Question

How does number of (i) valence electrons (ii) valency ;vary on the moving from left to right :

(a) in the second period of a periodic table ?

(b) in the third period of a periodic table ?

Answer 1

When we move from left to right electrons are added into the last orbital of the element as atomic no. increases electrons also increase.

In the second period , 1 electron gets added to the last orbital as we move from left to right. so number of valence electrons increase as shown
Li (1), Be(2), B(3) C(4) N(5) O(6) F(7) , Ne(8)

And the valencies increase as:

Li(+1) Be(+2) B(+3), C(+4 or -4), N(-3) O(-2) F(-1) and Ne (0)

We know that after 4, the valencies are obtained by subtracting the no.of electrons in the valence electrons from the octet
So valency of N is [8-5 =3] , O is [8-6=2] etc.

b) In the third period, valence electrons increase by 1 on moving from left to right
Na(1) , Mg(2), Al(3), Si(4), P(5), S(6), Cl(7), Ar (8)

But the valencies of these elements are;
Na(+1), Mg(+2), Al(+3), Si(+4), P(-3), S(-2), Cl(-1), Ar(0)