exact answers only
When we add extra metal to the lattice, it provide extra electrons to the lattice and the anions of the lattice leave their lattice points . These vacant sites are occupied by the electrons. And the free electron can easily absorb energy and can jump to higher level.
As these energy transitions are in visible spectrum of light so the crystal appears to be coloured.
For example in case of NaCl crystal, when we heat it the crystal appear yellowish.
I am M.Sc (chemistry ) from punjabi university. I am a science teacher with expertise in chemistry, with 8 years of experiance in teaching. i am getting a handsome salary , now i wish to help out the people who need me UNCONDITIONALLY. and I am constantly working on creating a new and easy way of learning the tough things in an effective way. I am constantly working to make authentic and reliable information to be shared with my students and widen the horizons of knowledge. View more posts
The Unconditional Guru