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To find the valence electrons in the d-block, you should include the s-block electrons from the period of that element as well as any d-block electrons in that period. You can do this by counting over from the left side of the periodic table towards the element. For example, Mn would have 7 valence electrons. I think I remember reading somewhere that questions regarding the number of valence electrons of the d-block elements will try to be avoided on exams but I am not completely sure about that.
Hi! I believe that valence electrons are all the electrons that are added to the prior noble gas notation if that is a helpful way to think about it. For example, Iron, which has 26 total electrons, has a noble gas notation of [Ar]3d64s2. This would make Fe have 8 electrons in its valence shell.
For most d-block elements in period 4, you would add have [Ar] 4s2 and some number of electrons in 3d orbital. As you go along the d block, add one electron to 3d. There are exceptions, such as Cr and Cu in which the 4s orbital is missing one electron. Cr is [Ar]4s1 3d5 and Cu is [Ar]4s1 3d10
So when you are taking into account the d block, you would count its valence electrons just like you would do for the s and p block, just make sure that for the 4th period, you count the d-block as it overlaps with the s and p blocks at that rate. Now, the only thing you would do differently, in terms of the electron configuration, subtract the n value or the period value by one (also you would subtract by 2 for the f orbital). Then, make sure that you place it below the other shell configuration values since it has a lower potential energy (lower shell value since you subtracted).
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