BECE Science 2026 Paper

Concept:In a stable compound the total combining power (valency) of one element equals that of the other. The criss-cross method swaps the valencies and writes them as subscripts.Solution:Valency of sodium $= 1$, valency of oxygen $= 2$.Cross the valencies: the $2$ of oxygen becomes the subscript of $\text{Na}$, and the $1$ of sodium becomes the subscript of $\text{O}$:$\text{Na}_2\text{O}_1 = \text{Na}_2\text{O}$ (a subscript $1$ is not written).Check: $2$ sodium atoms give $2 \times 1 = 2$ units of combining power, which equals the $2$ of one oxygen atom.Key point: $\text{NaO}$ leaves oxygen with one unused combining power, and $\text{Na}_2\text{O}_2$ (sodium peroxide) does not follow the simple valencies $1$ and $2$.Answer:Option B ($\text{Na}_2\text{O}$)