Let's talk about electrons. It's a long story. So hold on tight, and follow us.
We have already seen that electrons are staying in the empty space outside of the nucleus. They are held there by the attraction forces between their negative charges and the positively charged protons in the nucleus. However, they do not simply stay at a particular location. They do move around. So how are they moving, and how will multiple electrons be arranged in space? Is it like our solar system where the electrons are like the planets while the nucleus is like the sun? That's a good guess, and that's what Bohr believed.
Bohr's model
Bohr studied the electron behavior in hydrogen (\( \ce{H} \)) atoms as they only contain a single electron so that it would be easy to start with. Bohr was inspired by the solar system, and he suggested that electrons would orbit around the nucleus in defined trajectories. This could be illustrated by the figure below.
Subatomic particles in an atom
As can be seen in this illustration, the small black circle at the center of the atom represents the nucleus, whereas the big orange circles show the orbits that electrons would move in.
For a simple atom like hydrogen, Bohr's model would successfully describe the behavior of electrons. Although it fails to explain the electron behaviors for more complex atoms, Bohr's model has its merits of introducing us the concepts of energy levels.
Let's take a closer look at Bohr's model and try to understand the concept of "energy level". But before that, we have to choose a tool that allows us to study those super tiny atoms and even smaller electrons. That is "electromagnetic wave".