Physicochemical properties of elemental halogens

Structural features and chemical activity

Elemental substances formed by halogen subgroup elements have the general molecular formula $X_2$. Inside these molecules, atoms are bonded by a single covalent bond. The bond energy (denoted as X–X) determines the chemical reactivity of halogens: the easier it is to break the molecule into individual atoms, the more actively the substance enters into reactions.

The halogen-halogen bond energy is distributed as follows:

• Fluorine ($F_2$): 158.8 kJ/mol.
• Chlorine ($Cl_2$): 242.6 kJ/mol.
• Bromine ($Br_2$): 193.9 kJ/mol.
• Iodine ($I_2$): 152.5 kJ/mol.

It is precisely the relatively low bond strength in the molecule that causes the very high chemical reactivity of all members of this group.

Appearance and states of aggregation

All elemental halogens are colored compounds, and the intensity of their color and state of aggregation vary systematically. Under normal conditions (n.c.), halogens are represented in all three states of aggregation:

1. Fluorine ($F_2$): a pale yellow gas.
2. Chlorine ($Cl_2$): exists as a yellow-green gas.
3. Bromine ($Br_2$): a heavy reddish-brown liquid.
4. Iodine ($I_2$): a solid in the form of black crystals.

Iodine is of particular interest. Despite being a solid under standard conditions, it is highly volatile. Black iodine crystals are capable of evaporating completely, bypassing the liquid phase. This transition from the solid state directly to the gaseous state is called sublimation, producing violet vapors.

Melting and boiling points

Physical constants, such as melting ($t_{m}$) and boiling ($t_{b}$) points, are strictly individual for each halogen and systematically increase with the atomic mass of the element:

• Fluorine is characterized by extremely low temperatures: $t_{m}$ is –219.62 °C, and $t_{b}$ is –188.12 °C.
• Chlorine also melts and boils at sub-zero temperatures: $t_{m}$ = –101.5 °C, $t_{b}$ = –34.04 °C.
• Bromine melts at –7.2 °C and boils at 58.8 °C, making it a volatile liquid.
• Iodine has the highest values in the series: melting at 113.7 °C and boiling at 184.4 °C.

Organoleptic properties and water solubility

All halogens have a sharp, specific odor that is easily detected even at extremely low concentrations of these substances in the ambient air. For example, the smell of chlorine is associated by many with bleaching powder, while the scent of crystalline iodine resembles the familiar pharmacy tincture.

Regarding the interaction with water (solubility at 20 °C), halogens behave differently:

• Fluorine is such a strong oxidizing agent that it not only dissolves, but chemically decomposes water.
• Chlorine dissolves in water to form a solution with a concentration of about 1.4%.
• The solubility of bromine is slightly higher, at 3.5%.
• Iodine is practically insoluble in water, with its solubility reaching only 0.03%.

Class and exam questions

What else to study on this topic

• Mechanism of the reaction of fluorine with water
• Reasons for the change in bond energy in the chlorine-bromine-iodine series
• Crystal lattice of solid iodine
• Toxicology and safety when working with volatile halogens

Other topics in the section "Chapter 9. Group VIIA Elements — Halogens"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.