In this tutorial, you will learn how to find the electron shell configurations for various elements using the periodic table. If you enjoy this tutorial, be sure to check out our others linked below!
What are electron shell configurations?
Electron shell configurations tell us how electrons are distributed in an atom. Electrons exist in different shells as integers that increase in both energy level and numbers you move away from the positive nucleus. Furthermore, there are subshells. The four subshells you will work with the most in chemistry are the s,p,d, and f subshells. Each of these subshells can hold a set amount of electrons. Lastly, each sub-shell can be grouped into orbitals, which tell us where electrons are likely to be occupied. The outermost orbital shell contains the valence electrons which dictate chemical behavior and stability.
|Subshells||Number of Orbitals||Number of Electrons|
How to Write Electron Configurations
The electron configuration for a given element or atom can be determined using the periodic table. The periodic table consists of groups and periods. The periods, which are the horizontal rows that run left to right, tell us the principal energy level of an element. The groups, which are the vertical columns, tell us how filled each subshell is. Notice how in this image, each group in each block has a small designation in the top right corner. This is how full the electron subshell is. There are three guiding principles for electron configurations.
- Aufbau’s Principle
- The Aufbau principle states that the lower-energy electron orbitals will fill up first. This means the 4s shell will be filled before the 3d shell due to it having less energy.
- Hund’s Rule
- Hund’s rule states that if there is more than one orbital of equal energy available to fill, then it would be more stable to have one electron occupy each orbital individually rather than in pairs.
- Pauli Exclusion Principle
- The Pauli exclusion principle states that no more than two electrons can occupy an orbital and that each electron must spin in opposite directions.
Writing the Configurations
Let’s start with finding the electron configuration for carbon (C). First, we need to know how many electrons carbon has. Since carbon has the atomic number six, it will also have six protons and six electrons. Furthermore, carbon is located in the second period and the second group in block p. When finding electron configurations, we will work left to right. The corresponding electron shell configuration is, therefore: 1s22s22p2. The superscripts represent the number of electrons present and can be added to find the total number of electrons in an atom. To help visualize, this is how each subshell looks with the six electrons.
Now, let’s find phosphorus on the periodic table. It is located in the third period and within the p-block. Moreover, it has fifteen electrons. The electron shell configuration for phosphorous would therefore be 1s22s22p63s23p3.
Bromine has thirty-five total electrons. You can still write out every single subshell if you would like, but to save time it is good to know the shorthand method of electron configurations. The shorthand method uses the group 18 elements, the noble gases, as a bookmark. For example, bromine is located in period four of the p-block. Remember, for electron configurations you work left to right and down the periods until you get to the element you’re focusing on. The last noble gas that was passed for bromine was argon (Ar). Using the short-hand method, we will place argon in brackets like this [Ar] and then continue the electron configuration after argon. It would look like this: [Ar] 4s23d104p5. Though electron shell configurations can be confusing at first, it helps to practice.