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Electron Configurations Electron configurations shows how the electrons are distributed among the various orbitals. To understand the order take a look at how the sublevels are constructed. It would be nice if sublevels filled in a simple order, but that doesn't happen. Order in which sublevels are filled with electrons (increasing energy)
Cleaned up it looks like this 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p If we add the number of electrons that each sublevel holds it looks like this: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6 Placed into the periodic table it looks like this
It is very easy to determine an atom’s number of valence electrons in another way, from the periodic table. Look at the following table to see that we can tell the number of valence electrons an element has by looking at the group (vertical column) it belongs to on the periodic table:
The number of valence electrons an element has will become useful when we talk about chemical bonding. The diagram below really shows the overlap of the sublevels
Auf bau Principle - you must fill the lower energy sublevel before an electron can be placed in a higher one. So fill them in order.
Noble Gas Simplification Now, this is becoming a lot of work writing down these ever-expanding electron configurations! The more electrons we have in the atom, the longer the electron configuration. We have a method we can use to simplify writing these configuration as we become more familiar with them. If we look closely, we notice that the electron configuration for rubidium is the same as the previous element, argon, with a single 5s electron added on. For the electron configuration of argon, let us simply write [Ar]. The simplified electron configuration for rubidium then becomes:
We can do this for any element, BUT, we must use only noble gases in the brackets. I call this the noble gas simplification. In this method of writing electron configurations, the last noble gas before we get to the element of interest is the noble gas we put into the brackets. For instance, for the element aluminum we write
We may NOT use any element in the brackets, only noble gases. This notation for writing electron configurations helps us to highlight 2 different types of electrons in the atom. Those electrons in the brackets are called core electrons. These electrons do not participate in chemical reactions. The electrons written after the noble gas in brackets are called valence electrons. In many cases, “d” electrons will be present after the last noble gas, as in the element manganese : [Ar]4s23d5. We typically do not consider “d” electrons as valence electrons and therefore a more specific definition is needed: valence electrons are those electrons in the highest principal energy level. These electrons are important because they are the ones that are gained, lost or shared in chemical reactions. For the element aluminum, above, we see 2 electrons in the 3s orbital and 1 electron in the 3p orbital, so aluminum has a total of 3 valence electrons. Using the same method, calcium has 2 valence electrons. If we look at the electron configuration for manganese again:
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