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. What you'll notice is that the chemical symbol in the middle represents the nucleus of this carbon atom, but revolving around the nucleus are black circles, two of them, representing energy shells. Energy shells are just the two-dimensional representation of the electron orbitals, which are three-dimensional regions. These energy shells contain electrons, depicted as blue circles that orbit around the nucleus of the atom. Typically, an atom could have multiple energy shells as illustrated in our image below.
If we focus on the carbon atom, notice that it has two energy shells. The one that is closer to the nucleus and a second energy shell further from the nucleus. The energy shells closer to the nucleus are lower in energy than those more distant from the nucleus. Distant shells, being higher in energy, are more reactive. These higher energy shells are what scientists focus most of their attention on when looking at chemical bonds, which we'll discuss more in another video. The energy shell furthest from the nucleus is the highest in energy and the one that tends to form chemical bonds. This leads us to the concept of valence electrons, defined as the electrons in the outermost energy shell, also known as the valence shell. Electrons in the valence shell, in this case, the furthest shell, are termed valence electrons, and are higher in energy and more reactive. The other electrons closer to the nucleus are not as high in energy and are less likely to form chemical bonds with other atoms.
Below the atom, the chemical symbol is displayed. To the top left of the chemical symbol, we can show the mass number, which represents the total number of protons and neutrons. To the bottom left of the chemical symbol, the atomic number, which is the total number of protons in the nucleus. For the carbon atom shown here, it has 6 protons in its nucleus, symbolized by the letter C, and also has 6 electrons, maintaining a neutral net charge since the number of protons balances out with the number of electrons.
All the atoms we are showcasing here also have a neutral net charge. The first energy shell always holds a maximum of 2 electrons and the second energy shell holds up to 8 electrons. Upon examining the carbon atom further, the first energy shell holds 2 electrons and any additional electrons are in a different energy shell. Similarly, the hydrogen atom, which has an atomic number of 1, meaning it has one proton and no neutrons, has just one electron requiring only one energy shell. The nitrogen atom, having 7 protons and electrons, fills its first energy shell with 2 electrons, with the remaining electrons occupying the second shell which holds up to 8 electrons.
The oxygen atom, with an atomic number of 8 and a mass number of 16, indicating 8 neutrons, positions its first two electrons in the first energy shell, with the other electrons in the second shell holding 6 electrons. The phosphorus atom, with an atomic number of 15 and a mass number of 31, and the sulfur atom, with an atomic number of 16 and a mass number of 32, follow similar electron arrangements with their first and second energy shells.
This concludes our lesson on electron orbitals and energy shells, and we will be able to get some practice applying these concepts in our next few videos. I'll see you in the next one.