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Answer: First = 2, second = 8, third = 8, fourth = 2

Why:

• The theoretical shell capacities are \(2n^2\): 2, 8, 18, 32.
• But for elements with 1–20 electrons, the shells fill as 2, 8, 8, 2.
• Example: calcium has 20 electrons: 2, 8, 8, 2. Argon has 18: 2, 8, 8.
• The third shell can hold 18 in theory, but in the first 20 elements it never has more than 8 electrons before the fourth shell starts filling.

Check: The other options are wrong because the second shell can hold 8 electrons, not 2, and the third shell is filled to 8 before the fourth shell gets 8 electrons in this range.