What is a period on the periodic table?

• A. A vertical column of elements.
• B. A block of elements with similar properties.
• C. A sequence of chemical families.
• D. A horizontal row of elements.

Answer: (D)

A period is a horizontal row of elements on the periodic table. It represents the filling of a single electron shell as you move left to right across the table. Within a period, the outermost shell stays the same, so you’re adding protons and electrons to that same energy level, which is why the atomic number increases across the row and the elements’ properties change in a related way. In contrast, vertical columns are groups or families with similar behavior because they have the same number of valence electrons. The term block refers to the type of orbital being filled (s, p, d, or f), not a description of a single row of elements.