Lesson Video: Halogens Chemistry

In this video, we will learn how to describe the compounds and reactivities of halogens and trends in their physical and chemical properties.

17:50

Video Transcript

In this video, we will learn about the halogens and the trends in their physical and chemical properties. We will learn how to explain the trend in the reactivity of the halogens using examples of their reactions with metals to form salts and also by looking at halogen displacement reactions.

The halogens are the six nonmetallic elements that are found in group 17 or seven of the periodic table. This is the second column from the right-hand side of the periodic table. These elements are fluorine, chlorine, bromine, iodine, astatine, and tennessine. Astatine and tennessine are both radioactive elements, and their isotopes are not stable. Indeed, the Greek word “astatos” translates as unstable. Although astatine isotopes have been discovered in minute amounts in natural radioactive decay chains, astatine can be considered to be a synthetic element. In 2016, the discovery of the synthetic element with the atomic number 117 was recognized, and it was named tennessine after the state of Tennessee where research centers were located. Astatine and tennessine are both considered to be synthetic elements. And because their isotopes are very short-lived, you will not find an atomic mass value in the modern periodic table for these two elements.

The halogens are all very reactive elements, and they’re not encountered in nature as pure elements. Fluorine, chlorine, bromine, and iodine are all found combined with other elements in the Earth’s crust. Fluorine and chlorine are more common than bromine and iodine in this location. None of the halogens make the top 10 list for the most abundant elements found on planet Earth. As halogens are never found in nature as pure elements, they have to be manufactured. So, let’s take a look at each halogen in turn so that we can describe their physical appearance accurately.

Fluorine is the most reactive element of all known elements. It appears as a pale-yellow-to-green gas. Fluorine is very hard to contain due to its high reactivity. Iron wool bursts into flames when simply exposed to fluorine at room temperature. Iron three fluoride is formed in this very exothermic reaction. Although in its pure form fluorine exists as diatomic covalently bonded molecules, fluorine is more commonly found as a compound in useful products. Fluorine is found as the halide ion in toothpaste to help prevent tooth decay. And fluorine is also found in the compound PTFE, known under the brand name of Teflon, which is found in the nonstick coating on frying pans.

Chlorine is a dense yellow-to-green gas. It is very toxic, and it was used as a weapon during World War One. It is also found as diatomic covalently bonded molecules. Chlorine is a little less reactive than fluorine. And iron wool reacts rapidly when heated and placed into chlorine to produce dense brown fumes of iron three chloride. Chlorine is extracted via the electrolysis of brine, a concentrated solution of sodium chloride, the salt that we commonly call table salt. Much of the chlorine produced in this way is used directly as the element to kill bacteria to provide drinking water. Chlorine is also found in the compound hypochlorite, also known as bleach, which is often used to kill bacteria in swimming pools. Chlorine is also found in the compound PVC, a tough, flexible plastic often used as an electrical insulator.

Bromine is a dense reddish-brown liquid that gives off a dense red vapor. If we were to pick up a bottle of bromine liquid, we’d be surprised how heavy it were compared to a bottle of water of equal volume. Bromine is extracted from seawater. Like chlorine, it’s very toxic. The name bromine is derived from the Greek word “bromos,” meaning stench. Liquid bromine is best handled in the fume hood. Iron wool reacts when heated strongly and placed into liquid bromine but less violently than it does with chlorine. Aluminum foil will react very exothermically with liquid bromine, but the reaction takes a long time to get going. In both reactions, the metal salts iron three bromide and aluminum three bromide are produced. Bromine compounds are found widely used in pesticides and in drug molecules.

Iodine can be a difficult element to describe. Under normal conditions, it exists as a gray-to-black solid with a shiny crystalline structure. When heated, it sublimes to a purple vapor. When dissolved in water, it forms a brown solution. And dissolved in hexene, a purple solution is seen. Iodine can be extracted readily from seaweed. Like all the other halogens, iodine exists as covalently bonded diatomic molecules. Iodine has an important role as an antiseptic as the element. And it has an important function in the human body in small amounts in the thyroid gland located in your neck. The thyroid gland produces an important hormone called thyroxine. This is a complicated molecule that contains iodine atoms. If we mix powdered aluminum with powdered iodine, a reaction does occur after a long induction period. The product is aluminum three iodide, and the reaction is quite exothermic.

It’s clear that as we descend group 17, these halogens get darker in appearance. They also turn from gas to liquid to solid. By comparing their reactions with iron wool, we can see that the reactions get progressively less violent. If we had to make predictions about astatine based upon the trends and patterns emerging here, we would probably say that astatine is a black solid and it’s less reactive then iodine. If we could safely lay our hands on enough of this element, it might react with iron wool or aluminum only when heated very strongly.

From the descriptions of these halogens, we can see that there are some trends in their physical appearance as we move down group 17. The physical appearance, and in particular the color of these elements, is a physical property. As we progress down group 17, the colors progressively darken. We have also seen that as we move down group 17, the elements turn from gas to liquid to solid states at room temperature. This indicates that the melting and boiling points of these elements are progressively increasing. As elements, these halogens are all covalently bonded simple molecules.

In the solid state, these halogen molecules are all arranged in a regular way. If we were to melt these halogens to turn them from a solid state to a liquid state, we would have to overcome intermolecular forces that operate between these halogen molecules. Likewise, if we were to boil a liquid halogen to turn it into a halogen in the gas state, we also need to overcome intermolecular forces. Remember that when we melt or boil a simple molecular substance, we’re just separating the molecules apart from each other, and no covalent bonds are broken at all. This is also the case with these halogens. Melting or boiling a halogen sample does not involve breaking the covalent bonds within the halogen molecules at all.

As we move down group 17, the melting points and boiling points of these halogen molecules increase as the intermolecular forces that operate between the halogen molecules get increasingly stronger. It is also evident from the descriptions of how iron wool reacts with these halogens that the chemical reactivity of each halogen decreases as we move down the group. Fluorine reacts with iron wool when cold; it needs very little encouragement for the reaction to get started. The reaction is very exothermic, and the iron wool bursts into flames. Iodine and bromine, however, only react with iron wool when the iron wool is strongly heated first.

Fluorine is clearly the most reactive halogen, and iodine, the least. This trend in chemical reactivity can be explained by considering what is happening when halogens react with metals. Atoms of halogens each have seven valent shell electrons. When halogen atoms react with metals, they gain one electron to form a stable halide ion with a single negative charge. This gives the halogen the electronic configuration of the corresponding noble gas in the same period. Let us look at these electronic structures in more detail to see if we can link the electronic structure of each halogen to its chemical reactivity.

When halogens react with metals, the halogen atom gains one electron to form a stable halide ion. In the case of fluorine reacting with lithium, lithium loses one electron from its valence shell and fluorine gains one electron into its valence shell. A stable ionic compound, a salt called lithium fluoride, is formed. In the case of fluorine, the fluorine atom gains electrons very readily indeed. This is because it’s a very small atom. In the case of fluorine, the incoming electron is strongly attracted to the fluorine nucleus as it enters a shell close to the fluorine nucleus. It’s the protons in the fluorine nucleus that are attracting the incoming electron in the first place.

Another consequence of fluorine being a very small atom with only two shells occupied by electrons is that fluorine has less shielding. The concept of shielding is described as the effects of reduced attraction for valent shell electrons caused by inner shell electrons. The more occupied shells an atom contains, the more shielded the valent shell electrons are from the attractive effect of the nucleus by the inner shell electrons. So, as we move down group 17, the halogen atoms get larger as we add shells to the atoms. The outer shell is, therefore, further from the nucleus and shielded or screened from the nucleus by the inner shells present.

This means that there’s a reduction in the force of attraction between the nucleus and the valent shell or outermost electrons. It becomes harder for the halogens to attract an electron into the outer shell as we move down the group. Iodine is observed to be the least reactive halogen because it’s the largest atom with the most shielding. And it doesn’t gain electrons very easily.

Another important set of reactions that halogens take part in are the displacement reactions involving a more reactive halogen with the halide ion of a correspondingly less reactive halogen. When a more reactive halogen is placed into a solution of a less reactive halide ion, the more reactive halogen displaces the less reactive halide ion as the corresponding halogen. It’s important to note here that halide ion solutions are colorless. And when the halide ion is displaced to form the halogen, a color change is often observed.

An example of such a reaction would be a solution of chlorine dissolved in water being added to a solution of potassium iodide. In this reaction, chlorine reacts with potassium iodide to produce potassium chloride and iodine. As chlorine is more reactive than iodine, it displaces the iodide ions, and a brown solution develops.

If solutions of chlorine gas, bromine, and iodine are available, these reactions can easily be performed in a school or college lab. Fluorine is practically unobtainable in a school or college lab and reacts with water so will be omitted from these reaction descriptions. These reactions can be summarized in a table. Since halogens and halide ions of equal reactivity will not react with each other at all, these have been eliminated from the table.

If a solution of chlorine is added to a solution of bromide ions, bromine is displaced. An orange-to-red solution may be observed, or orange-to-red fumes may be evolved. If chlorine is added to a solution of iodide ions, iodine is displaced and a brown solution is formed. Both of these reactions happen because chlorine is more reactive than the respective halide ions concerned.

If a solution of bromine were added to a solution of aqueous chloride ions, there would be no reaction at all. Bromine is not reactive enough to displace chlorine from the chloride ions. The bromine solution would, therefore, stay orange to red. If bromine were added to aqueous iodide ions, iodine would be displaced. The solution would, therefore, turn brown.

Lastly, if an aqueous solution of iodine were added to aqueous chloride or bromide ions, there would be no reaction at all. Iodine is brown, and the solution will stay brown. We will now look a question to test your understanding of these reactions and any observations that you would make.

A halogen displacement reaction is shown in the following equation: X2 plus 2Br− aqueous makes Br2 gas plus 2X− aqueous. What is the color change that occurs when the bromide solution is converted to bromine gas? What halogen could X2 be? (A) Colorless to purple, chlorine; (B) colorless to brown, iodine; (C) colorless to brown, chlorine; (D) brown to colorless, iodine; and (E) brown to colorless, chlorine.

In this question, we see that we are starting with a solution of bromide ions. Bromide ions are halide ions. And all halide ion solutions are colorless unless there’s a positively charged cation present that gives the solution a color. So, the bromide ions solution starts off as a colorless solution. In the reaction, bromine gas is being displaced from bromide ions and another halide ion X− is being formed. The bromine gas displaced will have an orange-to-brown color, and the X− ions will be colorless.

So, the color change expected would be colorless to orange or brown. We can, therefore, eliminate responses (D), (E), and (A) as these do not contain the correct color changes expected. The halogen or X2 that displaced the bromine must be more reactive than the bromine itself. This is the halogen that was added to the bromide ions in the reaction at the start. The only halogen commonly found in the lab that is more reactive than bromine is chlorine. Response (B) suggests that iodine was used to displace the bromine, and this will not work as it’s less reactive than bromine. We can, therefore, reject response (B). The only response remaining is (C), which describes the correct color change and suggests chlorine a halogen that is more reactive than bromine as the displacing agents. This is the correct answer.

To finish off, let us look at the key points. As we move down group 17, there is a decrease in reactivity. There is an increase in atomic radius or the size of the atom. There is an increase in the number of filled electron shells. There is a decrease in the power of the halogen to attract an electron to form a negative ion. There is an increase in melting and boiling points, and the color of the elements tends to get darker.

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