This question is specifically regarding valence electron count in transition met

Question

This question is specifically regarding valence electron count in transition metals and higher level non-metals. I understand the concept of valence electrons in genreal, but I'm having trouble understanding valence electron configurations for transition metals. I know that due to energy levels, the orbitals are filled 1s>2s>2p>3s>3p>4s>3d>4p>5s>4d>5p etc etc.... right? And the valence electrons represent electrons outside of the stable "core", so for example, Oxygen, Sulfur, and Bromine all have 6 valence elctrons.

But what about the transition metals, and higher elements when d orbitals are involved. Based on the configuration for, for example lets take Technetium (Tc), why are there only 2 valence electrons in Tc and not 7, especially when we write the configuration as '[Kr]4s2 3d5'?? Does that mean ALL transition metals have 2 valence electrons?

What about say Tellurium, with the configuration [Kr]5s2 4d10 5p4. Does it have 6 valence electrons or 16 and why?

Thanks in advance.

Explanation / Answer

There is a simple concept of valence electrons . Valence electrons are all those electrons that are present in the outermost shell of any atom .

So for counting the valence lectrons , simply see the shell number of the highest order in that particular atom , count down the number of electrons present in all of its sub shells.

Now when you may see for oxygen the configuration is 1s2 , 2s2 , 2p4 , so the shell of highest order is 2 , and the electrons in 2nd shell = 2+4 = 6 , so 6 valence electrons.

For bromine the configuration is 1s2 ,2s2 , 2p6 ,3s2 , 3p6 ,3d10 , 4s2 , 4p5 , here the shell of highest order you see is 4 , so the number of valence electrons = 2+ 5 = 7 ( and not 6)

SO this is as simple as this !

Lets see for Techtenium , The configuration is [Kr] 4s2 3d5 , the highest order shell is 4 , it has 2 electrons , so the valence electrons = 2

For tellurium , configuration is [Kr] 5s2 4d10 5p4 , the shell of highest order you may see here is 5 , so the number of valence electrons = 2 + 4 = 6 ( and not 16)

Hope this helps !

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