How do we find the valency of periodic classification?

you mean how to find valency of an element from periodic table?
For neutral atoms, the number of valence electrons is equal to the atom's main group number. The main group number for an element can be found from its column on the periodic table. For example, carbon is in group 4 and has 4 valence electrons. Oxygen is in group 6 and has 6valence electrons.
Use the group numbers to determine the number of valence electrons. The Group number of a non-transition metal can be used to find the number of valence electrons in an atom of that element. The ones place of the group number is the number of valence electrons in an atom of these elements. In other words:
  • Group 1: 1 valence electron
  • Group 2: 2 valence electrons
  • Group 13: 3 valence electrons
  • Group 14: 4 valence electrons
  • Group 15: 5 valence electrons
  • Group 16: 6 valence electrons
  • Group 17: 7 valence electrons
  • Group 18: 8 valence electrons (except for helium, which has 2)

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with the help of atomic number.
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With the help of periodic table
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