To determine the correct order of ionisation enthalpy for the elements Li (Lithium), Na (Sodium), Cl (Chlorine), and F (Fluorine), we must understand the concept of ionisation enthalpy.
Ionisation enthalpy, also known as ionisation energy, is the energy required to remove an electron from a gaseous atom or ion. The related periodic trend is that ionisation energy generally increases across a period and decreases down a group in the periodic table.
Across a Period: Ionisation energy increases as the effective nuclear charge increases, making it harder to remove an electron.
Down a Group: Ionisation energy decreases because the additional electron shells increase the distance between the nucleus and the valence electrons, thus reducing the nuclear attraction.
Now, let us place the given elements in order of their positions in the periodic table:
Period 2: Li (Lithium).
Period 3: Na (Sodium).
Period 3: Cl (Chlorine).
Period 2: F (Fluorine).
Applying the periodic trend:
F (Fluorine) has the highest ionisation energy because it is the most electronegative and is located to the right of the periodic table.
Cl (Chlorine) comes next as it is in the same group as F but one period below, leading to a slightly lower ionisation energy.
Li (Lithium) has a moderate ionisation energy within its period, but greater than Na as it resides in the second period versus Na in the third.
Na (Sodium) has the lowest ionisation energy as it is in the third period and the furthest left among the listed elements.
This gives us the order of ionisation enthalpy as:
