Which pair of elements has the same number of energy levels (electron shells)?

• A. Na and Cl
• B. K and Br
• C. Li and Ne
• D. Mg and S

Answer: (A)

Verified step by step guidance

1. Recall that the number of energy levels (electron shells) in an atom corresponds to the period (row) of the element in the periodic table.
2. Identify the period number for each element in the given pairs. Elements in the same period have the same number of energy levels.
3. For example, Sodium (Na) is in period 3, and Chlorine (Cl) is also in period 3, so they have the same number of energy levels.
4. Check the other pairs similarly: Potassium (K) is period 4, Bromine (Br) is period 4; Lithium (Li) is period 2, Neon (Ne) is period 2; Magnesium (Mg) is period 3, Sulfur (S) is period 3.
5. Compare all pairs and confirm which pairs share the same period number, thus the same number of energy levels.