BECE Science 2017 Paper

Concept:In a neutral atom, the number of electrons equals the number of protons.Electrons fill shells in the order: first shell up to $2$, second shell up to $8$, and so on.Explanation:Proton number $= 17$, so the atom contains $17$ electrons.First shell can hold a maximum of $2$ electrons.Second shell can hold a maximum of $8$ electrons.After filling the first two shells, the total electrons used are $2 + 8 = 10$.Remaining electrons go to the third shell: $17 - 10 = 7$.Thus, the third shell contains $7$ electrons.Answer:D. $7$