Which part of the periodic table indicates the number of energy levels (principal energy shells) in the electron cloud of an atom?

• A. The atomic number of the element
• B. The element's symbol
• C. The group (vertical column) in which the element is located
• D. The period (horizontal row) in which the element is located

Answer: (D)

Verified step by step guidance

1. Understand that the periodic table is arranged in rows called periods and columns called groups.
2. Recall that each period (horizontal row) corresponds to the principal energy level (or shell) that the valence electrons of the elements in that row occupy.
3. Recognize that the period number tells you how many energy levels are filled in the electron cloud of an atom of that element.
4. Note that the atomic number indicates the total number of protons (and electrons in a neutral atom), but does not directly indicate the number of energy levels.
5. Conclude that the period (horizontal row) in which an element is located indicates the number of principal energy shells in its electron cloud.