The order of energy level in $ _{19}\text{K} $ is: · Atomic Structure MDCAT Past Paper MCQ

Chemistry Atomic Structure NUMS 2019

PMDC Verified Question 75 of 95

The order of energy level in $ _{19}\text{K} $ is:

$ 4s, 4p $

$ 4s, 3d $

$ 3s, 3d $

$ 3p, 4s $

Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:

Option D: $ 3p, 4s $

Concept: The filling of electrons into orbitals follows the Aufbau principle and the $ (n+l) $ rule.

Formula: $$ \text{Energy} \propto (n+l) $$

Solution:

Potassium (K) has $ Z = 19 $.
The electron configuration is: $ 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^1 $.
After filling the $ 3p $ subshell, the next available subshells are $ 4s $ and $ 3d $.
Calculate $ n+l $: For $ 4s $, $ 4+0=4 $. For $ 3d $, $ 3+2=5 $.
Since $ 4s $ has lower energy than $ 3d $, the electron enters $ 4s $. Thus, the progressive filling order at the valence boundary is $ 3p \rightarrow 4s $.

Why other options are incorrect: They do not represent the consecutive energy levels being filled for the 19th electron of Potassium.

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