Question
Write the electronic configuration of chlorine (Z = 17) using the shell method (K, L, M shells).
Solution — Step by Step
Atomic number = 17, which means chlorine has 17 electrons to distribute. We fill shells in order: K → L → M.
The K shell can hold a maximum of 2 electrons. We place 2 electrons here first.
Electrons placed: 2. Remaining: 17 − 2 = 15 electrons left.
The L shell can hold a maximum of 8 electrons. We fill it completely.
Electrons placed: 8. Remaining: 15 − 8 = 7 electrons left.
The M shell can hold up to 18 electrons, but we only have 7 left. Place all 7 here.
Electrons placed: 7. Remaining: 0.
The electronic configuration is 2, 8, 7.
Why This Works
Electrons fill shells from the innermost outward — this is the Bohr-Bury rule (also called the shell-filling rule in NCERT Class 9). Each shell has a maximum capacity: K = 2, L = 8, M = 18. We always fill each shell to its maximum before moving to the next.
The key number here is the 7 in the outermost shell (M). This tells us chlorine has 7 valence electrons. Since a full outer shell means 8 electrons, chlorine is just 1 electron short of stability — which is exactly why it’s so reactive. It desperately wants to gain 1 electron.
This also explains why chlorine forms Cl⁻ ions in reactions (gains 1 electron to become stable like argon) and why it has a valency of 1.
Alternative Method — Using Valence Shell Directly
If you know chlorine is in Group 17 (halogen family) of the periodic table, you immediately know it has 7 valence electrons. Group number directly gives valence electrons for Groups 1–18 in the main blocks.
From there, work backwards: outermost shell = 7, previous shell = 8 (L shell fills completely), innermost = 2 (K shell fills completely). This top-down approach is faster during exams when you just need valence electrons.
For any element up to Z=18, the shortcut is: subtract 2 (K shell), then subtract up to 8 (L shell), and whatever remains goes into M. This works 100% of the time for Class 9 NCERT problems.
Common Mistake
Putting more than 8 in the L shell. Many students know the M shell can hold 18, so they try to “save space” and put 9 or 10 in L. That’s wrong for shell-method problems. In the NCERT shell model, L shell maximum is 8 electrons — always. The 18-electron capacity of M is relevant only in advanced configurations (Class 11, orbital model). For Z = 1 to 18, L shell never exceeds 8.
The configuration 2, 9, 6 is a classic wrong answer you’ll see — 9 in L is impossible. If your L shell has more than 8, restart from that step.