Swimming, one of the oldest and most universal sports, has been practised as a sport since Ancient Greek times

These notes are designed as a guide on how to lead the session, and are written in a script format. If you wish to lead the session in a different way please feel free to do so.  

Probably the most recognised link between swimming pools and chemistry is the element chlorine. It is associated with the ‘chlorine’ smell many people have experienced at the swimming pool. At the end of the module we will learn how chlorine is actually used in pools but before that we will look at chlorine and the periodic group it belongs to. 

We will also start to explore electron configurations and ionic bonding. Chlorine is a halogen and is in group 7 of the Periodic Table. Other halogens that are commonly encountered are fluorine, bromine and iodine. The table below compares the appearances of these four elements at room temperature. 

 Halogen Appearance
 Fluorine, F  Pale yellow gas
 Chlorine, Cl   Yellowy green gas 
 Bromine, Br   Reddish brown volatile liquid 
 Iodine, I   Dark grey/purple crystalline solid

The halogens are regularly found in molecular form. An example of this is fluorine gas, F2. The two fluorine atoms are bonded covalently and share a pair of electrons in their outer energy level or shell. 

The halogens form ionic compounds with metal elements, an example of which is sodium chloride. Halogens react with metals to form salts - the name ‘halogen’ means salt producer. Chlorine forms chlorides, bromine forms bromides and iodine forms iodides. Ionic compounds are produced through ionic bonding. Ionic bonding occurs when a bond is formed between 2 atoms, one losing an electron and the other gaining that electron.

One atom will thus become a positive ion and the other a negative ion. Group 1 elements, such as sodium, will lose one electron when they react to form a positive ion. In sodium’s case this would be written as Na+. The halogens gain an electron where possible and become negative ions. In chlorine’s case this would be written as Cl−. As group 1 elements lose an electron and group 7 elements gain an electron it stands to reason that these elements regularly form ionic bonds with each other. Halogens also react with other metals and another example is the reaction between chlorine and iron.