The group to which elements belong in the periodic table is determined by the number?

Step 1: Analyze the question The question asks what determines the group (vertical column) an element belongs to in the periodic table.

Step 2: Explain the concept Elements in the same group have the same number of valence electrons, which are the electrons in the outermost shell. These electrons control how an element reacts, so elements in a group behave in similar ways.

• Group 1 elements (Li, Na, K) all have 1 valence electron.
• Group 2 elements (Mg, Ca) all have 2 valence electrons.
• Group 17 elements (F, Cl, Br) all have 7 valence electrons.

By contrast, the period (horizontal row) is determined by the number of electron shells.

Step 3: Check each option

• A. electrons: The total number of electrons equals the atomic number. It tells you the element's identity, not its group.
• B. core electrons: These are the inner electrons. They differ widely within a group, so they don't decide the group.
• C. valence shells: The number of occupied shells decides the period, not the group.
• D. valence electrons: This is correct. The number of outer-shell electrons decides the group.

Step 4: Example Sodium has the electron configuration 2, 8, 1.

• It has 3 shells, so it is in Period 3.
• It has 1 valence electron, so it is in Group 1.

Answer: D. valence electrons.

Exam tip: Remember "Group = Give the outer electrons" and "Period = Places (shells) occupied." Valence electrons give the group, and the number of shells gives the period.