the second energy level can hold one pair of electrons.
the second energy level can hold how many electrons?
one pair
each shell can contain only a fixed number of electrons: the first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on. the general formula is that the "n"th shell can in principle hold up to 2("n") electrons. since electrons are electrically attracted to the nucleus, an atom's electrons will generally occupy outer shells only if the more inner shells have already been completely filled by other electrons. however, this is not a strict requirement: atoms may have two or even three incomplete outer shells. (see madelung rule for more details.) for an explanation of why electrons exist in these shells see electron configuration.
energy level
the process of gaining and losing energy is normally random, so under typical conditions, a large group of atoms is unlikely to be in a suitable state for this reaction. lasers depend on some sort of setup that results in many electrons being in the desired states, a condition known as a population inversion. an easy to understand example is the ruby laser, where there is a metastable state where electrons will remain for a slightly longer period if they are first excited to even higher energy. this is accomplished through optical pumping, using the white light of a flash lamp to increase the electron energy to a blue-green or ultraviolet frequency. the electrons then rapidly lose energy until they reach the metastable energy level in the deep red. this results in a brief period where a large number of electrons lie at this medium energy level, resulting in a population inversion. at that point any one of the atoms can emit a photon at that energy, starting the chain reaction.
project excalibur