# Electron Shells --- **Electron shells**, also called atomic **orbitals** are the where different [[atom#Electrons Electron Shells|electrons]] live in the structure of the atom. This is actually a massive over-simplification of how electrons behave, and falls apart for large atoms...but truely understanding their behavior basically takes a quantum [[physics]] degree, and this is good enough. ## Electron Configuration The **electron configuration** of an atom is a representation of where the electrons of that atom are hanging out. Generally speaking, there are two rules: electrons go where it's the least expensive to join (either because it's closer to the nucleus or has an eaiser shell type to join), and electrons would rather be alone if they can be, but will pair up before jumping to a higher shell energy level. ### Shells & Subshells The **shell** number represents how close that shell is to the nucleus of the atom. It starts at 1 and goes up from there—sometimes they are refered to with letters instead, in which case it goes from the letter L upwards. Each shell is comprised one or more **subshells**, which are labeled in order *s*, *p*, *d*, *f*, *g*. Each subshell has a more complex shape and can hold more electrons than the one prior. The subshell names are written out with the shell number first then the subshell letter. For example, 1s, or 5d. subshell | max<br>electrons :-------:|:-------------: s | 2 p | 6 d | 10 f | 14 g | 18 shell | subshells | max<br>electrons :----:|:---------:|:-------------: 1 (L) |s | 2 2 (M) |s + p | 8 3 (N) |s + p + d | 18 4 (O) |s + p + d + f| 32 5 (P) |s + p + d + f + g|50 ### Valance Shell & The Octect Rule The **valance shell** is the outermost electron shell that an atom has. All atoms seek to fill it's valence shell, and how close to empty or full the shell is determines an atoms reactivity. [[element#Noble Gas|Noble gases]] get their name because they are so nonreactive, which is because their valence shells are naturally (at neutral charge) full. The **octect rule** is a chemical rule of thumb that states atoms are happiest if they have 8 (usually) electrons in their valance shell. There are some exceptions, such as in the lighter elements, like hydrogen and helium. ### Writing Out the Electron Configuration: The Aufbau Principle Unfortunately for us, writing out the electron configuration doesn't exactly go in in an intuitive order. But! There is a logic to it once you know what to look for. Essentially if you write down all the subshells in order and then connect them in a zig-zag pattern as shown below: ![[orbital energy order.png]] The next electron up will fill the subshells in this order. Written out it looks like this: <center><em>1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p</em></center> Finally, the number of electrons in the orbitals are written as a suberscript. Recall the max number of electrons per orbital. $\large1s^22s^22p^63s^23p^64s^23d^{10}4p^65s^24d^{10}5p^66s^24f^{14}5d^{10}6p^67s^25f^{14}6d^{10}7p^6$ Another way of determining the order of the blocks is to follow this periodic table in order: ![[periodic table electron configuration.png]] ___