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When you learn to write electron configurations in Class 9, you use a set of guidelines that predict where electrons go. These guidelines explain the stability of noble gases, how valency works, and why the periodic table is shaped the way it is.
What are the Bohr-Bury rules?
The scheme is named after Niels Bohr, who proposed the planetary model of the atom, and the English chemist Charles Rugeley Bury, who refined it. The distribution of electrons into different orbits follows three main rules:
The 2n² capacity rule: The maximum number of electrons a shell can accommodate is given by the formula 2n², where n is the shell number starting from the nucleus.
The octet rule for the outer shell: Bury suggested in 1921 that the maximum number of electrons in the outer shell of any atom is 8, no matter what its theoretical maximum might be.
Stepwise filling: Electrons do not enter a new shell until the inner shells are filled in order.
Using the 2n² formula, we can find the maximum capacity of the first four shells:
K shell (n = 1): 2 × 1² = 2 electrons.
L shell (n = 2): 2 × 2² = 8 electrons.
M shell (n = 3): 2 × 3² = 18 electrons.
N shell (n = 4): 2 × 4² = 32 electrons.
For the first 18 elements—from hydrogen (Z=1) to argon (Z=18)—these rules are straightforward. You fill the K shell to 2, then the L shell to 8, and finally the M shell to 8. Argon's configuration is 2, 8, 8.
Why do potassium and calcium have four shells?
The rules become more interesting when we reach atomic number 19. According to the 2n² rule, the third shell (M) can hold up to 18 electrons. You might expect potassium (Z=19) to have the configuration 2, 8, 9.
However, the second rule states that the outermost shell cannot contain more than 8 electrons. If potassium had a configuration of 2, 8, 9, its outermost shell would violate this limit. Instead, the M shell stops at 8 for the moment, and the 19th electron goes into a new N shell. Potassium and calcium begin filling the fourth shell before the third shell has reached its capacity of 18 electrons.
The correct configurations are:
Potassium (Z=19): 2, 8, 8, 1
Calcium (Z=20): 2, 8, 8, 2
This step-wise shifting is why calcium is placed in Group 2 rather than with the transition metals, and it determines the ions it forms. For more on what happens when these atoms lose electrons, see Electronic configuration of ions like Fe³⁺ and Cu⁺.
How do you apply the Bohr-Bury scheme to problems?
When solving textbook problems, use this checklist to write the configuration of any of the first 20 elements:
Find the atomic number (Z): This tells you the total number of electrons in a neutral atom.
Fill the K shell: Put up to 2 electrons in the first shell.
Fill the L shell: If you have electrons left, put up to 8 in the second shell.
Check the remaining electrons: If you have more than 8 left after filling the L shell, put exactly 8 into the M shell (for atomic numbers 19 and 20).
Start the N shell: Put the remaining 1 or 2 electrons into the fourth shell.
You can verify your answers using the Fermi Problem Library. If you paste a question about valency or atomic structure into the AI tutor, it acts as a step-by-step solver that explains the reasoning rather than printing an answer.
Understanding the Bohr-Bury scheme is the first step to mastering chemical bonding. Once you know how many electrons are in the outermost shell, you can predict whether an atom will lose, gain, or share electrons when forming compounds. (If you are moving on to organic chemistry, this is exactly how carbon's valency of 4 is determined—see Naming carbon compounds in Class 10 for more.)
