| Element | Colour | State at room temp |
|---|---|---|
| Chlorine (Cl2) | Green | Gas |
| Bromine (Br2) | Red | Liquid |
| Iodine (l2) | Grey | Solid |
Chlorine is a toxic gas, so should be handled in a fume cupboard.
If you look at the trends in the physical properties of the halogens, Cl2, Br2, I2 you can make predictions about the properties of the other halogens.
| Element | Colour | State at room temp |
|---|---|---|
| Fluorine (F2) | Yellow | Gas |
| Astatine (At2) | Black | Solid |
Group 7 elements are called the Halogens. As you go up group 7 (decreasing atomic number), the elements become more reactive. For example, fluorine is the most reactive and astatine is the least reactive.
A more reactive halogen will displace a less reactive halogen, e.g. chlorine will displace bromine:
By reacting a halogen solution with a potassium halide solution and making observations, the order of their reactivity can be deduced:
| Potassium chloride, KCl(aq) | Potassium bromide, KBr(aq) | Potassium iodide, KI(aq) | |
|---|---|---|---|
| Chlorine, Cl2(aq) | No change | Colourless to orange | Colourless to brown |
| Bromine, Br2(aq) | No change | No change | Colourless to brown |
| Iodine, I2(aq) | No change | No change | No change |
From the above results, chlorine displaces both bromine and iodine, and bromine displaces iodine. Therefore the order of reactivity is: chlorine is more reactive than bromine, which in turn is more reactive than iodine.
The higher up we go in group 7 (halogens) of the periodic table, the more reactive the element. The explanation concerns how readily these elements form ions, by attracting a passing electron to fill the outer shell.
In fluorine the outer electron shell is very close to the positively charged nucleus, so the attraction between this nucleus and the negatively charged electrons is very strong. This means fluorine is very reactive indeed.
However, for iodine the outer electron shell is much further from the nucleus so the attraction is weaker. This means iodine is less reactive.