Why colourless for main group metals?

[pivoxa15]

This is not a homeword question but an answer to a homework question of which I do not understand the reason for. The question was what are the differences between transition metals and main group metals. The answer was that transition metals can have colours but main group metals cannot. Why is this? When metal atoms combine with each other, they all have delocalised electrons?

 

[Gokul43201]

The answer was that transition metals can have colours but main group metals cannot.

This is wrong. I hope the source of this answer actually said: "salts of transition metals are usually colored, while salts of alkali and alkaline-earth metals are not".

The reason for the coloration of salts of transition metals lies in the energy difference between the two sets of degenerate valence d-orbitals (in a cubic, or octahedral, co-ordination geometry), labeled as the e_g and t_2g levels. It just happens that this energy difference for most transition metal ions lies in the visible spectrum, so it becomes easy to absorb visible photons through electronic excitations between these levels.

 

[GCT]

It would probably be good to consider charge transfer complexes, however, Gokul has proved the point that such statements (that main group metals do not exhibit color) isn't true in every sense.

 

[pivoxa15]

This is wrong. I hope the source of this answer actually said: "salts of transition metals are usually colored, while salts of alkali and alkaline-earth metals are not".

The reason for the coloration of salts of transition metals lies in the energy difference between the two sets of degenerate valence d-orbitals (in a cubic, or octahedral, co-ordination geometry), labeled as the e_g and t_2g levels. It just happens that this energy difference for most transition metal ions lies in the visible spectrum, so it becomes easy to absorb visible photons through electronic excitations between these levels.

The answers didn't mention what you say. So you are saying the metals in group 13-16 can form compounds that are coloured or not white?

The answers also suggesed that another difference was that only the transition metals have magnetic properties. Is this correct? Could you give a brief explanation for this?

 

[Cesium]

What is a "colorless" metal? Do you mean gray? Most of the transition metals are gray as well (gold being a notable exception). Cesium is goldish in color and stands out in my mind as a main block metal that isn't gray.

 

[scott_alexsk]

I think he is talking about when you burn a metal.

-scott

 

[Gokul43201]

I assumed the question was actually about salts of the metals (rather than the elemental metals themselves) because:

1. This question is being asked in chemistry, rather than in physics, and as a student of chemistry, you learn to guess what the cation is by the color of the salt. Salts whose cations are of transition metal elements, tend to be distinctly colored.

2. Most transition metals (elemental) are not colorful. The only ones that are clearly colored are Cu and Au. In addition, Ta and Os have a faint bluish tinge. The rest of the 30 or so transition metals are all metallic/silvery grey/white. Among the dozen s-block metals, Cs has a golden lustre.

The reason Cu and Au have reddish, yellowish hues is because they do not reflect high frequency light as well as they reflect low frequency light. The reason that high frequencies are absorbed by these metals has to do with a property known as the plasma frequency (and importantly, the dispersion relation of surface plasmons) - this is the characteristic frequency with which the free electron gas oscillates in the background of the positive lattice. Low frequency light will not be transmitted through a metal because the electron gas responds to the oscillating electric field and screens it out. But if the frequency is greater than the plasma frequency, the electron gas can not respond fast enough to damp the light. For most metals, the plasmon frequencies are far in the UV range (so most metals reflect all the visible frequencies almost equally well and end up looking greyish/whitish as a result), but for gold and copper, the effective plasma frequency is a little lower (making them appear yellowish/reddish). So, in short, the frequency dependence of the reflectivity decides whether something looks colored or not (and for most metals, this dependence is roughly the same, though there is a slight difference in the surface plasmon dispersion of main group metals and transition metals).