In the periodic table, what do the rows (periods) indicate?

• A. The chemical reactivity of the elements
• B. The number of electron shells in the atoms of the elements
• C. The phase (solid, liquid, gas) of the elements at room temperature
• D. The number of valence electrons in the atoms of the elements

Answer: (B)

Verified step by step guidance

1. Understand that the periodic table is organized into rows called periods and columns called groups or families.
2. Recognize that each period corresponds to the filling of a new electron shell (energy level) in the atoms of the elements in that row.
3. Recall that as you move from left to right across a period, electrons are added to the same principal energy level, increasing the atomic number by one for each element.
4. Note that the number of electron shells (energy levels) increases as you move down the periods, so elements in the same period have the same number of electron shells.
5. Conclude that the rows (periods) in the periodic table indicate the number of electron shells in the atoms of the elements.