Answer 1: Calcium has 20 electrons so the full electronic configuration is: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2. The 4s orbital is lower in energy than the 3d subshell and is therefore filled first. The shorthand version is [Ar] 4s 2 since argon is the nearest preceding noble gas to calcium which accounts for 18 electrons.
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