In this section, we will understand the different energy levels that electrons can occupy in an atom. After completion of this section, we can then go back to covalent bonding and understand how and why it occurs.

Imagine yourself spinning a golf ball connected to a spring around yourself. In this case, the golf ball represents the electron and the place where you stand is the nucleus. Now, if you spin the golf ball in a constant manner, the electron will be at the same distance from you, spinning at the same speed around you as well. If you start to spin harder by putting more energy into your spin, you'll notice that the golf ball will tend to move out away from you and the spring expands. Likewise, if you spin slower than usual, the golf ball will be closer to you than before and the spring contracts. In the same way, with changes in energy, an electron can occupy a different orbit.
The smallest of these orbits represents the lowest energy that the electron can have. This lowest energy state is known as the ground state. If the electron absorbs energy of the right amount, such as visible light, infrared (heat), or ultraviolet, the electron can jump to a higher orbit or "energy level" in the atom. With the electron in a higher orbit, the atom is said to be in the "excited state." At this point, the electron can fall back to a lower energy orbit or even the ground state. As it falls one orbit at a time, it emits a certain amount of energy, which may also be in the form of light, heat, or so on.

In the modern model of the atom, it is not possible to know the amount of momentum in an electron while knowing its direction; in other words, we can either predict the direction that an electron will take OR how much momentum it possesses. This is called the Heisenberg Uncertainty Principle, named after Werner Heisenberg. However, it is possible to know the probability, or chances, of where the electron is located. For example, look at the model of the hydrogen atom. Where the dots are condensed is where one would expect to find the electron; the spherical shell of dots is an average position of the electron. These shells are called principal energy levels and are numbered 1, 2, 3, etc.

The principal energy levels of the hydrogen atom are the regions in space that can be occupied by the single hydrogen atom. In every principal energy level, there exists at least one sublevel. The number of sublevels is the same as the principal quantum number. For example, the first principal energy level has one sublevel while the second principal energy level has two sublevels. We name the first sublevel as the 's' sublevel. The second sublevel is the 'p' sublevel. In the hydrogen atom, there is one principal energy level (for the only electron). Thus, the only sublevels available for that electron to move about is the 1s sublevel.

Pause for a minute before you continue to the next page. Take some time to look at the diagrams and understand what principal energy levels are and what sublevels are. When you are ready, click on 'Next Section.'