Group 7: the halogens
Definition
Diatomic molecule
A molecule containing two atoms, such as chlorine, Cl2\text{Cl}_2Cl2, or oxygen, O2\text{O}_2O2.
- Group 7 contains the halogens: fluorine, chlorine, bromine and iodine.
- They exist as diatomic molecules, such as Cl2\text{Cl}_2Cl2 and Br2\text{Br}_2Br2.
- Each atom has seven outer electrons, so it gains one electron to complete its outer shell, forming a −1-1−1 halide ion.
- At room temperature their colour and state change down the group:
- Fluorine is a pale yellow gas.
- Chlorine is a pale green gas.
- Bromine is a red-brown liquid.
- Iodine is a grey-black solid.
- So going down the group the halogens get darker and denser, and their melting and boiling points increase as the molecules get larger.
Key Idea
- All halogens form −1-1−1 ions by gaining one electron.
- They react with metals to form ionic compounds and with non-metals by sharing electrons.
Reactivity decreases down Group 7
Definition
Shielding
The reduction in the nucleus’s attraction to an outer or incoming electron caused by electrons in inner shells.
- Reactivity of the halogens decreases down the group, the opposite of Group 1.
- Reacting means gaining an electron, so a halogen is more reactive when it can attract an electron more strongly.
- Down the group, the outer shell is further from the nucleus and there is more shielding from inner shells.
- This weakens the attraction for an incoming electron, so the electron is gained less easily and the halogen is less reactive.
Note
Group 1 gets more reactive down the group; Group 7 gets less reactive, but both trends come from the same idea of distance and shielding.
Displacement reactions show the order of reactivity
Definition
Displacement reaction
A reaction in which a more reactive element replaces a less reactive element in a compound.
- A more reactive halogen will displace a less reactive halogen from a solution of its salt.
- For example, chlorine displaces bromine from potassium bromide solution.
- Cl2+2KBr→2KCl+Br2\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2Cl2+2KBr→2KCl+Br2
- The solution changes colour as the displaced bromine is formed.
- The results follow the reactivity order chlorine > bromine > iodine:
- Chlorine displaces bromine and iodine.
- Bromine displaces iodine only.
- Iodine displaces neither, because it is the least reactive.
Self review
- Give the colour and state of chlorine, bromine and iodine.
- Answer: chlorine a pale green gas, bromine a red-brown liquid, iodine a grey-black solid.
- What is the trend in reactivity down Group 7, and why?
- Answer: it decreases, because the outer shell is further from the nucleus and more shielded, so an electron is gained less easily.
- Write the equation for chlorine displacing bromine from potassium bromide.
- Answer: Cl2+2KBr→2KCl+Br2\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2Cl2+2KBr→2KCl+Br2.
- Can iodine displace chlorine? Explain.
- Answer: no, because iodine is less reactive than chlorine.
