In this video, we're going to continue to talk about atoms by focusing on their electron orbitals and energy shells. Now, electron orbitals are really defined as three-dimensional regions around the nucleus of an atom where electrons can be found. Although electron orbitals are three-dimensional regions, they can still be envisioned in two dimensions as energy shells. In our course, we're mainly going to focus on the energy shell aspect. You'll learn more about the three-dimensional shapes of electron orbitals in a chemistry class. Let's take a look at our example image here of the carbon atom on the far left to get a better understanding of this idea of energy shells. You'll notice that the chemical symbol right in the middle represents the nucleus of this carbon atom, but revolving around the nucleus, what we have are these black circles, two of them for that matter, that represent energy shells. Once again, energy shells are just the two-dimensional representation of the electron orbitals, which are really three-dimensional regions. The energy shells contain electrons, and these little blue circles you see around here are the electrons orbiting around the nucleus of the atom in these energy shells. Any typical atom could have multiple energy shells as you can see from our image. Focusing on the carbon atom here, notice that it has two energy shells: one that is closer to the nucleus and a second energy shell that is further away from the nucleus. The energy shells that are closer to the nucleus are lower in energy than the energy shells that are more distant from the nucleus. The distant shells, being higher in energy, are more reactive, which is why scientists tend to focus on them when looking at chemical bonds. This leads us to the idea of valence electrons, defined as the electrons found in the outermost energy shell or the valence shell. For carbon, the valence shell is the one furthest from the nucleus. Electrons in the valence shell are the ones that are higher in energy and more reactive. Other electrons that are closer to the nucleus are not nearly as reactive and are not the ones to form chemical bonds with other atoms. Below the atom, you'll find a different way to represent atoms: the chemical symbol is shown with the mass number to the top left, representing the total number of protons and neutrons, and the atomic number to the bottom left, representing the total number of protons in the nucleus. The carbon atom has 6 protons in its nucleus and also has 6 electrons, resulting in a neutral net charge. All atoms shown have a neutral net charge. The first energy shell always holds a maximum of 2 electrons, and the second energy shell holds a maximum of 8 electrons. Our biology course requires knowing that the first shell holds up to 2 electrons, and the second shell holds up to 8 electrons.
Moving on to the hydrogen atom, it has an atomic number of 1, meaning it has 1 proton, and its mass number is also 1, indicating it has 1 proton and no neutrons. Since it has only one electron, it only needs one energy shell. For the nitrogen atom, it has 7 protons in its nucleus, and its energy shell configuration is such that the first shell is filled with 2 electrons, with remaining electrons in the second shell. The oxygen atom, with an atomic number of 8 and a mass number of 16, fills its first shell with 2 electrons, with others moving to the second shell, which can hold up to 8 electrons. For the phosphorus atom, with an atomic number of 15 and a mass number of 31, its first shell is filled with 2 electrons, its second shell with 8, and additional electrons move to the third shell. Similarly, the sulfur atom, with an atomic number of 16 and a mass number of 32, follows the same shell-filling pattern. This concludes our lesson on electron orbitals and energy shells and sets the stage for applying these concepts in our upcoming videos. I'll see you in our next one.