To determine the correct order of ionisation energy among Cl, S, P, Al, and Si, it is essential to understand the general trends of ionisation energy across the periodic table:
Ionisation Energy Trend: Ionisation energy generally increases across a period from left to right due to an increase in the effective nuclear charge, which holds the electrons more tightly.
Group Consideration: Going down a group in the periodic table, ionisation energy decreases as electrons are added to higher energy levels further from the nucleus, resulting in weaker nuclear attraction.
Let's place the given elements in the periodic table to predict the order of their ionisation energies:
Aluminium (Al): It is in Group 13 and is expected to have the lowest ionisation energy as it is located further left in Period 3.
Silicon (Si): It is to the right of Al in Group 14.
Phosphorus (P): Belonging to Group 15, its ionisation energy is expected to be higher than that of Al and Si.
Sulfur (S): Positioned next to P in Group 16, S typically has higher ionisation energy due to the added electron-electron repulsion in its p-orbitals.
Chlorine (Cl): In Group 17, Cl has the highest ionisation energy among these elements because it is furthest to the right in Period 3.
Considering these trends, here is the order of ionisation energies in decreasing order:
\(\text{Cl}\) (highest due to its position in Group 17)
\(\text{P}\) (higher than S due to more stable half-filled 3p subshell)
\(\text{S}\) (after P, but before Si)
\(\text{Si}\) (lower than P and S)
\(\text{Al}\) (lowest due to its leftmost position in Period 3)
This confirms that the correct order of ionisation energy is \( \text{Cl} > \text{P} > \text{S} > \text{Si} > \text{Al} \).
