Which atom has the electron configuration [Kr] 4d¹⁰ 5s² 5p⁶ in its ground state?
If in an electron configuration, you see an element symbol surrounded by brackets, that indicates that it’s a summary of the electron configuration of that element. Here, the symbol for krypton, Kr, is enclosed in brackets. If we go to the periodic table, we can find out the electron configuration for krypton. Krypton is a noble gas. It’s found in group 18, otherwise known as group eight. It could be found on the fourth row, otherwise known as period four.
There’s a neat trick for figuring out the electron configuration of an element based on the periodic table. The first two elements on the periodic table are hydrogen and helium. The first two electrons we add to helium are the 1s subshell. So we know that further down the periodic table, krypton will have at least this as its core. Next, over in the s block, we have lithium and beryllium, representing the 2s subshell. So we know that krypton will have at least 2s² in its configuration. We can do the same thing with the p block in period two, where by the end, we have a full 2p subshell, 2p⁶, and so on with 3s², 3p⁶, 4s², 3d¹⁰, and finally 4p⁶. This gives us the electron configuration of krypton in energy order 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶. Given how long this is, it’s easy to understand why the shorthand of krypton in brackets is used.
Now that we understand what this shorthand means, we can move on to analyzing our given electron configuration. Now, it may be slightly confusing that our electron configuration is given in number order, not in energy order. We can dispel this confusion by remembering the AUFBAU principle, which tells us the order in which subshells are filled. Fundamentally, the AUFBAU principle is that the lowest energy orbital is filled first. The lowest energy orbital is 1s followed by 2s, followed by 2p and then 3s, followed by 3p and then 4s, and so on 3d 4p 5s 4d 5p 6s, and so on. Sometimes we write things in number order because we aren’t worried about the energies. In this case, we’re not going to worry too much about it.
After filling all the energies that krypton has, we get the 5s subshell. In the given electron configuration, the 5s subshell is completely full. So we can move on. Next, in energy order is the 4d subshell, which has a total occupancy of 10. Again, this subshell is full in our electron configuration. So we can move on. The last electrons we have to account for are the 5p electrons, of which there are six in this electron configuration. One, two, three, four, five, six brings us to an element in group 18.
Remember, the question tells us that we’re looking for an atom, which has to be neutral. Therefore, this electron configuration is the natural configuration of an element. As we’ve been told, it’s an atom in its ground state. That’s where all the electrons have as low an energy as they possibly can.
All we need to do now to get the answer is have a look on our periodic tables and take one step down from krypton. The atom, which has an electron configuration of krypton 4d¹⁰ 5s² 5p⁶ in its ground state, must be an atom of xenon, with symbol Xe.