BECE Science 2023 Paper

Concept:A period is a horizontal row of the periodic table. Elements in the same period have the same number of occupied electron shells.Explanation:Electron configurations:$_{5}\mathrm{B}$: 2,3 (period 2); $_{13}\mathrm{Al}$: 2,8,3 (period 3)$_{1}\mathrm{H}$: 1 (period 1); $_{3}\mathrm{Li}$: 2,1 (period 2)$_{4}\mathrm{Be}$: 2,2 (period 2); $_{5}\mathrm{B}$: 2,3 (period 2)$_{11}\mathrm{Na}$: 2,8,1 (period 3); $_{19}\mathrm{K}$: 2,8,8,1 (period 4)Only Be and B both have two shells.Key point: B and Al, and Na and K, are in the same group (vertical column), not the same period.Answer:Option C ($_{4}\mathrm{Be}$ and $_{5}\mathrm{B}$)