Group 7: properties and reactions

In a nutshell

The elements of Group $$7$$​ are called halogens. Chlorine, bromine and iodine are three examples of halogens. They exist as diatomic molecules; each halogen has different appearances at room temperature and in solution. They can form metal and non-metal halides. Halogens undergo displacement reactions.

Appearance

At room temperature: chlorine is a green gas, bromine is a brown liquid and iodine is a grey solid. In aqueous solution: chlorine is pale green, bromine is orange and iodine is dark brown.

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Diatomic structure​​

The octet rule specifies that atoms are stable when they have a 'full' outer electron shell. A full outer shell is equal to eight electrons. This can be achieved by covalent bonding. A covalent bond is a shared pair of electrons between two non-metal atoms. 

Group numbers (columns) in the periodic table indicate how many electrons are in the outer shell of an atom. Halogens are in Group $$7$$, which means there are seven electrons in the outer shell of a halogen atom. Halogens achieve a full outer shell by forming diatomic molecules ($$X_2$$​) via a covalent bond

Examples

• Chlorine $$(Cl_2)$$
  
• Bromine $$(Br_2)$$​​
  
• Iodine $$(I_2)$$​​

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Melting and boiling points

Going down Group $$7$$​, the relative atomic mass ($$A_r$$​) increases. This means the relative molecular mass ($$M_r$$​) of each diatomic halogen molecule ($$X_2$$​) increases down the group. This is associated with a greater number of electrons, which increases the strength of forces between $$X_2$$​ molecules. This increases the melting and boiling points down Group $$7$$​, as a higher amount of (heat) energy is required to break the intermolecular forces between halogen molecules. 

Example 

• Iodine ($$I$$​) $$A_r$$​ = $$127$$​ so $$M_r$$​ of $$I_2$$​ = $$254$$​​
• Bromine ($$Br$$​) $$A_r$$​ = $$80$$​ so $$M_r$$​ of $$Br_2$$​ = $$160$$​​

There are stronger intermolecular forces between $$I_2$$​ molecules compared to $$Br_2$$, due to a higher $$M_r$$​. $$I_2$$ therefore has a higher melting and boiling point, compared to $$Br_2$$.

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Non-metal halides

Non-metal halides are formed by covalent bonding. 

Examples

Hydrogen chloride ($$HCl$$​) and hydrogen bromide ($$HBr$$​)

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Metal halides 

Halogen atoms have seven electrons in their outermost shell but atoms require eight electrons in their outer shell to be stable. Halogen atoms satisfy this rule by gaining one electron to form $$1-$$​ ions. These ions are called halide ions ($$X^-$$). 

Halide ions can form ionic bonds with positive metal ions. Ionic bonding is electrostatic forces of attraction between two oppositely charged ions

Example

​$$calcium + bromine \rightarrow calcium\,bromide$$​​

$$Br_2+ Ca \rightarrow CaBr_2$$​

Displacement reactions 

Halogen molecules ($$X_2$$​) can react with halide compounds according to the following rules:

• The reactivity of halogens decreases down the group
• Chlorine is above bromine in the periodic table
• This means it can displace bromine in a metal halide compound
• Iodine is below bromine in the periodic table
• This means it cannot displace bromine in a metal halide compound
• This rule applies to the rest of the halogens