Question

first tell the valency of nitrogen and oxygen then answer!

NO3(Nitrate)

now valency of nitrogen is -3
oxygen -- -2
then the valency should be -3+(3*-2)
=-3+(-6)=-3-6=-9

but why it's -1????
like this
valency of HCO3 is
h-+1
c-+4
o-. -2*3

so 1+4+(-6)
= 5-6 = -1
then why not in case of NO3??? ​

Answer 1

Yes, there is some logic behind it, but your excellent question shows that the answer is not obvious.

Looking at oxygen first, each oxygen atom has 2 electrons in its inner shell, and 6 in its second shell. For quantum mechanical reasons, oxygen has 'room' for two more electrons in its second shell, because the second shell has room for 8 electrons. (This is partly because the second shell is close to its positively charged nucleus, and so we can informally think of oxygen as 'wanting' two electrons. Technically, we say that oxygen is highly electronegative).

So three oxygen atoms 'want' 6 electrons as you indicated in your question.

Now comes the harder part. Nitrogen has a total of 7 electrons - two in its inner shell and 5 in its second shell. So by the reasoning above, you'd think that nitrogen has room for 3 more electrons in its second shell. And indeed it does. And some compounds, such as lithium nitride, Li3N, follow this pattern. (The nitrogen takes an electron from each of the three lithium atoms to complete its outer shell).

But now consider this. With 5 electrons in nitrogen's outer shell, it can also be 'happy' by giving away these 5 electrons, leaving only its two inner shell electrons. After all that's the sort of thing sodium atoms do when they react with chlorine atoms. A sodium atom gives its single outer shell electron to a chlorine atom, forming table salt.

So nitrogen gives away its five outer shell electrons to the three oxygen atoms, because the oxygen atoms pull more strongly on the nitrogen's outer electrons than nitrogen's own nucleus does.

Now we're getting close to the answer to your question. The nitrogen atom loses 5 electrons to the three oxygen atoms. But these three oxygen atoms want 6 electrons. So there's still a strong need for one last electron.

And so when, for example, sodium forms the compound sodium nitrate, NaNO3, the single outer-shell sodium electron transfers over to the NO3, making it a nitrate ion, giving it a -1 charge. (And to keep things balanced this also forms a sodium ion with a +1 charge).