Chemistry

Question Analysis
This question tests the student's ability to use shorthand notation for electron configurations, specifically recognizing 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, followed by the valence shell configuration.

Step-by-step Solution
1. Identify the atomic number of calcium: 20.
2. Write the full electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
3. Use the noble gas argon (Ar, atomic number 18) to re...