If you need to see how electron shells are filled, click here.
Orbitals are filled with electrons as shown in this graphic:
So, for example, if we wanted to know the electron configuration for sodium (atomic number 11), we start at the top left and follow that arrow to 1s2 (we can only add two electrons to an “s” orbital).(See orbital list in the lower right of the graphic).
Following the next arrow, we fill another “s” orbital 2s2. Following the third arrow down we add 6 electrons to a “p” orbital (2p6). We only have one more electron to fill and we place that in an “s” orbital 3s1.
So, the electron configuration for sodium is:
Note that the calculator also shows an abbreviated way of expressing electron configuration.
Basically, for sodium, it shows the atomic symbol (in brackets) of the inert gas (neon) in the previous periodic table row plus the electrons needed for the current row. [Ne] 1s1
Let’s calculate the abbreviated electron configuration for calcium (atomic number 20).
Calcium is in row 4 of the periodic table. The inert gas in row three is Argon, atomic number 18.
Now we have 2 more electrons to place into orbitals. Looking at the graphic, we see the next orbital to be filled is 4s and so the abbreviated electron configuration for calcium is [Ar] 4s2.
There are 21 elements that do not follow the rule shown in that graphic, and those elements are listed below.
When you input the atomic number of one of these elements, you will get the incorrect configuration displayed in the first row and the correct configuration will be shown in row 3. We just thought you’d like to see the elctron configuration if that element followed the rule.
| Element | Atomic Number |
|---|---|
| Chromium | 24 |
| Nickel | 28 |
| Copper | 29 |
| Niobium | 41 |
| Molybdenum | 42 |
| Ruthenium | 44 |
| Rhodium | 45 |
| Palladium | 46 |
| Silver | 47 |
| Lanthanum | 57 |
| Cerium | 58 |
| Gadolinium | 64 |
| Platinum | 78 |
| Gold | 79 |
| Actinium | 89 |
| Thorium | 90 |
| Protactinium | 91 |
| Uranium | 92 |
| Neptunium | 93 |
| Curium | 96 |
| Lawrencium | 103 |