Which statement about a period in the periodic table is correct?

• A. A vertical column
• B. The number of protons
• C. A horizontal row
• D. The atomic mass increases left to right

Answer: (C)

Periods are horizontal rows on the periodic table. They run left to right, and elements in the same period share the same number of electron shells. As you move across a period, the atomic number (and thus the number of protons) increases, but the defining feature is that the elements form a row, not a column. A vertical column is a group, which groups elements by similar properties. Atomic mass isn’t the defining trait of a period because masses vary with isotopes, even though masses tend to increase across a period. So the correct description is that a period is a horizontal row.