What’s the excited state electron configuration of potassium?

Cameron Jacobs | Certified Educator

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An electronic excited state of an atom refers to any electronic state above the ground state, and there is not just one such state but many. Let us start by defining the electronic ground state, show what it is for potassium, and then look at the differences between ground and excited states.

Quantum mechanics describes how electrons in atoms exist in specific states, called orbitals, and how each state has a specific energy level associated with it. The quantum number rules tell us how many electrons can occupy each energy level or sublevel. The sublevels 2s, 3p, and so on include all of the possible electron states that have the exact same energy level and are what we list in electron configurations.

A ground state atom has all of its electrons in the lowest-energy available sublevels. If you look at the sublevels in order of increasing energy, the lowest ones will all be filled, and there will be no gaps, no unfilled spaces with one or more electrons at higher energy levels. The structure of electronic energy levels is reflected in the structure of the Periodic Table. The first row contains only elements in the s block, so it reads 1s. The second row has elements in the s and p blocks, so it reads 2s 2p. The third row also has elements in the s and p blocks, so add 3s 3p. The fourth row includes elements in the d block. If we know the rules, the principal energy level of d block elements is one less than the row number, so these elements correspond to the 3d sublevel. Thus, the fourth row adds 4s 3d 4p.

We now have enough sublevels to accommodate potassium’s 19 electrons in the ground state (assigning them to the lowest level sublevels available). Each s sublevel can hold two and each p sublevel can hold six, and since potassium is in the fourth row in the s block, we know we will end in the 4s. For the ground state electron configuration, then, we get `1s^2 2s^2 2p^6 3s^2 3p^6 4s^1.`

Since there is only one possible ground state electron configuration for a neutral element, any other arrangement of potassium’s 19 electrons has a higher energy and is an excited state. You could promote any of the electrons to the 4p, for example: `1s^2 2s^1 2p^6 3s^2 3p^6 4s^1 3d^0 4p^1`

The presence of an odd number of electrons in a lower sublevel is a tip-off that you are looking at an excited configuration. Count the electrons and see that the total still adds up to 19. Moving one of the lower electrons into the 4s would make another excited state: `1s^2 2s^2 2p^5 3s^2 3p^6 4s^2.`

Again, there are 19 electrons, but notice the unfilled 2p sublevel. Unless there are instructions in your assignment describing a particular excited electronic state, you can probably choose any electron to promote to any higher energy sublevel, as shown in the examples.

I am attaching a good interactive periodic table. If you click around the tabs and options, you can get it to display ground-state electron configurations, orbital diagrams, and much more.