Core and Valence Electrons

Question

Determine the number of core and valence electrons in each of the following neutral atoms:

 

Fe

Co

Na

Ag

Rb

Answer 1

I think the answer to this question really depends on how you define "valence" electrons.  Do you define them as the electrons in the outer most shell, i.e. the shell with the highest principal quantum number?  Or do you define them as the electrons that take place in bonding and in chemical reactions?  The first definition is fine for main group elements (s and p block), but not so much for transition elements (d block) and for f block elements (Fe, Co, Ag) in your question. 

Fe has 8 valence electrons and 18 core electrons, or as probably taught in your class it has 2 valence electrons and 24 core electrons. It is [Ar]4s2 3d6 and since 4 is the highest principal quantum number and there are 2 electrons.

Co has 9 valence electrons and 18 core electrons, or as probably taught in your class it has 2 valence electrons and 25 core electrons. It is [Ar] 3d7 4s2. and again 4 is the highest principal quantum number and there are 2 electrons.

Na has 1 valence electron and 10 core electrons.  Simple because it is an s block element

Ag has 11 valence electrons and 36 core electrons, or as probably taught in your class, it has 1 valence electron and 46 core electrons

Rb has 1 valence electron and 36 core electrons. Simple because it is an s block element.

 

So, as you can see, it is not as straight forward as some might think.