Dear Pooja

Elements are put into groups according to how many valence  electrons they have. You'll recall that the valence electrons are the  electrons in the outer most shell of an atom. It is the number of  valence electrons that generally determine the chemistry of atoms, and  that's what the periodic table is all about.
A quick way to determine the number of valence electrons for a  representative element is to look at which group is it in.  Elements in  group Ia have 1 valence electron which is equal to the valence electron of Hydrogen.  Elements in group IIa have 2 valence  electrons.  Can you guess how many valence electrons elements in group  VIa have?  If you guessed 6 valence electrons, then you are correct!   The only group of representative elements that this method doesn't work  for is group 0.  Those elements certainly have more than 0 valence  electrons; in fact, all of them except for helium have 8 valence  electrons.  Why doesn't helium have 8 valence electrons?  Think for a  moment about how many electrons helium has - it has a total of only two  electrons, so helium only has 2 valence electrons.
So generally speaking, the number of valence electrons stays the same as you go up or down a group



All the best.                                                           
AKASH GOYAL
iitd.akash@gmail.com
AskiitiansExpert-IIT Delhi
 
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