The way I would explain it is quite difficult to understand, so this is what Google says. "The wavelength (or equivalently, frequency) of the photon is determined by the difference in energy between the two states. These emitted photons form the element's spectrum. The fact that only certain colors appear in an element's atomic emission spectrum means that only certain frequencies of light are emitted." I hope this helped.
Answer:
Nickel(II) cyanide is an inorganic compound with a chemical formula Ni(CN)₂
Explanation:
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The answer is four hundred
When an electron occupies an orbital of an atom singly, rather than an electron pair such electron is said to be an unpaired electron. Each atomic orbital of an atom specified by quantum numbers can contain a maximum of two electrons having opposite spins, electron pair.
The numbers of unpaired electrons are determined by the electronic configuration of an atom. The electronic configuration of calcium, 
is:
or ![[Ar]4s^{2}](https://tex.z-dn.net/?f=%5BAr%5D4s%5E%7B2%7D)
Since, from the electronic configuration it is clear that the number of electrons in valence orbital that is 
is 2 that means the electrons are paired up in the valence orbital so, there are no unpaired electrons present in .
Hence, the number of unpaired electrons in is 0.
I think it is d. Because the process is not reversible and it a chemical process
