If the atomic number of krypton is 36, what is the correct electronic configuration of this noble gas element?

Introduction / Context:
This general chemistry question tests your understanding of electronic configuration and how electrons are arranged in shells (energy levels) for a neutral atom. Krypton is a noble gas with atomic number 36, and exam questions often use such elements to check whether students really know how to distribute electrons across different shells in the correct order. A solid grasp of electronic configuration helps you understand chemical reactivity, valency, and periodic trends such as atomic radius and ionisation energy.

Given Data / Assumptions:

• Element considered: krypton (Kr).
• Atomic number of krypton = 36.
• The configuration is to be written in terms of shells: K, L, M and N.
• We assume a neutral atom, so number of electrons = atomic number = 36.
• Maximum electrons in a shell are given by the formula 2 * n^2, where n is the shell number.

Concept / Approach:
The main concept is that the atomic number of an element gives the number of electrons in a neutral atom. These electrons are arranged in shells in order of increasing principal quantum number, subject to the capacity rule for each shell. For school level shell notation the K shell can hold up to 2 electrons, L up to 8, M up to 18, and N up to 32. For krypton we need to place 36 electrons into these shells, filling each shell to its allowed maximum before moving to the next, while respecting the total of 36 electrons.

Step-by-Step Solution:
Step 1: Total electrons in neutral krypton atom = 36. Step 2: Fill the K shell (n = 1). Maximum capacity is 2 * 1^2 = 2 electrons. So K shell gets 2 electrons. Step 3: Remaining electrons = 36 - 2 = 34. Step 4: Fill the L shell (n = 2). Maximum capacity is 2 * 2^2 = 8 electrons. So L shell gets 8 electrons. Step 5: Remaining electrons = 34 - 8 = 26. Step 6: Fill the M shell (n = 3). Maximum capacity is 2 * 3^2 = 18 electrons. So M shell gets 18 electrons. Step 7: Remaining electrons = 26 - 18 = 8. Step 8: Put the remaining 8 electrons into the N shell (n = 4). So N shell has 8 electrons. Step 9: Collecting these values we get K = 2, L = 8, M = 18, N = 8, written as 2,8,18,8.

Verification / Alternative check:
Add up the electrons in all shells and verify that the total equals the atomic number. Calculation: 2 + 8 + 18 + 8 = 36, which matches the given atomic number of krypton. Also, krypton is a noble gas in period 4 and group 18, so it should have a completely filled outer shell. An outer shell with 8 electrons (octet) is consistent with a stable noble gas configuration.

Why Other Options Are Wrong:
Option B, 2,8,8,18, gives a total of 36 electrons but gives an unrealistic distribution because the N shell is shown as containing 18 electrons while the M shell has only 8, which does not follow the usual filling rule for shells in this notation. Option C, 2,18,8,8, places 18 electrons in the L shell, which exceeds its maximum capacity of 8 in the simple shell model. Option D, 2,8,16,10, has the wrong distribution and also gives a total of 36, but does not match the allowed shell capacities. Option E, 2,10,18,6, violates the usual capacities and also does not correspond to the standard shell filling for krypton.

Common Pitfalls:
A common mistake is to mix up subshell notation (like 3d and 4s) with simple shell notation. At this level, the question only expects the K, L, M, N shell distribution. Another frequent error is to forget the maximum capacity of the L shell and incorrectly put more than 8 electrons there. Students may also add correctly to 36 but ignore the rule that lower shells must be filled before higher ones in this notation scheme.

Final Answer:
The correct electronic configuration of krypton in shell notation is 2,8,18,8, which uses all 36 electrons and respects the maximum capacity of each shell.