Two elements, X and Y are in the same group on the periodic table because they both have the same

Step 1: Analyze the question The question asks what property elements in the same group (vertical column) of the periodic table share.

Step 2: The concept In the modern periodic table:

• Groups are the vertical columns.
• Periods are the horizontal rows.

Elements in the same group have the same number of electrons in their outermost shell (valence electrons). This is why they have similar chemical properties. For example, Li, Na and K in Group 1 each have 1 valence electron, so all three react vigorously with water and form +1 ions.

Step 3: Check each option

• A. Number of electronic shells: This is the same for elements in the same period, not the same group. Going down a group, the number of shells increases. Wrong.
• B. Number of valence electrons: This is the same for elements in the same group. Correct.
• C. Atomic size: This increases down a group, so it differs. Wrong.
• D. Atomic number: Each element has a unique atomic number. Wrong.

Step 4: Answer The correct option is B. number of valence electrons.

Tip: Remember "Group = Gets the same outer electrons" and "Period = Places (shells) are the same". If a question asks about the same group, think valence electrons. If it asks about the same period, think number of shells.