Which of the following pairs of elements are in the same period of the periodic table?

• A. C and Si
• B. Na and Mg
• C. Li and Na
• D. O and S

Answer: (B)

Verified step by step guidance

1. Recall that elements in the same period of the periodic table are located in the same horizontal row.
2. Identify the period number for each element in the given pairs by referring to their positions in the periodic table.
3. For the pair C (carbon) and Si (silicon), note that C is in period 2 and Si is in period 3, so they are not in the same period.
4. For the pair Na (sodium) and Mg (magnesium), both elements are in period 3, indicating they are in the same period.
5. For the pairs Li (lithium) and Na (sodium), and O (oxygen) and S (sulfur), check their periods: Li is in period 2, Na in period 3; O in period 2, S in period 3, so neither pair is in the same period.