Why the element in periodic table which have more than three electron valens must plus ten in order to know the group in periodic table ?

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If an element has 3 or more valence electrons then the element belongs to p block of the periodic table. For p block elements the group number is found out by adding the number of electrons in valence shell + 10.

The reason for this is because in the middle of the periodic table there are the d block elements ranging from group 3 to group 12 a total of 10 groups in between elements having 2 valence electrons and 3 valence electrons

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