BECE Science 2025 Paper

Concept:Elements in the same group of the periodic table have the same number of valence (outermost-shell) electrons.Solution:Magnesium ($Z = 12$): $2, 8, 2$Calcium ($Z = 20$): $2, 8, 8, 2$Both have $2$ valence electrons, so both are in Group 2 (the alkaline earth metals).Why the others are wrong: their proton numbers differ (12 and 20); Mg has 3 shells while Ca has 4, so they are in different periods; their atomic structures are not the same.Key point: Same group = same number of valence electrons. Same period = same number of shells.Answer:Option B (valence electrons)