Group 17: The Halogens
Table of contents
Figure 1.1 A visual to place the halogens relative to the periodic table. (Not drawn to scale.)
The halogens can be found on the left-hand side of the noble gases. These five toxic, non-metallic elements make up Group 17 of the periodic table and consist of: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Although astatine is radioactive and only has short-lived isotopes, it behaves similar to iodine and is often included in the halogen group. Since the halogen elements have seven valence electrons, they only require one additional electron to form a full octet. This characteristic makes them more reactive than other non-metal groups.
Halogens form diatomic molecules (X2 where X refers to a halogen atom) when they are in their pure states. The bonds in these diatomic molecules are non-polar covalent single bonds. However, halogens readily combine with most elements and are never seen uncombined in nature. As a general rule of thumb, fluorine is the most reactive halogen and astatine is the least reactive. All halogens form sodium salts that have similar properties. Halogens are present as halide anions with a negative one charge (i.e. Cl-, Br-, etc.). Replacing the -ine ending with an -ide ending forms the names for these halide anions. (For example, Cl- becomes chloride.) In addition, halogens serve as oxidizing agents, because they exhibit the property to oxidize metals. Therefore, most of the chemical reactions that involve halogens are oxidation-reduction reactions in aqueous solution. The halogens often form single bonds, when in the -1 oxidation state, with carbon or sometimes nitrogen in organic compounds. When a halogen atom is substituted for a covalently-bonded hydrogen atom in an organic compound, the prefix halo- can be used in a general sense, or the prefixes fluoro-, chloro-, bromo-, or iodo- for specific halogen substitutions. Halogen elements could cross-bond with each to form diatomic molecules with polar covalent single bonds.
Chlorine (Cl2) was the first halogen to be discovered in 1774, followed by iodine (I2), bromine (Br2), fluorine (F2), and astatine (At), which was discovered last in 1940. The name "halogen" is derived from the Greek roots hal- (which means "salt") and -gen (which means "to form"). Together these words combine to mean "salt former", which is in reference to the fact that halogens form salts when they react with metals. Halite is the mineral name for rock salt, which is a natural mineral consisting essentially of sodium chloride (NaCl). Lastly, the halogens are also relevant to real-life, whether it be the fluoride that goes in toothpaste, the chlorine that disinfects drinking water, or the iodine that is responsible for the production of thyroid hormones in one's body.
Figure 1.2 Shows all of the halogen elements in respect to its mass and atomic numbers.
Table 1.1 Electron configurations of the halogens.
The periodic trends observed in the halogen group:
Melting and Boiling Points: increases down the group
The melting and boiling points increase down the group because of the van der Waals forces. The size of the molecules increases down the group. This increase in size means an increase in the strength of the van der Waals forces.
F < Cl < Br < I < At
Table 1.2 Melting and Boiling Points of Halogens
Atomic Radii: increases down the group
The size of the nucleus increases down a group because there is a higher number of protons and neutrons. Also, more energy levels are added on after passing each period. This results in a bigger orbital, and therefore a bigger radius.
F < Cl < Br < I < At
Table 1.3 Atomic Radii of Halogens
Ionization Energy: decreases down the group
If the outer valence electrons are not near the nucleus, it will not take as much energy to remove them. Therefore, the energy required to pull off the outermost electron will not be as high for the elements at the bottom of the group since there are more energy levels. Also, the high ionization energy makes the element appear non-metallic. Iodine and astatine display metallic properties so ionization energy should decrease down the group.
At < I < Br < Cl < F
Table 1.4 Ionization Energy of Halogens
Electronegativity: decreases down the group
The number of valence electrons increases due to the increase in energy levels as the elements progress down the group. The electrons are not as near to the nucleus anymore. Therefore, the nucleus and the electrons are not as attracted to each other as much. An increase in shielding is observed. Electronegativity will therefore decrease down the group.
At < I < Br < Cl < F
Table 1.5 Electronegativity of Halogens
Electron Affinity: decreases down the group
Since the atomic size increases down the group, electron affinity will decrease. An electron will not be as attracted to the nucleus, resulting in a low electron affinity. However, fluorine has a lower electron affinity than chlorine. This can be explained by the small size of fluorine, compared to chlorine.
At < I < Br < F < Cl
Table 1.6 Electron Affinity of Halogens
Reactivity of Elements: decreases down the group
Down a group, the atomic radius gets bigger, and there is an increase in the amount of energy levels. This results in less attraction of the valence electrons. It also decreases because electronegativity decreases down a group, which means that there will be less interactions with the electrons in terms of "pulling". Also, since there is a decrease in oxidizing ability down a group, the reactivity of the elements will decrease as well.
At < I < Br < Cl < F
Hydrogen Halides and Halogen Oxoacids
Halide- Halogen reacts with another element [which does not exceed that halogen's electronegativity] to form a binary compound. More relevant to look at the hydrogen halides; a halogen reacts with hydrogen to form a halide "HX" (where X refers to a halogen element):
These hydrogen halide compounds can readily dissolve in water to form hydrohalic (hydrofluoric, hydrochloric, hydrobromic, hydroiodic) acids in a range of concentrations. Acidic strength of the hydrogen halides:
Hydrofluoric acid can etch glass, as well as certain inorganic fluorides over a long period of time.
A halogen oxoacid is an acid which has hydrogen, oxygen, and halogen atoms. Halogen oxoacids demonstrate that the acidity can be found by analyzing their structures. The halogen oxoacids consist of the following:
When finding the acidity of the oxoacid, the length of the bond is no longer a factor. This is due to the fact that for each acid, there is a similar foundation: a hydrogen atom is bonded to an oxygen atom. Electronegativity becomes the key factor in figuring out the acidity of the oxoacid. Acidic strength will increase depending on the number of oxygen atoms bound to the central atom.
States of Matter at Room Temperature
Table 1.7 States of Matter and Appearance of Halogens
Explanation for Their Appearance
The color of the halogens is based on the absorption of visible light by the molecules, which leads to excitation. Fluorine absorbs violet light, and therefore appears light yellow. Iodine, on the other hand, absorbs yellow light and appears violet (color wheel). This shows that the color of the halogens becomes darker down the group:
In closed containers, liquid bromine and solid iodine are in equilibrium with their vapors, which can often be seen as colored gases. *Although the color for astatine is unknown, it is assumed that astatine must be darker than violet (iodine), i.e. black, based on the periodic trend for colors observed in the halogens.
V. Oxidation States of Halogens in Compounds
Rule: Halogens have an oxidation state of -1. Although if the halogen is bonded to oxygen or to another halogen that is more electronegative, its oxidation state will not be -1.
Another Exception: If fluorine exists in its elemental form (F2), its oxidation state is zero.
Table 1.8 Oxidation States of Halogens
Question: Why does fluorine always have an oxidation state of-1 in its compounds?
Applications of Halogens
Fluorine: Although fluorine is very reactive, it still serves many purposes in the real world. For example, it is a key component of the plastic polytetrafluoroethylene (called Teflon-TFE by the DuPont company) and certain other polymers, often referred to as fluoropolymers. Chlorofluorocarbons (CFCs) are organic chemicals that were used prior to the discovery that they were depleting the ozone layer in the atmosphere, especially in polar regions of the earth. They were used as refrigerants, as well as propellants in aerosols. Hydrochlorofluorocarbons (HFCs) were then used as an alternative. Fluoride is used in other compounds to produce many results. For example, it is used in toothpaste and water to help reduce tooth decay. Fluorine also exists in clay, which is used to make some ceramics. Fluorine is associated with generating nuclear power as well. In addition, fluorine is used to produce fluoroquinolones, which are antibiotics. Below is a list of some of fluorine's important inorganic compounds.
Table 1.9 Important Inorganic Compounds of Fluorine
Chlorine: Chlorine has many uses in modern-day life. For example, chlorine is used to disinfect drinking water as well as swimming pools. Compounds which generate chlorine in water are also used, such as sodium hypochlorite (NaClO) in bleach. Hydrochloric acid (a solution of hydrogen chloride, HCl, dissolved in water), sometimes referred to as muriatic acid, is a very commonly used acid in industry and laboratories. Chlorine is also present in polyvinyl chloride (PVC), and a couple other polymers. PVC is used in wire insulation, pipes, as well as in electronics. In addition, chlorine is very useful in relation to its use in medicine. For example, medicinal products that contain chlorine help fight infections, allergies, and diabetes. Various compounds used in medicine are used as the neutralized hydrochloride form. Chlorine is also used to help sterilize the machines in hospitals, limiting the growth of infections in patients. Chlorine is used in agriculture as well. For example, there are many pesticides which contain chlorine. DDT (dichlorodiphenyltrichloroethane) was used as an agricultural insecticide, but use was discontinued.
Bromine: Bromine is often used in flame retardants because of its ability to resist fire. They are also found in the pesticide, methyl bromide, which can help store crops and eliminate the spread of bacteria. However, the excessive use of methyl bromide has been shown to destroy the ozone layer. Due to this realization, this pesticide was eventually banned because of its negative effects on the environment. Bromine is involved in gasoline production as well. Other uses of bromine include: the production of photography film, the content in fire extinguishers, as well as in particular drugs to treat pneumonia and Alzheimer's disease.
Iodine: Iodine is important in the proper functioning of the thyroid gland in one's body. If the body does not receive adequate iodine, a goiter (enlarged thyroid gland) will form. Table salt now contains iodine to help promote proper functioning of the thyroid hormones. Iodine is also used as an antiseptic (kills germs). Solutions used to clean open wounds will most likely contain iodine in them. Iodine is also found in disinfectant sprays. In addition, silver iodide is important for photography development.
Astatine: Since astatine is radioactive, and not common, there are no proven uses for this halogen element. However, there is speculation that this element could possibly aid iodine in regulating the thyroid hormones. Also, astatine 211 has been used in mice to aid the study of cancer.
VII. Outside Links
Color wheel referenced to in the text: http://www.wou.edu/las/physci/ch462/c-wheel.gif
1. Why does fluorine always have an oxidation state of -1 in its compounds?
(please highlight to see text) Answer: Electronegativity increases across a period, and decreases down a group. Therefore, fluorine has the highest electronegativity out of all of the elements. Since fluorine has seven valence electrons, it only needs one more electron to acheive a noble gas configuration (eight valence electrons). Therefore, it will be more likely to pull off an electron from a nearby atom.
2. Find the oxidation state of the halogen in each problem:
3. What are three uses of chlorine?
Answer: disinfecting water, pesticides, and medicinal products
4. Which element(s) exist(s) as a solid in room temperature?
Answer: iodine and astatine
5. Do the following increase or decrease down the group of halogens?
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