Student Resources

Figure 2.12 Electron Energy Filling Diagram. Orbitals with the lowest energy are filled with electrons before orbitals at higher energy levels.
Note that in the energy diagram above that there are many orbitals that share the same energy level. For example the three 2p orbitals have the same energy level. Thus, when these orbitals are filled with electrons, they are filled according to Hund’s Rule which states that single electrons will occupy orbitals at the same energy level before they will pair up. (If it helps, you can think to yourself…’electrons find each other repulsive and like to live alone’) The following example shows the correct and the incorrect way to fill the 2p orbitals.

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We can use the electron energy filling diagram to complete an example of how electrons in an element are arranged within the electron orbitals (Figure 2.13). Let’s take the example of Iron, (Fe). Iron has an atomic number of 26 or 26 protons. Thus in the elemental state, iron will also have 26 electrons. We will use arrows to represent our electrons within our diagram. Arrows pointing in opposite directions denote electrons that have opposite spin states. Note that the first set of electrons fills the 1s orbital. The next 8 electrons then fill the 2s and 2p orbitals. Note that when filling orbitals that have the same energy level, that the electrons have to follow Hund’s Rule, and fill these orbitals as single electrons first, and then pair up when there are no other options to exist in an orbital as a single electron. This filling pattern is noted in the lefthand diagram of the iron electron configuration. The lefthand diagram shows the active filling of the 2p orbitals with one electron at a time. After all three orbitals receive an electron, they will then pair up before filling the higher energy 3s orbital. Electron filling continues until all 26 electrons are placed. Note that in the case of iron, that the final electrons are placed in the 3d subshell orbitals following Hund’s Rule.