Lesson Video: Halogens Science

In this video, we will learn how to identify and name the halogens, describe their physical properties and explain their chemical activity.


Video Transcript

In this video, we will learn how to identify and name the halogens, describe their physical properties, and explain their chemical activity.

The halogens are a group of nonmetal elements located towards the right-hand side of the periodic table. The halogens form group seven A of the periodic table. This group is also often referred to as group 7 or group 17 of the periodic table. Group seven A is found within what is known as the p-block in the periodic table. The halogens are composed of fluorine, chlorine, bromine, iodine, and astatine. Although the halogens have the chemical symbols F, Cl, Br, I, and At, they ordinarily exist as diatomic molecules in their elemental form. Molecular astatine has not been observed or studied by scientists.

A diatomic molecule is a molecule made of two atoms chemically bonded together. So, the chemical formulae of the halogen elements are F2, Cl2, Br2, and I2. Following this pattern, the presumed molecular formula for astatine could be At2, although this has not been discovered yet. Astatine is a radioactive element, and it is very rare. Not much of it exists on Earth at any given time. So, we will not mention astatine for the rest of this video.

In the next section of this video, we shall look at the physical properties of the halogens. Firstly, we’ll take a look at the appearance of the halogens as elements. Elemental fluorine is found as a pale-yellow gas. Chlorine is a green gas at room temperature, whilst bromine is a dark reddish-brown colored liquid. Bromine is a very dense liquid, but it gives off a red-to-orange vapor with a very strong and unpleasant smell. Iodine is found as an almost black solid with a slightly shiny appearance. Overall, we can see that the colors of the elements darken as we move down the group. When iodine is heated, it sublimes, and a purple vapor is released.

Sublimation is the change of matter from solid to gas state with no intermediate liquid stage. We can see that as we move down the group, the physical states of the elements change from gases to a liquid to a solid. In fact, as we move down the group, the molecules get larger and the melting points increase.

In the next section of this video, we shall look at the chemical properties of the halogens. The halogens are all very reactive, nonmetal elements. Because of this, they are ordinarily not found as pure elements in nature. Fluorine to iodine react very readily with metals to make salts by direct combination of the elements. Fluorine reacts rapidly with iron wool simply by coming into contact with the metal. This is a very fast reaction that happens at room temperature.

Chlorine reacts with iron wool when iron wool is heated very strongly. In this reaction, brown fumes of iron(III) chloride are formed. This reaction happens at a fairly fast rate, but only at high temperature. We can see by comparing these two reactions that fluorine is clearly more reactive than chlorine. Similar reactions happen with iron wool and liquid bromine and with iron wool and solid iodine. In both cases, the reactants must be heated very strongly. These reactions are slower reactions, and they only happen at high temperature.

In the reaction of iron with bromine, iron(III) bromide is formed. This can be represented in a symbol equation where two atoms of iron react with three molecules of bromine to produce two units of iron(III) bromide. The reaction of iodine with iron is similar. The product is iron(III) iodide in this case.

From the descriptions of the reactions of iron wool with the halogens, we can see that chemical reactivity decreases as we move down the group. This is the opposite to the trend in reactivity seen in group one A, the alkali metals. In this group, the most reactive metals are found at the bottom of the group. In terms of electronic configurations within the atoms, all of the halogens have seven outer shell electrons. For example, an atom of chlorine has atomic number 17. It has two electrons in the first shell, eight electrons in the second shell, and seven electrons in the outer shell.

When halogens react, they gain one more electron from another atom and form an ion with a negative one charge state. Halogens ordinarily get eight outer shell electrons and the same electronic configuration at a noble gas atom. In this example, a chlorine atom has changed into a chloride ion. The negative ions of halogen atoms are called halide ions. Halogen atoms gain one electron and form a halide ion. A halide ion is a single halogen atom that has gained one electron and a net negative charge state. Halide ions are found in metal halide salts. Metal halide salts are easily formed by direct combination of a halogen with a metal.

Potassium will react rapidly with bromine to form potassium bromide. Group one A metal halides are usually white salts, and they are often very soluble in water, giving solutions containing anions and cations. Whilst solutions of halide ions are usually colorless, solutions of the halogens in water are very different in appearance. The halogens chlorine, bromine, and iodine are all soluble in water to varying extents. The aqueous solutions have noticeable colors.

Chlorine forms a very pale-green solution in water. It is reasonably soluble, and it’s used to kill bacteria in drinking water in this way. Bromine forms an orange solution, whilst iodine forms a brown solution. It is often used as an antiseptic in this form.

When the halogen solutions are added to halide ion solutions, sometimes displacement reactions can occur. We will look at some of these in the next section of this video.

A displacement reaction is a chemical reaction in which one reactant displaces part of another reactant. A more reactive halogen can displace a less reactive halogen from its halide salt during a displacement reaction. An example of this is the reaction of aqueous chlorine with aqueous bromide ions. The chlorine solution is a pale-green color, and the potassium bromide solution is colorless.

Chlorine is more reactive than bromine and bromine will be displaced. The green color caused by the chlorine disappears as it displaces bromide ions from potassium bromide. An orange color appears as bromine molecules are formed. Bromine is more reactive than iodine. Bromine solution is orange. So, bromine can displace iodide ions from a solution of potassium iodide. Iodine is formed in this reaction. There is a color change in this reaction. The orange color of the bromine water disappears and is replaced by the brown color of the iodine solution. This reaction can be represented by a symbol equation where one molecule of bromine reacts with two units of potassium iodide. This reaction produces one molecule of iodine and two units of potassium bromide.

Now that we have learned about the physical and chemical properties of the halogens, let’s take a look at some questions.

Based on their position in the periodic table, which of the following elemental types can the majority of halogens be described as? (A) Metals, (B) metalloids, (C) inert gases, (D) nonmetals.

The halogens are located in group seven A of the periodic table towards the right-hand side. This part of the periodic table is where the majority of the nonmetals are located. In fact, the halogens are all nonmetals. Answer (A) is therefore incorrect. Answer (B) is also incorrect as metalloids form the boundary between metals and nonmetals. Elements like silicon and germanium form the boundary between conventional metal and nonmetal elements. Silicon and germanium are metalloids.

At room temperature, fluorine and chlorine are both found as gases. So, answer (C) could be correct. However, fluorine and chlorine, as with the other halogens, are both highly reactive elements. This means they are not inert, which means unreactive. Answer (C) is therefore an incorrect description of the halogens in general.

The best answer to the question “Which of the following elemental types can the majority of halogens be described as?” is nonmetals.

Which of the halogens is the most reactive?

The halogens are located in group seven A of the periodic table. From the top of the group to the bottom of the group, they are composed of fluorine, chlorine, bromine, iodine, and astatine. All of these elements are highly reactive nonmetals, and they react with metals like iron. If fluorine comes into contact with iron wool at room temperature, a fierce reaction starts immediately.

A similar reaction happens with iodine and iron wool, but the iron wool must be heated very strongly before it will react. This is a much slower reaction compared with the reaction between fluorine and iron wool. In fact, we see the reactivity of the halogens steadily decrease as we move down the group. So, fluorine is the most reactive of the halogens, whilst astatine, although it’s extremely rare, could be predicted to be the least reactive of the halogens.

To answer the question “Which of the halogens is the most reactive?” fluorine is the correct answer.

Now, it’s time to review the key points that we have learned in this video. The halogens are a group of reactive nonmetals that form group seven A of the periodic table. Reactivity decreases down the group. The colors of the halogens darken, and the melting points increase moving down the group. Halogens ordinarily exist as diatomic molecules. Halogen atoms contain seven electrons in their outer shell. Halogen atoms gain one electron when they react and become one minus charge state halide ions. A more reactive halogen can displace a less reactive halide ion from its solution.

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