What term refers to the horizontal rows in the periodic table?

• A. Group
• B. Corrosion
• C. Reactive substance
• D. Periods

Answer: (D)

Horizontal rows on the periodic table are called periods. Each period reflects the filling of the same electron shell as you move across the table from left to right, with the atomic number increasing along the row. As you go across a period, the elements' properties change gradually due to increasing nuclear charge while sharing the same shell structure. In contrast, vertical columns are groups, where elements have similar valence electron configurations and similar chemical properties. Corrosion describes a chemical process of metal deterioration, not a way the table is organized. A “reactive substance” is a general term for how substances behave chemically and isn’t a label for rows.