What are the rules for electron configuration?
There are two basic rules that need to be followed while distributing electrons of an atom in shells:
Rule 1: The maximum number of electrons in the outermost orbit can be 8
Let us take the example of calcium atom to understand this rule.
The number of electrons in a shell is given by the formula, 2𝑛2and a calcium atom has 20 protons, 20 neutrons and 20 electrons.
Applying the formula, the electrons will be distributed in the first two shells as follows:
- K shell has 2 electrons
- L shell has 8 electrons.
This leaves 10 electrons. Since M can take upto 18 electrons, the remaining 10 electrons would be in this shell. This means there will be no fourth orbit, making M with 10 electrons, the outermost shell.
But this contradicts the rule. So, we allot 8 electrons to the third shell and the remaining 2 to the fourth shell.
The electron configuration of calcium is therefore: 2, 8, 8, 2
Electron configuration for calcium
Rule 2: The inner shells need to be filled first before proceeding
That is, filling up of electrons occurs in a stepwise manner. If we take the example of the calcium atom again, it means first the K shell will be filled with 2 electrons, followed by 8 electrons in the L shell. Then the M shell will be filled and lastly the N shell.