Question

Why hydrogen spectrum shows four lines in the visible region, helium shows eight, sodium shows two and neon shows sixteen?

Answer 1

There are actually a lot more than 4,8,2,16 in hydrogen, helium, sodium and neon respectivel
but those are the most prominent ones within the visible region. One atom has multiple excited states,
and this emission spectrum is the energy difference between excited states and normal states.
When activated, atoms will absorb enough energy to have a level-up transition. but you see , a
higher level for any atom is not stable so it will jump down to a lower energy level quickly
emitting a photon and forms spectrum.
Hence only those atoms which cannot stay in the excited states show up in the spectrum