C3aL4 Trends 2: The Halogens
Key Words
Covalent - bonds that share electrons.
Displace - knock out of the way.
Halide ion - negatively charged halogen atom
Halogen - element in Group 7
Ionic salt - neutral compound held together by attraction of opposite charges
Grade E
The halogens are a family of coloured gases that have seven outer shell electrons. They are in Group VII. They have the following properties:
- They are reactive, readily forming halide ions;
- They exists as molecules of two covalently bonded atoms;
- They are denser than air.
The group get less reactive as you go down, because the outer shell wants to gain an electron to achieve the noble gas structure.
Fluorine is a very toxic and reactive gas, while iodine is not that reactive, Fluorine has the outer shell near near to the nucleus, so that the extra electron is pulled in quickly.
The table shows some properties:
| Element | Symbol | Atomic Number | Melting Point (oC) | Boiling Point (oC) | Colour |
| Fluorine | F | 9 | -219 | -188 | Pale yellow |
| Chlorine | Cl | 17 | -101 | -34 | Yellow-green |
| Bromine | Br | 35 | -7 | 59 | Brown |
| Iodine | I | 53 | 114 | 184 | Purple |
| Astatine | At | 85 | 302 | 337 | Black |
Astatine is very radioactive, with a half life of a about 10 minutes.
Click HERE to watch a 15 minute film that sums up the chemical properties of both the alkali metals and the halogens.
Grade C
Hydrogen reacts with the halogens to form hydrogen halide molecules that are covalently bonded. Hydrogen halide molecules form gases. In this picture, you can see the full outer shell.
These gases dissolve readily in water, dissociating into positive and negative ions to form strong acids:
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Hydrogen fluoride becomes hydrofluoric acid;
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Hydrogen chloride becomes hydrochloric acid;
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Hydrogen bromide becomes hydrobromic acid;
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Hydrogen iodide becomes hydriodic acid.
Hydrogen + bromine ® hydrogen bromide
H2(g) + Br2(g) ® 2HBr(g)
In water, the molecules dissociate into ions:
HCl(g) + H2O(l) ® H+(aq) + Cl-(aq)
(Metals react with halogens to form metal halide salts, for example:
magnesium + chlorine ® magnesium chloride
Mg(s) + Cl2(g) ® MgCl2(s)
Many of these reactions are exothermic.
The trend as we go down the halogens is that the elements get less reactive. Fluorine is a very reactive element, while iodine is not very reactive. In the same way as there is a reactivity series for the alkali metals, there is also a reactivity series for the halogens.
If you blow chlorine gas onto potassium iodide soaked onto filter paper, you will see a brown patch where the iodine is displaced.
chlorine + potassium iodide ® potassium chloride + iodine
Cl2(g) + 2KI(aq) ® 2KCl(aq) + I2(s)
Grade A
Fluorine is the most reactive non metal. It readily picks up electrons from other atoms to form a negative ion. It achieves the electronic structure of neon.
The outer shell is near the nucleus, so electrons are powerfully attracted.
As the other elements have outer shells that are several layers out, they are further from the nucleus. Additionally there is shielding by other electrons of the nucleus. Therefore there is a lot less attraction for outside electrons, hence iodine is very much less reactive than the others.
Astatine would be even less reactive, but it doesn't last long enough to get involved.