A) is it possible to say that a wave pulse( waves packet) has one particular wavelength? b) how are wave packets built (mathematically speaking)?

A) we can write a wave packet like Y = ∑aₙ*cos(w*n*t)

So you have lots of different wavelengths here, and you think that a wave packet has a unified frequency, but not wavelength.

B) mathematically speaking you will write a wave packet like i did up there, has a sum of many waves, where for each one you can have an intensity aₙ and a different frequency, the only thing you must see is that all the waves you are suming are moving in the same direction.

when it gains or loses electrons

myth. it was once thought that animals, including cats and dogs, could only see in black and white. however, scientists have proven this to be a myth. in all animals, including humans, the perception of colour is determined by the presence of cells in the eye called cone photoreceptors.