Chemistry

Question Analysis
This question evaluates the student's understanding of shorthand notation and the application of the noble gas core.

Key Concept Explanation
Shorthand notation uses the symbol of a noble gas (in square brackets) to represent the filled inner electron configuration, with only the valence shell configuration written.

Step-by-step Solution
1. Identify the atomic number of potassium, which is 19.
2. Write the full electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
3. Identify the noble gas before potassium, which is argon (Ar, atomic number 18): [Ar].
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