The properties of a substance depend on what atoms are present and how its atoms are bonded (held together) . For your GCSE Chemistry, you need to know about three types of bonding - ionic, covalent and metallic. This is the second of four quizzes on bonding and it is all about covalent bonding, in which molecules are formed by atoms sharing electrons.
When non-metals join together they form covalent bonds. A covalent bond involves sharing a pair of electrons between atoms to form a molecule. Some molecules are small and consist of just two atoms joined together but others are huge and can contain thousands of atoms bonded with each other. It is estimated that a human DNA molecule could contain over two billion individual atoms!
A good example of how the properties of covalently bonded molecules depend on their size is seen when you consider crude oil. This is a mixture of different sized molecules that are mainly hydrocarbons from the alkane family of chemicals. Any differences in the properties of the different hydrocarbons must therefore be due to the size of the molecule as they all contain the same two atoms - hydrogen and carbon. The smallest molecules are methane, ethane, propane and butane which have a total of 5 to 14 atoms covalently bonded together. These chemicals have very low melting and boiling points and are highly flammable gases. Octane (a key chemical in petrol) has 26 atoms in its molecule and has a higher melting and boiling point than the smaller molecules but is still highly flammable. The molecules in tar have hundreds of atoms and it is a thick, sticky liquid that is very difficult to light.
You would normally associate giant structures with ionic bonding, however, a few covalently bonded substances have giant structures too. One example is silica which you probably know better as the hard mineral called quartz, the main component of many forms of sand. Another example of these giant covalent structures is the allotropes of carbon. Graphite is used in lubricant greases and to make pencil lead as it is one of the softest solids, a diamond is a very precious stone that is used in jewellery and for making diamond tipped cutting wheels as it is the hardest solid. They are very different substances, however they are both the same element. The differences in their properties are due to how the atoms are held together. Diamond is a giant three dimensional lattice where each carbon atom is joined to its neighbours by very strong covalent bonds. Graphite consists of layers of carbon atoms that can slide easily over each other.
Try this quiz and see how much you know about atoms sharing electrons in covalent bonding.
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