Electron configuration
1s22s22p4
As atomic number increases these get rather lengthy. There is further shorthand to write electron configurations using the last noble gas configuration;
Helium has electron configuration 1s2, oxygen can be written [He]2s22p4
This shorthand is much more effective with higher atomic numbers!
1. Noble gases configurations, outer electron shells are filled with no “extra” electrons. This is why the noble gases will not react with any other elements. They don’t want to share, and don’t want to “steal” any more electrons.
2. Here is a way to help remember the order in which the orbitals are filled:
4f 5f
3d 4d 5d 6d
2p 3p 4p 5p 6p 7p
1s 2s 3s 4s 5s 6s 7s
This Pyramid will help you remember the order in which electron orbitals are filled. Build the pyramid starting at the bottom with “1s” then 2s etc up to 7s. Then start the p blocks at 2p go up to 7p, then d block starts at 3d, and f starts at 4f. The d block only goes up to 6d, and the f block to 5f.
Now these orbitals are filled along the solid diagonal arrows. When you come to the end of the solid arrow, follow the dashed arrow to the top of the next solid arrow, etc.
1s22s22p63s23p64s23d64p65s24d105p66s24f145d106p67s25f146d107p6
Remember…
The s block holds 2 electrons (there are two groups in the s block)
The p block holds 6 electrons (there are 6 groups in the p block)
The d block holds 10 electrons (there are 10 groups in the d block)
The f block holds 14 electrons (there are 14 groups in the f block)
The sum of the exponents in the electron configuration is the number of electrons in the configuration. Fill out the orbitals in the order using the diagonals of the pyramid until you get to the atomic number of the element you are doing the configuration for.
Another much easier way to remember the order that orbitals are filled is to simply follow along the PT starting left to right top to bottom! The d-block starts at row 3, even though it’s on row four of the table. The f-block starts with 4, even though it is on row 6 of the table.
“ d starts with 3, f starts with 4”
Just follow along left to right top to bottom until you reach the element you have. In this way the outer electron configuration becomes a “grid coordinate” of the element.
Example Iron is in the top row of the d-block 6 columns over, so its outer configuration is:
3d6
“d” for d-block, “3” for top row (d starts with 3), “6” for sixth column of the d-block.
(there are exceptions to the rule of orbital filling that are based on quantum mechanics beyond the scope of this class, you may notice them on your PT, but you will be responsible to know them)