WAEC Chemistry 2025 Paper

Concept:Species that are isoelectronic have the identical number of electrons.Explanation:A neutral neon atom has atomic number $10$. Therefore, it contains exactly $10$ electrons. An ion is isoelectronic with neon only if it also has $10$ electrons. Now check each option by subtracting the positive charge from the atomic number. $K^+$: atomic number $19$, electrons $= 19 - 1 = 18$ $Mg^{2+}$: atomic number $12$, electrons $= 12 - 2 = 10$ $Ca^{2+}$: atomic number $20$, electrons $= 20 - 2 = 18$ $Li^+$: atomic number $3$, electrons $= 3 - 1 = 2$ Only $Mg^{2+}$ has $10$ electrons, matching neon.Answer:B. $Mg^{2+}$