BECE Science 2021 Paper

Concept:The number of electrons in the outermost shell of an atom is given by its electronic configuration.Explanation:Write the electronic configuration for each element by distributing its atomic number electrons into shells $K, L, M, \ldots$).For $\mathrm{Ca}$ $(Z = 20)$: configuration is $2, 8, 8, 2$, so outermost shell has $2$ electrons.For $\mathrm{Cl}$ $(Z = 17)$: configuration is $2, 8, 7$, so the outermost shell has $7$ electrons.For $\mathrm{Ne}$ $(Z = 10)$: configuration is $2, 8$, so the outermost shell has $8$ electrons.For $\mathrm{S}$ $(Z = 16)$: configuration is $2, 8, 6$, so the outermost shell has $6$ electrons.Thus, the element with seven electrons in the outermost shell is chlorine, $\mathrm{Cl}$.Answer:Option B. $\;\; {}_{17}\mathrm{Cl}$