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Complete Guide On How To Determine Electron Configuration And Noble Gas Core Electron Configuration

Electron Configuration

Step 1: Divide Periodic Table into s, p, d and f blocks as shown. Move helium next to hydrogen.

Step 2: Number the rows on the Periodic Table on the left and right sides. Number the d block 1 less then S block as shown. Number f block as well.

Step 3: Locate the element on the periodic table. Travel starting from the top left to the right through each row until you reach the element needed. As you travel through the table, write out the electron configuration. For example, as you travel though the first row, you will get 1s^2. 1 is the number of the row, s is the name of the block and the superscript 2 is the number of squares in the block you are getting through.

Let's try some examples:

Oxygen electron configuration

As I go through the first row, I get 1s2. Then, I have to start at the second row and I get 2s2, followed by 2p4 (since oxygen is in the fourth square on the p block row 2).


Exceptions: must be memorized

Noble Gas Abbreviated Electron Configuration

We can also write out the electron configuration as a noble gas configuration. All we need to do is find the noble gas (Group 18 on the very right of the Periodic Table) that is before our element. We put that noble gas into brackets and follow by writing the rest of the electron configuration. For oxygen, we would have [He] 2s^2 2p^4

Electron Configuration Of Ions

Ions are atoms that gained or lost electrons. If an atom loses electrons, it becomes positive and is called a cation. If an atom gains electrons, it become negative and is called an anion.


If you have a negatively charged ion, then you have to add electrons. For example -2 means two more electrons. On the periodic table shift to the right, number of boxes equal to the number of electrons added.


A positively charged ion is an atom that lost electrons. Write the electron configuration for the neutral atom and then take off the number of electrons equal to the charge from the highest energy orbital. Please note: For transition metal ions: Take off from the S before D.



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