IE, EN & Electron Gain Enthalpy
Master the thermodynamic properties and the famous Fluorine-Chlorine anomaly.
Group 17 elements (Halogens) have the valence shell electronic configuration of $ns^2 np^5$. Because they are merely one electron short of the next highly stable noble gas configuration, their desire to gain an electron is the strongest in the entire periodic table. This dictates all of their thermodynamic values.
| Element | Atomic Number | First IE ($\Delta_i H_1$ kJ/mol) | Electronegativity (Pauling) | Electron Gain Enthalpy (kJ/mol) |
|---|---|---|---|---|
| Fluorine (F) | 9 | 1680 | 4.0 | -333 |
| Chlorine (Cl) | 17 | 1256 | 3.2 | -349 |
| Bromine (Br) | 35 | 1142 | 3.0 | -325 |
| Iodine (I) | 53 | 1008 | 2.7 | -296 |
| Astatine (At) | 85 | 890 | 2.2 | -270 |
1. Ionization Enthalpy ($\Delta_i H$)
Ionization Enthalpy is the energy required to remove the most loosely bound electron from an isolated gaseous atom.
The Highest in Their Periods
Because halogens lie at the far right of the periodic table (just before noble gases), they possess the maximum effective nuclear charge and smallest atomic radii in their respective periods. Consequently, they have very high ionization enthalpies, meaning they have very little tendency to lose electrons.
The Down-the-Group Trend
As you move down Group 17 from Fluorine to Astatine, atomic size increases. The valence electrons are further from the nucleus's pull, meaning the ionization enthalpy gradually decreases down the group.
2. Electronegativity
Electronegativity is the tendency of an atom to attract a shared pair of electrons in a covalent bond. Halogens are the absolute champions of this property.
- Fluorine (4.0): With a Pauling scale value of 4.0, Fluorine is the most electronegative element in the entire periodic table. This leads to extremely polar bonds and unique properties, like bringing out the highest oxidation states of other elements (e.g., $IF_7$, $SF_6$, $PtF_6$).
- The Trend: Electronegativity decreases down the group. However, even the heavier halogens (like Br and I) remain quite electronegative compared to the rest of the periodic table.
3. Electron Gain Enthalpy ($\Delta_{eg} H$)
Electron Gain Enthalpy is the energy change when an electron is added to a neutral gaseous atom. Halogens have the maximum negative electron gain enthalpy in their respective periods because gaining just one electron gives them a highly stable noble gas configuration.
Generally, the value becomes *less negative* as you go down a group because the incoming electron feels less attraction from the shielded nucleus. But, Group 17 has the most frequently tested anomaly in inorganic chemistry.
The Grand Anomaly: Fluorine vs. Chlorine
Look at the data table: Fluorine's value is $-333\text{ kJ/mol}$, but Chlorine is $-349\text{ kJ/mol}$. Fluorine has a less negative electron gain enthalpy than Chlorine! Consequently, Chlorine has the highest negative electron gain enthalpy in the entire periodic table.
Why does Fluorine release less energy?
Fluorine's atomic size is extremely small. The 7 valence electrons are packed tightly into a compact $2p$ orbital. When an incoming electron attempts to enter, it suffers from intense inter-electronic repulsion. The atom actively resists the extra electron, so less energy is released. Chlorine's larger $3p$ orbital is much more spacious, easily accommodating the new electron without severe repulsion.
Figure 1: Comparison of the steep drop in Ionization Enthalpy ($\color{#10b981}{\text{green dashed line}}$) and Electronegativity ($\color{#a855f7}{\text{violet solid line}}$) from Fluorine to Astatine.
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