Group 7 (Halogens)

The halogens are found in group 7 of the Periodic Table and are one electron away from a full outer shell of electrons. The halogens include the elements chlorine, bromine and iodine which all behave in similar ways due to similarities in their electron configurations.

What are the halogens?

The halogens are the elements found in the second-to-last group of the Periodic Table. They all have seven electrons in their outer shell and since they need only one more to complete their octet they are fairly reactive. The halogens typically form ionic compounds with elements in group 1 which have one outer electron that the group 7 elements can’t resist taking.

Properties of the group 7 elements

As you go down the halogens, from fluorine to astatine, the elements become darker in colour and have a higher boiling point. Boiling point increases as you go down the group because the mass of each element increases and they have more electrons around their nuclei. The more electrons an element has, the more intermolecular forces (London dispersion forces) it can form. Each halogen has the following characteristics at room temperature:

• Fluorine is a pale yellow gas

• Chlorine is a poisonous green gas

• Bromine is a toxic red-brown liquid

• Iodine is a dark grey solid which gives off a purple vapour when heated

• Astatine is a black solid

Unlike the group 1 metals, reactivity decreases as you go down the halogens. This means that fluorine, at the top of the group, is the most reactive. Fluorine is so eager to react with anything that it is almost never found as a pure element and it is so dangerous to work with that scientists avoid handling it in reactivity experiments. Chlorine is less reactive and much more manageable, and added to water in small quantities to kill microorganisms to make it safe to drink.

Explanation for trends in reactivity

The group 7 elements want to gain one more electron so that they have a stable electronic structure. The smaller the atom, the easier it is to grab an electron from another atom, making the atom more reactive. As you go down group 7, the atomic radius increases and it becomes more difficult to attract another electron.

Displacement reactions involving halogens

In displacement reactions, a more reactive element will displace (replace) a less reactive one. Think of it as the introduction of an attractive geordie on love island, forcing someone else out of a relationship and leaving them by themselves. Displacement reactions involving halogens and halogen ions (halides) can be used to provide evidence for the order of reactivity of the halogens.

An example is the addition of chlorine to a solution of potassium iodide. Chlorine is more reactive than iodine so will take its place in the molecule, resulting in the formation of potassium chloride and iodine. The equation for the reaction would look like this:

Let’s say we add iodine to a solution of potassium bromide. This time we’re adding an element which is less reactive than the halogen in the compound so no reaction will take place.

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