Which statement best explains how periods on the periodic table are organized?

• A. Increasing number of neutrons from left to right
• B. Increasing atomic number from left to right
• C. Decreasing number of electrons from left to right
• D. Decreasing atomic mass number from left to right

Answer: (C)

Periods are horizontal rows in the periodic table, and they are arranged so that each step to the right adds one proton to the nucleus. In neutral atoms, that also means one more electron. So as you move left to right across a period, the atomic number increases, and the number of electrons in the atom increases as well, while the number of electron shells stays the same. That consistent increase in atomic number is what lines up the elements in a period.

The idea that periods are organized by neutrons, decreasing electron count, or decreasing mass number doesn’t fit how the table is built. Neutrons vary and don’t define the period, isotopes can affect mass but aren’t the organizing factor, and across a period the number of electrons does not decrease.