WAEC Chemistry 2017 Paper

Concept:Across a period, ionization energy generally increases from left to right due to increasing nuclear charge and decreasing atomic radius.Explanation:For period 3 elements, the order of increasing ionization energy is: $\mathrm{Na} < \mathrm{Al} < \mathrm{P} < \mathrm{Cl}$.This is because $\mathrm{Na}$ has the lowest nuclear charge and largest atomic radius, so its outermost electron is removed most easily.Moving to $\mathrm{Al}$, $\mathrm{P}$, and $\mathrm{Cl}$, the nuclear charge increases while the atomic size decreases, making electrons harder to remove.Therefore, the correct decreasing order is the reverse of the increasing order.So, the decreasing order is: $\mathrm{Cl} > \mathrm{P} > \mathrm{Al} > \mathrm{Na}$.Answer:B. $\mathrm{Cl}$, $\mathrm{P}$, $\mathrm{Al}$, $\mathrm{Na}$