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The halogens show trends in color and in melting and boiling points as summarized in table 6. Notice that there are distinct trends in these properties that change with halogen size. For example, the smallest halogen, F2, has the lowest melting point and boiling point, and it exists as a gas at room temperature. Cl2 is larger and has higher melting and boiling points than F2, although it also exists as a gas at room temperature. Br2 is the next largest halogen in the group and is a liquid due to its melting and boiling points, which are higher than those of Cl2 or F2. It is interesting that only two elements, bromine and mercury, are liquids at room temperature. The largest halogens, I2 and At2, are solids due to their high melting and boiling points.
The halogens show trends in color and in melting and boiling points as summarized in table 6. Notice that there are distinct trends in these properties that change with halogen size. For example, the smallest halogen, F2, has the lowest melting point and boiling point, and it exists as a gas at room temperature. Cl2 is larger and has higher melting and boiling points than F2, although it also exists as a gas at room temperature. Br2 is the next largest halogen in the group and is a liquid due to its melting and boiling points, which are higher than those of Cl2 or F2. It is interesting that only two elements, bromine and mercury, are liquids at room temperature. The largest halogens, I2 and At2, are solids due to their high melting and boiling points.
Table 6: Physical properties of the halogens
Earlier, you observed that the reactivity of alkali metals increases as you move down through the group. The opposite trend is observed for halogens—halogen reactivity decreases as you move down through the group. Fluorine is the most reactive, followed by chlorine, then bromine, and then iodine. Astatine, the largest halogen, is the least reactive. This trend is due to the tendency of halogens to gain electrons from other atoms to complete their outer valence shell. Smaller halogens exert a greater attractive force to gain electrons because the distance between the nucleus and the incoming electron is smaller than in a larger halogen.
You see this trend in reactivity in reactions where halogens take electrons from other substances such as hydrogen (H2), water (H2O), and metals. Fluorine reacts explosively with alkali metals, followed by chlorine, which reacts less vigorously, and so on down to iodine, which reacts very mildly. In all of these reactions, electrons are transferred rather than shared. The halogen atom gains an electron, converting it to a halogen ion with a charge of −1. Such ions are very stable because they have noble gas configurations. Chemists use the term halide when they refer to compounds with halogen ions.
The pattern of reaction and trend in reactivity are also observed in reactions between two different halogens. A halogen higher in the group has a greater ability to gain an electron than a halogen below it as long as the lower halogen has an electron it can give up. Thus, any halogen in its elementary state can gain an electron from any other halide ion, X–, below it in the group.
For example, fluorine, F2, reacts with the chloride ions, Cl−, by attracting the electrons from it to form chlorine, Cl2, and fluoride ions, F−. Note that bromine, Br2, does not react with chloride ions, Cl−, because chlorine is more reactive than bromine. Electrons can only pass from a less reactive halogen to a more reactive halogen.
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