transitional metal valence e- applications

[hoss19]

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does anyone know how to determine the number of valence electrons of a transition metal when required to answer a question about the molecule's formal charge, molecular shape, geometry, etc.

For instance, i ran across a question asking for the shape for MnO4-. One needs to know the number of valence electrons in order to draw the lewis dot diagram; from there one can determine the shape. But how do you know how many valence electrons do you start with for Mn?

My text tells me that the shape is tetrahedral...