To which group and period respectively does an element with 15 electrons belong?

Analyze the question: We need the group and period of an element with 15 electrons (atomic number 15, which is phosphorus).

Concept:

• The period equals the number of occupied electron shells.
• The group (for main-group elements) is given by the number of valence electrons. In the older Nigerian syllabus, Group = number of outer-shell electrons, so 5 valence electrons means Group 5 (Group 15 in the modern IUPAC numbering).

Solution:

Electron configuration of 15 electrons: 1s² 2s² 2p⁶ 3s² 3p³

Shell arrangement: 2, 8, 5

• Number of shells = 3, so Period 3
• Valence electrons = 5 (3s² 3p³), so Group 5 (Group VA)

So the group is 5 and the period is 3, respectively. This is phosphorus (P).

Answer: C. 5 and 3

Tip: Write the electron arrangement as 2, 8, 5. The number of numbers (3) gives the period, and the last number (5) gives the group. Always answer in the order the question asks: group first, then period.