As we know, some metal ions like Calcium ion , potassium ion and sodium ion are colorless, while Cobalt ion, Iron(II)/(III) ion have color. Why ? What is the physical nature beneath that phenomenon ? What theories are involved ?

It depends on how much light the ions absorb , and if the rest light (the light the ions havent absorbed) is between 400nm and 800nm we can see a color.

 

At least that what i think.

As we know, some metal ions like Calcium ion , potassium ion and sodium ion are colorless, while Cobalt ion, Iron(II)/(III) ion have color. Why ? What is the physical nature beneath that phenomenon ? What theories are involved ?

 

As Ilja has already said it sbecause the wavelengths of light the ions absorb varies, even the colourless ones absorb light but its notmally somewhere in the infrared spectrum

 

Have a look on Wikipedia or Google for more informaiton on this subject

 

Cheers,

 

Ryan Jones

Most of the time, the color of complex ions comes from d->d transitions. That is, the absorbtion of a specific wavelenght of light excites electrons in one d orbital (e.g. from the t_2g orbitals to the e_g^* orbitals). Calcium, potassium, and sodium are all in the first two columns of the periodic table and therefore, their valence electrons are only in the s orbital. Transition metals like cobalt, iron, and manganese have d electrons and therefore can have d->d transitions. It is also worth noting that despite the fact that zinc is a transition metal, Zn²⁺ ions are generally colorless because they have a full d subshell, so no d->d transitions can occur.