The correct answer is A: krypton and neon.
Krypton (Kr) and neon (Ne) are both in the same period of the periodic table, with krypton being in Period 4 and neon in Period 2. However, if we consider their electron shells, both elements have the same characteristic of being in the same "block" of the periodic table concerning their respective periods.
However, more accurately, B: lithium and francium would be correct as they both have electrons in the same number of energy levels or shells, which is 2 in their outermost shell, as they both are in Group 1.
If we are strictly looking at the number of energy levels or shells, electrons are organized into different energy levels based on their principal quantum number (n), and elements in the same group have the same outer electron configuration.
Thus, to clarify further:
- Krypton and Neon belong to different periods.
- Lithium (Li) and Francium (Fr) both have one valence electron in the outermost shell, which reflects their electron configuration depending on the period.
Confirming the correct option strictly, B: lithium and francium would be the best choice indicating they share the same outer electron energy level, while also considering that both are alkali metals in Group 1.
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