So elements in period 1 (hydrogen and helium) need 2 electrons to fill their valence shell. So you're 2,8,8,2 rule looks like it shows the maximum amount of valence electrons elements can have in a period up to atomic number 20 (calcium). This means that we have a total of 8 electrons in the 2nd energy level (2 from the s subshell, and 6 from the p subshell).Īs you go further, you will get practice with identifying and conceptualizing electron configuration. In every p subshell there are 3 p orbitals. In the 2nd energy level, we have a p subshell in addition to the s subshell. This specific type of orbital is called the s orbital, and we have 1 s orbital for every s subshell. In the 1st energy level, we have 1 subshell, which basically means we have 1 type of orbital. Each orbital then has 2 electrons, which are said to have different and opposite spins. Finally, within each subshell there are individual orbitals referencing a specific region of space around the atom's nucleus. We can organize these electrons into different subshells based upon the shape of the region they occupy. The pattern that we observe results in 3 classifications as follows (there are other considerations that you will learn about later as well).Ī Shell / Energy Level is a region or set of regions that have the same energy.Īlthough we cannot predict the exact location of the electrons at any time, we can map out the regions of space that they occupy. This makes it easier to understand and predict how atoms will interact to form chemical bonds.As electrons are added to the space around the atom's nucleus they are arranged in a way as to minimize repulsions. The configuration notation provides an easy way for scientists to write and communicate how electrons are arranged around the nucleus of an atom. This give us the (correct) configuration of:įor the Cr 2+ ion we remove one electron from 4s1 and one from the 3d5 leaving us with:įor the Cr 3+ ion we remove a total of three electrons (one from the 4s1 and two from the 3d5) leaving us with Therefore, one of the 4s2 electrons jumps to the 3d5 so that it is half-filled (see video below). ![]() Half-filled and fully filled subshell have got extra stability. Therefore we have (still incorrect) 1s 22s 22p 63s 23p 63d 44s 2Ĭorrect Electron Configuration for Chromium (Cr) Both of the configurations have the correct numbers of electrons in each orbital, it is just a matter of how the electronic configuration notation is written ( here is an explanation why). Note that when writing the electron configuration for an atom like Cr, the 3d is usually written before the 4s. Therefore the expected electron configuration for Chromium will be 1s 22s 22p 63s 23p 44s 23d 9. ![]() After the 4s is full we put the remaining four electrons in the 3d orbital and end with 3d4. We now shift to the 4s orbital where we place the remaining two electrons. Since the 3s if now full we'll move to the 3p where we'll place the next six electrons. We'll put six in the 2p orbital and then put the next two electrons in the 3s. The p orbital can hold up to six electrons. The next six electrons will go in the 2p orbital. Since 1s can only hold two electrons the next 2 electrons for Chromium go in the 2s orbital. In writing the electron configuration for Chromium the first two electrons will go in the 1s orbital. Video: Cr, Cr 2+, and Cr 3+ Electron Configuration Notation
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