Bromine (Br) is a chemical element with atomic number 35, belonging to the halogen group, Group 17, of the periodic table. At room temperature, it appears as a volatile, deep reddish-brown liquid that easily transforms into a similarly colored vapor. It stands out as the only nonmetallic element that remains liquid under normal conditions. This element is recognized for its general reactivity.
Bromine’s Fundamental Reactivity
Bromine’s reactivity stems from its position in Group 17 of the periodic table, where it is classified as a halogen. Like other elements in this group, bromine has seven valence electrons in its outermost shell. This electron configuration means it requires only one additional electron to achieve a stable octet, a highly favorable state.
Bromine exhibits high electronegativity, with a Pauling scale value of 2.96. This property describes its strong ability to attract electrons towards itself in a chemical bond. Consequently, bromine functions as a strong oxidizing agent, readily accepting electrons from other elements to complete its outer shell.
Reactions with Elements
Bromine readily reacts with various elements, often in exothermic processes that release heat. With highly reactive metals, such as the alkali metals like sodium or potassium, reactions can be quite vigorous, sometimes even violent, leading to the rapid formation of metal bromides. Bromine also combines with other metals, including aluminum, arsenic, and antimony, though these reactions are generally less intense compared to those with alkali metals.
Bromine also interacts with nonmetals, frequently forming covalent molecular bromides. It reacts with hydrogen, for instance, to produce hydrogen bromide (HBr), a compound that exists as a colorless gas at room temperature. This conversion can be industrially achieved by combining hydrogen and bromine gases at elevated temperatures, often within the range of 200–400 °C, utilizing a platinum catalyst. Furthermore, bromine reacts with phosphorus, forming phosphorus bromides such as phosphorus tribromide (PBr₃) or phosphorus pentabromide (PBr₅), depending on the specific reaction conditions and the ratio of reactants.
Reactions with Compounds
Bromine’s reactivity extends to a variety of chemical compounds. In organic chemistry, bromine participates in distinct types of reactions depending on the hydrocarbon’s saturation. With unsaturated hydrocarbons like alkenes and alkynes, bromine undergoes addition reactions. In these reactions, the bromine molecule breaks across the double or triple carbon-carbon bonds, resulting in the formation of dibromoalkanes or tetrabromoalkanes, respectively.
For saturated hydrocarbons, or alkanes, bromine can engage in substitution reactions, but these typically require specific conditions, such as the presence of ultraviolet (UV) light. In this process, a hydrogen atom on the alkane is replaced by a bromine atom, often producing hydrogen bromide as a byproduct. While bromine reacts with hydrocarbons, it does so less readily than chlorine.
When bromine dissolves in water, it undergoes a disproportionation reaction, forming hydrobromic acid (HBr) and hypobromous acid (HBrO). This solution is known as bromine water. However, bromine water can decompose in sunlight, releasing oxygen.
Bromine can also participate in displacement reactions with other halides. As a more reactive halogen than iodine, bromine can displace iodide ions from their salts. This reaction results in the formation of bromide ions and elemental iodine.
Safety Considerations
Given bromine’s reactive nature, handling it requires strict safety protocols. Bromine is highly corrosive to skin and can cause severe burns upon contact. Its reddish-brown vapors are toxic and act as a strong irritant to the respiratory system, eyes, and mucous membranes, even at low concentrations, causing symptoms like eye-watering and coughing at concentrations below 10 ppm. Exposure to concentrated bromine vapor, even for a short period, can be fatal.
Due to these hazards, proper ventilation is necessary when working with bromine to prevent the buildup of harmful vapors, especially since bromine gas is heavier than air and settles in low-lying areas. Individuals handling bromine must use appropriate personal protective equipment, including chemical-resistant gloves, eye protection, and respiratory protection if necessary. Bromine should always be handled with extreme caution in a controlled environment, such as a laboratory fume hood, and is not suitable for untrained individuals.