You are given four substances: a fuming red liquid, a dark metallic-looking solid, a pale-yellow gas, and a yellow-green gas that attacks glass. You are told that these substances are the first four members of Group \(7 \mathrm{~A},\) the halogens. Name each one.

Fluorine, the lightest member of the halogen group, exhibits some unique chemical properties. It is a pale yellow gas at room temperature, characterized by its highly reactive nature. As the most electronegative element, fluorine forms bonds eagerly, particularly with hydrogen, forming hydrogen fluoride, which is notable for its ability to etch glass.

Another characteristic of fluorine is its small atomic radius, which contributes to strong interatomic forces when forming compounds. Due to its high reactivity, it's rarely found in its elemental state in nature, instead, it's commonly found in mineral forms like fluorite and apatite.

Chlorine, recognized by its distinct yellow-green color, is a highly reactive gas. Its ability to react with a wide range of substances is seen in its applications in water purification, disinfectants, and bleach. Chlorine's reactivity stems from its drive to gain an additional electron to achieve a stable octet configuration, fitting with noble gases.

This prompts it to form ionic and covalent bonds with many different elements. Its reaction with organic matter is so strong that chlorine can degrade many types of materials, which is why it attacks glass. Chlorine's presence in household compounds, such as sodium chloride (table salt), also underlines its industrially significant reactivity.

Bromine is unique among the halogens as it's a liquid at room temperature. It presents as a reddish-brown fuming liquid, emitting vapors that are harmful and can cause irritation.

Fuming Nature

Bromine vaporizes easily, due to its high vapor pressure, which is why it can be seen fuming at room temperature.

Utilization

Its characteristics make it useful in various industries, from agriculture in pesticides to the pharmaceutical industry in certain medications. However, the same volatility and reactivity that make it useful also require careful handling due to its corrosive and toxic nature.

Iodine often surprises with its appearance. It's a dark, shiny solid that can appear metallic, although it is a non-metal. When heated, iodine skips the liquid phase and sublimates directly into a violet-colored gas, a property known for its dramatic aesthetic.

Iodine's lustrous appearance and its tendency to form dark, shiny crystals are peculiarities among the halogens. While iodine's reactivity is less aggressive than fluorine and chlorine, it still plays an essential role in biology, notably in the production of thyroid hormones, which are crucial for healthy metabolism. Iodine which is a requirement for human health, is typically sourced from dietary intake, often fortified in items like table salt.