## 12.1 Halogens The elements in **Group 17** (or Group VII-A) of the periodic table are known as the **halogens**. The name is derived from the Greek words *halous* (meaning "salt") and *gen* (meaning "former"), as they readily react with metals to form salts. The halogens include: - Fluorine (F) - Chlorine (Cl) - Bromine (Br) - Iodine (I) - Astatine (At) - Oganesson (Og) *Note: Astatine is a radioactive element, and Oganesson is an artificial radioactive element. Little is known about their properties.* All halogens are reactive non-metals and exist as diatomic molecules ($X_2$) at standard conditions. Their follows a specific pattern. - **Fluorine ($F_2$):** Pale yellow gas - **Chlorine ($Cl_2$):** Greenish-yellow gas - **Bromine ($Br_2$):** Reddish-brown liquid - **Iodine ($I_2$):** Shiny, greyish-black solid ### 12.1.1 Volatility of Halogens As you move down Group 17, the halogens become **less volatile** and their colors become darker. **Trend:** Volatility decreases down the group ($F_2 > Cl_2 > Br_2 > I_2$). **Reason:** The decrease in volatility is due to the increasing strength of intermolecular forces. 1. Halogen molecules ($X_2$) are non-polar. 2. The only intermolecular forces present are weak **instantaneous dipole-induced dipole forces** (also known as London dispersion forces). 3. As you go down the group, the number of electrons and the size of the electron cloud in each molecule increases. 4. This leads to stronger temporary dipoles and therefore stronger intermolecular forces. 5. More energy is required to overcome these stronger forces, resulting in higher melting and boiling points. This explains why Bromine ($Br_2$) is a liquid and Iodine ($I_2$) is a solid at room temperature, while Fluorine and Chlorine are gases. ### 12.1.2 Bond Strength of Halogens The bond energy is the energy required to break the covalent bond in one mole of diatomic halogen molecules ($X-X$). **General Trend:** As you go down the group from chlorine to iodine, the covalent bond becomes weaker. This is because the atomic radius increases, leading to a longer and weaker bond. **Fluorine Anomaly:** The $F-F$ bond is unexpectedly weaker than the $Cl-Cl$ bond. This is due to the small size of the fluorine atom. The non-bonding electrons (lone pairs) on the two fluorine atoms are very close together, causing significant **inter-electronic repulsion**, which weakens the covalent bond. | Halogen Bond | Bond Energy ($kJ/mol$) | |:------------:|:----------------------:| | $F-F$ | 156 | | $Cl-Cl$ | 243 | | $Br-Br$ | 193 | | $I-I$ | 151 |