Q.Considering the elements F, Cl, O and N, the correct order of their chemical reactivity in terms of oxidizing property is:

The key idea is that oxidizing property (the ability to gain electrons) follows the same periodic trend as electronegativity and electron gain enthalpy — it increases across a period and decreases down a group.

Reasoning:

1. Fluorine is the strongest oxidising agent because it has the highest electronegativity and the most negative electron gain enthalpy (ignoring the small anomaly due to its small size).
2. Oxygen is next: it is in the same period as F but one group left, so it is less oxidising than F but more than Cl. …

The oxidizing property of an element is its ability to accept electrons and get reduced itself. In the periodic table, this property is closely tied to two factors: electronegativity (how strongly an atom attracts electrons) and electron gain enthalpy (the energy change when an electron is added). For non-metals like fluorine, chlorine, oxygen, and nitrogen, the strongest oxidizing agent is the one that most readily gains electrons.

A common trap here is to assume that oxidizing power follows the same trend as electronegativity in a straightforward way. While fluorine is the most electronegative element, chlorine has a more negative electron gain enthalpy than fluorine — yet fluorine is still the strongest oxidizer. Why? Because the small size of fluorine leads to strong repulsion in its electron cloud, but its high electronegativity and low bond dissociation energy in $F_2$ make it exceptionally reactive. Let’s break this down step by step.

1. Identify the key property: Oxidizing power is directly proportional to the tendency to gain electrons. For elements, this is best gauged by electron gain enthalpy (more negative means stronger oxidizer) and electronegativity (higher means stronger pull on electrons). However, for fluorine, a special case arises: its small size causes high electron-electron repulsion when adding an electron, making its electron gain enthalpy less negative than chlorine’s. Despite this, fluorine is the strongest oxidizer due to its extremely high electronegativity and the weak $F-F$ bond in $F_2$, which makes it easy to break and release fluorine atoms.

2. Compare fluorine and chlorine: Fluorine has a less negative electron gain enthalpy ($-328 \text{ kJ/mol}$) than chlorine ($-349 \text{ kJ/mol}$), but its electronegativity (4.0 on the Pauling scale) is higher than chlorine’s (3.0). In practice, fluorine oxidizes almost everything, including water, while chlorine is less aggressive. So, F > Cl in oxidizing power.

3. Place oxygen in the order: Oxygen has an electronegativity of 3.5 and an electron gain enthalpy of $-141 \text{ kJ/mol}$ (for adding one electron to form $O^-$). It is a strong oxidizer but not as strong as fluorine. However, oxygen is a better oxidizer than chlorine in many contexts — for example, oxygen can oxidize chlorine in some reactions (like forming $Cl_2O$). The key is that oxygen’s small size and high electronegativity give it an edge over chlorine. So, O > Cl. …