Please fill in the blanks.

The electrons was originally thought to exist as orbits which are in __ shape, much like (a). It was thought that the path and location could be known in the (b) model later determined to be inadequate to explain electron movement. However, the idea that electrons were located at different (c) around the nucleus having discrete (d) levels is still thought to be true. The researcher proposing this model was (e) ,basing his conclusions on the line spectrum of the element hydrogen. 

An electron without any excess energy is said to be at (f) state but becomes (g) when energy is absorbed. An electron having excess energy jumps to a/n (h) Here, it is unstable and will immediately (i), releasing the (j) it had absorbed as a (k) of light. 

We believe electrons exist in discrete (l) at different distances around the nucleus because if they were located at ANY distance from the nucleus, with any amount of energy we would not see an (m) Spectrum with (n) lines but rather, a (o) of colors, representing the energies of the (p) electrons falling back to (q) state. 

Scientists named (r) came up with an alternative model based on the premise that electrons behave as both a (s) and as a (t) . It is a mathematical model using the term (u) to describe the space where the electrons exist and does not specify its (v). For each electron, a unique set of (w) numbers exist. The descriptors of these four "numbers are (x). (y). (z), and (aa).