Question

a green flame is seen when copper (ii) is burned. how is the green light (photon) emitted?
when the protons, after being excited, fall to a lower energy level.
when the electrons are excited and move to a higher energy level.
when the protons are excited and move to a higher energy level.
when the electrons, after being excited, fall to a lower energy level.

Explanation:

When atoms are heated, electrons gain energy and move to higher energy levels (excitation). When these excited - state electrons return to lower energy levels, they release energy in the form of photons. The color of the light (photon) emitted depends on the energy difference between the levels. In the case of copper(II) burning, it is the electrons that, after being excited, fall to a lower energy level and emit green - colored photons. Protons do not play a role in this light - emission process.

Answer:

When the electrons, after being excited, fall to a lower energy level.