[Yusuf72]

Hi guys, I have a question about valance electrons and orbitals.      Here in Turkey when we      number the groups,  we do this  like 1-B  2B  3B ..8B       1A 2A 3A ...8A  and there is also another system from 1 to 18 anyway my question is that why we don't consider the   filled orbitals , for example if there is a configuration like this  4s'2 3d'1      we say iths group number is four okey I know but if there is a configuration like this 4s'1 3d'10  we say its group number is 1B(group11)   but why we don't consider the other filled orbitals (10 electron of 3d 10) and if you use the system from number 1-18 the question may seem absurd   I mean why we dont consider thwe filled orbitals,

[AWK]

4s'2 3d'1  we say iths group number is four

Sc - 4s²3d¹ is group 3B

This is an old numbering. Most of periodic tables still show both numbering since the new numbering is recommended by IUPAC (not yet decided by your government as currently obligatory).

[Yusuf72]

Why we consider only the last energy level and the  orbital that is not filled

[Burner]

We put different elements in the groups in the periodic table because they have similar chemical properties - which are mainly governed by the valence electrons in the outermost unfilled orbitals.

[Yusuf72]

Guy I know that why do we use group number my questions is about orbital configuration, which orbitals should we consider when we calculate the group number,   and why we consider the electrons of orbitals at t last energy level or  incomplete ones

[Enthalpy]

These are the electrons that can rearrange in chemical reactions. The other ones are more strongly bonded to the nuclei and stay where they are. More in detail, rearranging these would cost energy and doesn't happen (in a first analysis).

Then, you have all added subtleties of almost-outer electrons that can react too. It's the case in carbon for instance, where the 2s electrons are the shallowest (and carbon does show the valence 2) but the 2p, a bit deeper in energy, often react too (so carbon often has the valence 4).

It gets more fuzzy with transition elements where many orbitals are shallow, so they can fill more or less and in a varying sequence. Such elements show a broad set of valences and often serve as catalysts as they can pick or release a molecule fragment just by changing their oxidation state.