Bonding - Covalent

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!

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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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  1. Methane is covalently bonded. How many bonds are involved?
    The carbon forms one covalent bond with each hydrogen atom, the formula of methane is therefore CH4
  2. Which of these has molecules with two atoms joined by a covalent bond?
    Bromine is described as a diatomic molecule
  3. Some covalent substances have huge numbers of atoms bonded together by a network of covalent bonds. This is called...
    Diamond is an example of a substance with a giant covalent structure
  4. A water molecule is covalently bonded. How many covalent bonds does it have?
    Oxygen has a valency of 2 so it only ever forms two covalent bonds. You can work this out for yourself from the electron structure
  5. How many covalent bonds can nitrogen form?
    It has 3 unpaired electrons in its outermost energy level (electron shell) therefore will form 3 covalent bonds
  6. How many electrons are involved in a covalent bond?
    Covalent bonds are formed from pairs of electrons, one from each atom involved
  7. Covalent bonding occurs when what type of atoms join?
    A metal and a metal form an alloy with metallic bonding present. When a metal and non-metal bond, it is always ionic bonding and for the GCSE, noble gases are regarded as being completely unreactive
  8. Where do the electrons for a covalent bond come from?
    At GCSE level, bonding always involves the outermost electrons
  9. What happens to electrons in covalent bonding?
    Non-metals can only lose electrons under extreme conditions, it is much easier for them to share electrons to become stable
  10. Pick the correct combination of properties of most covalent compounds.
    Most covalent compounds do not conduct electricity because they do not contain any ions and have low melting and boiling points as the bonds between the molecules are weak and can be easily broken by increasing the temperature

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