Group 17 / VII Elements – The Halogens
In this lesson, we look at Group 17 elements, also called the halogens.
Note: Singapore O-Level / SEC uses Group 17. Cambridge IGCSE still writes Group VII. They are the same group.
The elements in this group are fluorine, chlorine, bromine, iodine and astatine. They are all non-metals.
Structure of halogen molecules
Halogens exist as molecules. Each molecule is made of two atoms covalently bonded together. That is why we call them diatomic molecules.
The forces between the molecules are weak, so only a little energy is needed to overcome these weak intermolecular forces. Hence, halogens have relatively low melting and boiling points.
Melting and boiling points down the group
As we go down the group, the molecules get bigger. The intermolecular forces between bigger molecules are stronger, so the melting and boiling points increase.
At room temperature:
- fluorine and chlorine are gases
- bromine is a liquid
- iodine is a solid
Colour of the halogens
All the halogens are coloured. The colour gets darker down the group.
At room temperature and pressure:
| Element | Appearance |
|---|---|
| Fluorine | pale yellow gas |
| Chlorine | pale yellow-green gas |
| Bromine | red-brown liquid |
| Iodine | grey-black solid |
This table is for the element itself, not the aqueous solution.
Electronic configuration and halide ions
Each halogen atom has 7 electrons in its outermost shell. It only needs to gain 1 electron to achieve a stable noble gas octet electronic configuration.
That is why halogens form negative ions with a charge of –1. These ions are called halide ions.
When a halogen forms a halide, the ending changes from -ine to -ide:
| Halogen | Halide ion |
|---|---|
| fluorine | fluoride, F⁻ |
| chlorine | chloride, Cl⁻ |
| bromine | bromide, Br⁻ |
| iodine | iodide, I⁻ |
Reactivity of the halogens
Halogens are very reactive non-metals. They react with metals to form ionic compounds.
For example, when sodium burns in chlorine, sodium chloride is formed, and a bright flame can be seen.
Reactivity decreases down Group 17 / VII.
Fluorine is the most reactive non-metal in the Periodic Table. Chlorine is more reactive than bromine, and bromine is more reactive than iodine.
Displacement reactions (preview)
A more reactive halogen can displace a less reactive halogen from its halide solution.
We will look at these displacement reactions in the next lesson.
Summary
- Halogens are reactive non-metals that exist as diatomic molecules.
- Their melting and boiling points are low, but increase down the group.
- They are coloured, and the colour intensity increases down the group.
- They are all reactive, and reactivity decreases down the group.
- A more reactive halogen can displace a less reactive halogen from its halide solution.
- All halogens are strong oxidising agents.
Watch the lesson
Watch the video lesson on this topic, then go on to the displacement reactions video.
