Sodium burns with a brilliant flame if heated and placed into chlorine producing white sodium chloride (common salt) crystals Chlorine is a highly reactive non-metal; steel wool will burn in the gas if it is gently heated. Sodium is a highly reactive metal, dissolving exothermically in cold water, sometimes bursting into flames. Why are compounds formed? What is the driving force behind these reactions? Why do the elements combine?

To find the answer, we must look carefully at the Noble or Inert gases. The word inert means that these gases do not undergo any chemical reactions. These Noble gases will not react with fluorine for example, the most reactive non-metal, or even caesium, the most reactive metal. Leave me alone Noble gases are different from other elements because they have full outer shells of electrons. Other atoms react, because they are trying to achieve a state where they too can have full outer shells. He Ne Ar These shells are full! Noble Gases

How does it work in practice? Metals can loose electrons in order to have full outer shells and non-metals can gain electrons in order to have full outer shells. Sodium has an atomic number of 11 which means that it has 11 protons (p + ) in its nucleus. As a neutral atom, it must also have 11 electrons (e _ ). Losing one electron would leave it with a full outer shell. Na is “written” as 2,8,1

How does it work in practice? Metals can loose electrons in order to have full outer shells and non-metals can gain electrons in order to have full outer shells. Sodium has an atomic number of 11 which means that it has 11 protons (p + ) in its nucleus. As a neutral atom, it must also have 11 electrons (e _ ). Losing one electron would leave it with a full outer shell.

How does it work in practice? Metals can loose electrons in order to have full outer shells and non-metals can gain electrons in order to have full outer shells. Sodium has an atomic number of 11 which means that it has 11 protons (p + ) in its nucleus. As a neutral atom, it must also have 11 electrons (e _ ). Losing one electron would leave it with a full outer shell.

The sodium atom has now become a sodium ion. It has more protons than electrons (one more!) so it must have a positive charge. It is written as Na + + Chlorine has an atomic number of 17 which means that it has 17 protons (p + ) and 17 electrons (e _ ). Cl is “written” as 2,8,7 Na + is “written” as 2,8

Suppose the electron lost by the sodium atom was gained by the chlorine atom. It would now have a full outer shell. But now, chlorine would have more electrons than protons (one more!) and it would become a negative ion. It is written as Cl - - Cl - is “written” as 2,8,8

According to the laws of electrostatics, opposite charges attract and so the sodium ions and the chloride ions join together. It is called ionic bonding. Cl - Na + Cl - Na + Billions of ions bond in this way until massive crystals of salt are formed. The crystal shape is cubic because of the arrangement of the ions.

These pictures, “downloaded” from the Internet give you a better idea of the crystal formation Which is the more realistic? Can you see why the formula for sodium chloride is NaCl?

Formation of Lithium Fluoride Lithium, Li has an atomic number of 3. Its electron arrangement is 2,1 Fluorine, F has an atomic number of 9. Its electron arrangement is 2,7 Each Li atom loses an electron to a F atom to form the compound lithium fluoride. Li + F -

Formation of Lithium Fluoride Lithium, Li has an atomic number of 3. Its electron arrangement is 2,1 Fluorine, F has an atomic number of 9. Its electron arrangement is 2,7 Each Li atom loses an electron to a F atom to form the compound lithium fluoride. Li + F - + -

Formation of Magnesium Chloride Mg is 2,8,2; Cl is 2,8,7

Mg Cl -  MgCl 2

Now try some for yourself! Can you do potassium oxide (K 2 O) or aluminium oxide (Al 2 O 3 )?