It was taught to us by our teacher that;

I A has a valence of +1;
II A has a valence of +2;
III A has a valence of +3;
IV A has valence of combination of +4 and -4;
V A has a valence of of combination of +5 and -3;
VI A has a valence of combination of +6 and -2;
and VII A has a valence combination of of +7 and -1.

Please refer to the groups of periodic table.

Furthermore, our teacher added that I A has s^1, II A has s^2, III A has p^1, IV A has p^2, V A has p^3, VI A has p^4 and VII A p^5.

Please refer to the electron distribution mnemonics.

So here's my questions:

1) How do I know if I'm going to use either the positive or negative one? (For groups IV A up to VIII A)

That will be only.

pls. help

 

Not sure I understand the question Catapult.

You do not use them, the element does. It depends on the compound the element is in. The valence(s) of the particular elements define the number and type of bonds the elements can make, and in a compound define the ratios those elements will take on.

A group V11A element will either 'lend' 7 electrons to other elements or 'borrow' an electron from another element. As an example, Chlorine (Cl) has valences (+7, -1). If it combines with a 1A element like Sodium (Na) that has a valence of +1, the Na will 'lend' and electron to the Cl, which will 'borrow' it. In this case the Cl is in it's -1 valence.


However, in a more complex compound like Cobalt(II) Chlorate;

\(Co (ClO_{3})_{2}\)

The Chlorine is in its +7 valence, with each Chlorine 'lending' 2 electrons to each Oxygen (valence -2) and 1 to the Cobalt. (valence -2)

In the diagram below, the Cl is green, the Co is dark grey and the O is light grey.

Does this help?