Elements in the same period in the periodic table have the same?

Step 1: Analyze the question We're asked what elements in the same period (a horizontal row) of the periodic table have in common.

Step 2: Explain the concept

• A period is a horizontal row. A group is a vertical column.
• Moving across a period from left to right, the atomic number increases by one each time, and electrons are added to the same outermost shell (energy level).
• The period number equals the number of occupied electron shells.

Step 3: Check each option

• A. Number of shells: Correct. Sodium (2,8,1) and chlorine (2,8,7) are both in Period 3, and both have 3 shells.
• B. Atomic number: Wrong. Atomic number increases across a period, so each element has a different one.
• C. Chemical properties: Wrong. Chemical properties depend on the number of valence electrons, which changes across a period. Elements in the same group share similar chemical properties.
• D. Physical properties: Wrong. These vary across a period, for example from metals (Na) to non-metals (Cl) to gases (Ar).

Step 4: Answer Elements in the same period have the same number of (electron) shells.

Exam tip: Remember "Period = Plates (shells) across a row". Period shows the number of shells, while group shows the number of valence electrons and similar chemical properties.