The valance electrons determine the valance charge of an atom.
Determine how many electrons an atom should gain or lose to achieve stability.
The IA group have 1 valance electron. Losing that electron would provide the atoms with a stable electron configuration equivalent to the group VIII A or inert gases.
The IIA group have 2 valance electrons. Losing those electrons would provide the atoms with the stable electron configuration equivalent to the inert gases.
Group IV A atoms have four valence electrons to be stable these atoms can lose four or gain four electrons to achieve stability.
Losing 2 electrons can create a filled s subshell. This electron configuration is not as stable as losing 4 but is found in nature.
As example is Lead
Group VI A atoms have six valance electrons. Atoms like sulfur can lose six electrons as well as gaining two to reach stability.
As well as looking at the valance numbers it is important to look at the periodic table to see where levels of stability are located.
Group B atoms are a good example of this.
Group IIIB have three valance electrons. Which means they can become stable by losing 3 electrons. Group VI B can not easily gain enough electrons to become stable so must only lose electrons to become stable. But Group VI can become stable by losing the 2s electrons to become +2 as well as +6 losing all six electrons or by losing the 4 d electrons to become +4.
Knowing the valance electrons can provide a good basic idea of what the valance of an atom will be. This knowledge must be combined with an understanding of the periodic stable to get a complete understanding of the valance of the elements.
Here's what I find.
Let's consider only the Main Group elements: those in Groups 1, 2, and 13 to 18.
The valency of an atom is the number of
Atoms in Groups 14 to 17 can get a stable octet by gaining electrons. For these atoms,
Group 18 atoms have a complete octet. They need no more electrons, so their valency is zero.
Here's a summary of the situation.