The combining power of an element, especially as measured by the number of hydrogen atoms it can displace or combine with.
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valency is the combining capacity of an element or the number of electrons loosed or gained by an element is called it's valency ....

if in the outermost shell number ( l,m...) of valence electrons is 1 , 2 , 3 then the valency of the element will be 1 , 2 , 3 respectively but here electrons will be loosed and if in the outermost shell number of valence electrons is 5 , 6 , 7 then valency will be 3 , 2 , 1 respectively  ....................

in k shell shell the maximum number of electrons that can make the element stable is 2 so if any element i.e. helium only has 2 electrons in it's outermost shell so it's valency is 0 ... and in argon also atomic number 10 it has 0 valency because 2,8 this is argon's electronic configuration so here L shell which has capacity of 8 electrons is full so it's valency is 0 ......

i hope it helps............
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