In this lesson, we will learn:
• To recall properties of the halogens
• To understand the trend in properties found in the halogens
• To apply knowledge of electronic structure and bonding to explain the trends in halogens.
• We have already seen that the Periodic Table is arranged, top-left to bottom-right, by proton number and number of outer shell electrons. The number of outer shell electrons dictates the chemical properties of an element.
Therefore, it is easy to see which elements have similar properties to each other – they will be in the same column of the table as each other, the columns which we call groups!
• Just like with the alkali metals, the halogens are another example of a well-studied group of elements which display trends in their common properties as you go down the group.
• The halogens have the following properties: ∘ They are non-metals stable as diatomic molecules (this means at room temperature and pressure, they exist as molecules made of two atoms, e.g. Cl2). ∘ They are colored. ∘ They have relatively low melting and boiling points compared to other non-metals (except the noble gases). ∘ Halogens in elemental form are relatively toxic, reactive substances. ∘ They do not conduct heat or electricity. ∘ They are brittle as solids.
• As you go down the group, the properties of the elements change in the following ways: ∘ The melting and boiling point gets higher – starting as gases, bromine is a liquid while iodine is a solid. ∘ The color of the halogens gets darker – fluorine is pale yellow, followed by green chlorine, brown/purple bromine and purple iodine. ∘ The halogens get less reactive – fluorine, top of the group, is the most reactive element known. Iodine is the least reactive halogen (besides astatine which is often ignored due to its rarity).
• A more reactive halogen can displace a less reactive halogen in a salt, in a displacement reaction, for example:
∘ The equivalent reaction would not work if Br2 was reacted instead of F2 because Br is not as reactive as Cl. The halogens get less reactive going down the group because halogens react by attracting an electron into their outer shell. ∘ As we saw in lesson on Periodic Trends: Electronegativity, electronegativity decreases going down the group so fluorine is much more able to attract an electron and fill its outer shell than chlorine, which is more able than bromine, which is more able than iodine. ∘ Therefore, fluorine is the most reactive and iodine the least reactive.
• The halogens have a valence of 1, and can form covalent bonds with other non-metals or ionic bonds with metal atoms, where they receive an electron and form a single negative charge – ions of halogen atoms will nearly always be -1 charge.
Halogens: A summary
Recall the properties of the halogens.
Sort the halogens according to the properties listed below:
Explaining the trend in properties of the halogens.
A student in a school laboratory has a solution of sodium bromide (NaBr) in a beaker. To this, she adds an equal amount of chlorine water (chlorine dissolved in water).
Group 7: The halogens
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