Electronic Configuration Explanation for Krypton (Z=36)

Let's find the electronic configuration of Krypton (Kr), which has an atomic number (Z) of 36. The atomic number tells us the number of protons in the nucleus, and for a neutral atom, it's also equal to the number of electrons. So, a neutral Krypton atom has 36 electrons.

Electrons occupy different energy levels or shells around the nucleus. These shells are denoted by letters K, L, M, N, and so on, starting from the shell closest to the nucleus (K shell).

Each shell has a maximum capacity for electrons. The maximum number of electrons a shell 'n' can hold is given by the formula \(2n^2\), where 'n' is the shell number (n=1 for K, n=2 for L, n=3 for M, etc.).

• K shell (n=1): Capacity = \(2 \times 1^2 = 2\) electrons.
• L shell (n=2): Capacity = \(2 \times 2^2 = 8\) electrons.
• M shell (n=3): Capacity = \(2 \times 3^2 = 18\) electrons.
• N shell (n=4): Capacity = \(2 \times 4^2 = 32\) electrons.

Electrons fill these shells in order of increasing energy. For the purpose of writing simple shell configurations, we generally fill the shells sequentially up to their maximum capacity, or until all electrons are accommodated, keeping in mind that the outermost shell typically doesn't hold more than 8 electrons for stable configurations like noble gases (except for the first period). Let's distribute the 36 electrons for Krypton:

1. K shell (n=1): This shell fills first and can hold a maximum of 2 electrons.
   Remaining electrons: 36 - 2 = 34.
2. L shell (n=2): This shell fills next and can hold a maximum of 8 electrons.
   Remaining electrons: 34 - 8 = 26.
3. M shell (n=3): This shell can hold up to 18 electrons. Since we have 26 electrons remaining, we fill the M shell completely with 18 electrons before starting the N shell.
   Remaining electrons: 26 - 18 = 8.
4. N shell (n=4): The remaining 8 electrons go into the N shell. This shell can hold many more, but only 8 are left.
   Remaining electrons: 8 - 8 = 0.

So, the distribution of electrons in the K, L, M, and N shells is 2, 8, 18, and 8, respectively.

This gives the electronic configuration of Krypton (Z=36) as 2, 8, 18, 8.

Analyzing the Options for Krypton Electronic Configuration

• Option 1: 2, 18, 16 (Total: 2 + 18 + 16 = 36). This distribution is incorrect because the L shell (second shell) should fill before the M shell (third shell) is completely filled, and the L shell's capacity is 8, not 18 in this position.
• Option 2: 2, 8, 18, 8 (Total: 2 + 8 + 18 + 8 = 36). This configuration correctly fills the K, L, and M shells to their capacities (in order for the first few shells) and places the remaining electrons in the outermost shell, which holds 8 electrons, characteristic of a noble gas like Krypton.
• Option 3: 2, 18, 8, 8 (Total: 2 + 18 + 8 + 8 = 36). This is incorrect. The second shell (L) holds 8 electrons, not 18. The third shell (M) can hold up to 18 and should be filled before the fourth shell (N) gets 8 electrons in this manner for Krypton.
• Option 4: 2, 8, 20, 6 (Total: 2 + 8 + 20 + 6 = 36). This is incorrect. The M shell (third shell) effectively holds a maximum of 18 electrons in this type of representation before the N shell is significantly populated, especially for noble gases.

Therefore, the correct electronic configuration for Krypton (Z=36) is 2, 8, 18, 8.

Revision Table: Key Concepts in Electronic Configuration

Additional Information on Electron Shells and Subshells

While the shell configuration (2, 8, 18, 8) is a simplified representation, the actual filling of electrons follows the Aufbau principle, Hund's rule, and the Pauli exclusion principle, which involve subshells (s, p, d, f) within each shell.

• K shell (n=1) has 1s subshell (capacity 2). Configuration: \(1s^2\).
• L shell (n=2) has 2s (capacity 2) and 2p (capacity 6) subshells. Configuration: \(2s^2 2p^6\). Total = 8.
• M shell (n=3) has 3s (capacity 2), 3p (capacity 6), and 3d (capacity 10) subshells. Configuration: \(3s^2 3p^6 3d^{10}\). Total = 18.
• N shell (n=4) starts with 4s (capacity 2), then comes 3d (part of M shell but filled after 4s), then 4p (capacity 6). For Krypton (Z=36), the electron filling order is:
  • \(1s^2\) (2 electrons in K)
  • \(2s^2 2p^6\) (8 electrons in L)
  • \(3s^2 3p^6\) (8 electrons in M so far)
  • \(4s^2\) (2 electrons in N)
  • \(3d^{10}\) (10 electrons in M, completing it to 8+10=18)
  • \(4p^6\) (Remaining 6 electrons in N, totalling 2+6=8 in N)
  The full subshell configuration for Krypton is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6\).

Summing the electrons in each principal shell from this subshell configuration gives the shell configuration:

• n=1 (K): \(1s^2\) → 2 electrons
• n=2 (L): \(2s^2 2p^6\) → 2 + 6 = 8 electrons
• n=3 (M): \(3s^2 3p^6 3d^{10}\) → 2 + 6 + 10 = 18 electrons
• n=4 (N): \(4s^2 4p^6\) → 2 + 6 = 8 electrons

This confirms the shell configuration 2, 8, 18, 8 for Krypton, which is a stable noble gas configuration.