Please help me understand the whole valence electron shenanigan

[Monkeymaniac]

I am thoroughly confused when it comes to answering questions that deal with valence electrons of atoms which have d orbitals filled.

1) for Fe ([Ar]4s^23d^6), which are considered valence electrons?
I read that valence electrons are ones in orbitals with highest n value, and they are the ones that get ionized first, so are the 2 electrons in 4s only valence electrons?

2) I think colors we see when metals are mixed with water have to do with electrons being excited from lower energy d orbitals to higher energy d orbitals. But is it when the excited electron is de-excited and when a photon is subsequently release that we actually see the color?

3) For a neutral atom, it seems to be rather straightforward to decide from its electron configuration whether it has unpaired electrons, which makes it paramagnetic. But if we were asked to determine if a molecular compound (covalent bonds) or ionic compounds are paramagnetic substance, how do we know if it has unpaired electron or not? Do we decide this by looking at orbitals not used in forming hybridized bonding orbitals and see if unpaired electorns do exist?

Thanks in advance!

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[MT Headed]

(1) The valence electrons are those that can participate in chemical bonds. You know that Fe3+ exists as an ion. Therefore Fe has 8 valence electrons. Zinc, which only exists as Zn2+, has two electrons. Valence electrons as they apply to transition metals is quite messy, you can read about it here: http://en.wikipedia.org/wiki/Valence_electron#Valence_electrons_and_electron_configuration

(2) No, the colors in transition metal complexes have to do with photon absorption, not emission. These test tubes do not glow in the dark.

(3) I believe molecular orbital theory, and predicting results without a molecular orbital diagram, is the most difficult part of first year general chemistry. While I could attempt to explain it here, I seriously question why. It is not a PS.Chemistry.Bonding topic. The only other place you'd run across it is analyzing aromatics, but BS.Organic.Aromatics was removed years ago. This topic is not mcat worthy.

 

[Monkeymaniac]

MT Headed, thank you, that helped a lot. For (2), you said the color we observe has to do with photon absorption? Would you please elaborate on that?

Is it like the complementary color thing, where when white light is shone on transition metal, it absorbs certain EM frequency, emits complementary color, and this is what we observe?

 

[MT Headed]

Yes, it's a complimentary color thing. They are pigments.

Much like a leaf is green, not because it emits green light, but because a leaf absorbs red and yellow light. Your eyes perceive the colors that are reflected back as green light.