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So yesterday(10/5/15) in lecture Professor Lavelle went over the conceptual basics behind atomic orbitals and electron configurations. I was a little confused after lecture because I hadn't covered that material in a while. As I think about it more and more I am beginning to grasp the concepts discussed in lecture. However, I looked maybe one or two pages ahead in the course reader and saw some electron configurations that read as something like 3d14s2. My question is why does the configuration start all the way at the 3d orbital? When writing electron configurations, are we only concerned with the valence electrons since those are the electrons determining how atoms of a specific element will react? Also in the electron configuration 3d14s2 what do the 1 and 2 signify? In terms of the Chapter 1 homework problems, to where should we be able to do to? I just finished problem #27 and am not able to do #33's and above.
You might recall from Dr. Lavelle's lecture that for many electron systems, the energies of the 2s and 2p orbitals split and the energy of 2p is actually a little higher than 2s. The same idea caries over to the 3rd shell, where the energy of the 3s orbital is less than the 3p orbital which is less than the 3d orbital (3d is highest in energy). The energy difference is actually so significant for the 3rd shell, that the 4th shell s orbital is actually LOWER in energy than 3d, so 4s is filled before 3d. 1s22s22p63s23p64s1 is the correct electron configuration for potassium (or [Ar]4s1). You can refer to figure 2.14 in the textbook for a visual representation of the energy splitting levels. However, when the 3d orbital is filled with electrons, semantically we write the 3d orbital before 4s i.e. [Ar]3d14s2. This tells us that there are two electrons occupying the 4s orbital and 1 electron occupying the 3d orbital.
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