In the periodic table, periods are horizontal rows. What characteristic do these rows share?

• A. They have the same number of electron rings.
• B. They contain the same number of protons.
• C. They have identical atomic masses.
• D. They show the same chemical properties.

Answer: (A)

In a period, atoms share the same number of electron shells. That means they have the same number of electron rings. As you move across a period, electrons are added to the same outer shell, and the nuclear charge increases, but no new shell is formed until the next period. So the defining feature of a period is the constant number of electron rings.

The other ideas don’t fit because the number of protons increases across a period, not stays the same; atomic masses aren’t identical and generally rise across the row; and chemical properties don’t stay the same throughout a period—they change as you move from left to right.