In 1774, Swedish chemist Carl Wilhelm Scheele discovered chlorine. While conducting a chemical reaction, a pale green gas was produced that he could not identify. It was later identified to be diatomic chlorine gas, the first element ever discovered from group 17, which contains some of the most reactive elements on the periodic table.
Halogen: The ‘Salt Generator’
One year later, German chemist Johann Schweigger proposed the name ‘halogen’—literally, ‘salt generator’ or ‘salt maker’. He noted that chlorine was often found together with metals in ionic compounds called salts—for example, compounds like sodium chloride, commonly known as table salt.
With the discoveries of iodine in 1811 and bromine in 1826, the list of elements found that combined with metals to produce salts was growing.
In 1829, German chemist Johann Döbereiner recognized the chemical similarities of these elements in one of his triads, even though at room temperature chlorine is a gas, bromine is a liquid and iodine is a solid. Around that same time, the Swedish chemist Jon Jacob Berzelius weighed in to propose that the salt-generator name ‘halogen’ might instead be used to refer to this entire group of salt-forming elements.
Diatomic Molecules of Halogens
In their pure forms, all four of the common halogens form diatomic molecules with a single bond. Naturally, having two atoms increases the size of their electron clouds in their diatomic form. And larger electron clouds increase the amount of very slight charge imbalances in the clouds that weakly draw the clouds together via the Van der Waals attractions.
The result is a series of diatomic molecules, each larger in size than the last, with higher and higher melting and boiling points.
Being diatomic, each is also much larger than a single atom of the neighboring noble gas. Thus, diatomic fluorine’s boiling point is more comparable to a larger single atom of monatomic argon, rather than to fluorine’s monatomic neighbor, neon.
Molecular Properties of Chlorine
Molecules of chlorine occupy a large volume because of their additional electron shell. A more distant third valence shell of electrons is screened from the nuclei by the inner second shell of electrons. This makes chlorine molecules interact a bit more effectively with one another.
At ordinary pressure, liquid chlorine forms at minus 34° Celsius. Moreover, chlorine can be stored and transported as a liquid at higher temperatures if it is kept under more pressure.
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Density of Chlorine
The fact that chlorine is a third-row element with two atoms per molecule also makes it a remarkably dense elemental gas. With an atomic mass of about 34.5, a molecule of two chlorine atoms has a molecular mass of about 71 amu. On comparing that to diatomic oxygen at 32 or diatomic nitrogen at just 28 amu, we can understand why the more massive chlorine molecules would actually sink into the air.
The density of chlorine was exploited by German chemist Fritz Haber during World War I. Haber counted on the density of chlorine gas to make it drift across the battlefield and sink into the trenches of the enemy as he ordered the release of thousands of buried tanks of pressurized chlorine gas at Ypres, Belgium in 1915.
Fluorine and Bromine
Fluorine gas is even more reactive than chlorine. Why not? Diatomic fluorine has a mass of 19 x 2 = 38 amu. with a density barely more than oxygen and nearly the same density as air. Had Haber considered using fluorine during the world war, any release of fluorine gas would have been carried by wind currents too easily and dispersed.
Chlorine’s larger neighbor, bromine, is a liquid that would never have moved as a gas across the battlefield at all. With a larger electron cloud and even larger associated Van der Waals attractions, bromine remains a deep purple liquid until it boils at 60° Celsius, 140° Fahrenheit.
Iodine: The Trickster of a Halogen
The larger molecules of iodine stay together even more than bromine. Iodine might trick one into believing that it is a liquid, given the liquid solutions called ‘iodine’ sold at the pharmacy. But pure iodine is solid, with very large electron clouds that make its molecules stick to one another so well they form a solid crystalline substance at room temperature. It’s almost like a metal.
But heat pure iodine even gently, and it becomes clear that this is no metal! Iodine melts at 114° Celsius, just above the boiling point of water. And that same iodine will boil at just 184° Celsius, where it forms a beautiful, violet gas with properties and reactivities similar to chlorine and fluorine.
Tendency to Take Electrons
The halogens’ position on the table just before the noble gases results in fierce electron-seeking behavior that decreases in a regular way as we descend to larger and larger members of the group.
But while the intensity of the reactivity declines, each of the halogens is on a quest for one additional valence electron that drives them to form diatomic molecules when pure, charge-balancing anions in metal salts, and acids when combined with hydrogen.
Compounds of Halogens
Covalent compounds with carbon form a host of useful compounds of the halogens, many of which sadly have considerable environmental and health impacts.
Their combination with oxygen results in some truly angry molecules—potent oxidizing agents of varying strength, from bleaches and drinking water additives, to rocket fuel enhancers.
Because of the many similarities across the halogens, substituting one halogen for another also offers the potential for additional technologies, like using radioactive astatine to act as an iodine impersonator that may deliver targeted radiation to thyroid cancer patients.
Common Questions about Halogens
Chlorine was the first halogen to be discovered in 1774 when Swedish chemist Carl William Scheele was conducting a chemical reaction. He unknowingly produced a pale green gas that he could not identify, but which was later recognized as the diatomic chlorine gas.
Chlorine is dense. With an atomic mass of about 34.5, a molecule of two chlorine atoms has a molecular mass of about 71 amu.
Covalent compounds with carbon form a host of useful compounds of the halogens, many of which have considerable environmental and health impacts. Their combination with oxygen results in some truly angry molecules—potent oxidizing agents of varying strength, from bleaches and drinking water additives, to rocket fuel enhancers.