Which elements have two electron shells (i.e., electrons occupying only the $n=1$ and $n=2$ shells in their ground-state electron configurations)?

• A. Elements in Period 3 (Na through Ar)
• B. Elements in Period 1 (H through He)
• C. Elements in Period 2 (Li through Ne)
• D. All noble gases (He, Ne, Ar, Kr, Xe, Rn)

Answer: (C)

Verified step by step guidance

1. Understand that the principal quantum number \( n \) represents the electron shell number, where \( n=1 \) is the first shell, \( n=2 \) is the second shell, and so on.
2. Recall that elements are arranged in periods on the periodic table, where each period corresponds to the filling of a particular electron shell.
3. Identify that elements with electrons only in the first and second shells (\( n=1 \) and \( n=2 \)) must have their electrons filling only these two shells and no higher shells.
4. Recognize that Period 1 elements (H and He) have electrons only in the first shell (\( n=1 \)), so they do not have electrons in the second shell.
5. Note that Period 2 elements (Li through Ne) have electrons filling the first shell and the second shell, but no electrons in the third shell or beyond, matching the condition of having two electron shells occupied.