
  • Position of hydrogen in Mendeleev’s Periodic Table can be a debatable issue because it could be placed along with alkali metals as well as with halogens.
  • Modern Periodic Law: “The properties of elements are periodic function of their atomic number.”
  • The Modern Periodic Table has 18 vertical columns, i.e. groups and 7 horizontal rows, i.e. periods.
  • The group number to which a particular element belongs is same as the number of valence electrons in the element.
  • The period number to which a particular element belongs is same as the number of shells in an atom of the element.
  • When we move from left to right; across a period; valency first increases from 1 to 4 and then decreases to zero.
  • Atomic radius decreases across a period and increases down a group.
  • Metallic property decreases across a period and increases down a group.
  • Non-metallic property increases across a period and decreases down a group.

These main points are highly helpful for last minute preparation for examination. You can glance through them while going for examination. You can view them in the school bus, in car, in elevator, etc.