Once again, showing how covalent bonds are formed using dot and cross diagrams is simpler at A level than at GCSE as you only need to show outer shell electrons. You need to be confident in working them out for unfamiliar molecules and in identifying lone pairs of electrons (this is going to be really important when we move on to shapes of molecules).
Covalent bond is a shared pair of electrons between two non-metal atoms (predominantly), and there is a strong electrostatic attraction between the nuclei of each atom and the shared pair. Just be aware that the shared pair of electrons is not always shared equally between the two atoms, in which case we will have a polar bond. This has a significant effect on the properties of many molecules.
An atom does not necessarily share all of its outer-shell or valence electrons.
Nitrogen can use its lone pair of electrons to make a dative covalent bond (both electrons in the shared pair are donated by one atom).
e.g. NH3 + H+ ⇾ NH4+
You should also be aware that some molecules such as boron trichloride and aluminium chloride form dimers using a dative covalent bond.
Elements in Period 3 and above have the option to expand their octet using empty d orbitals, which means they can form many more bonds and they have more than 8 electrons in their outer shell as a result. Sulphur is a good example of this.
Finally, when drawing a dot / cross diagram for an anion, we need a third symbol to indicate the electron that has been gained:
Practice questions
Draw dot and cross diagrams for the following covalent molecules and ions:
- H2O
- HI
- CH2Cl2
- F2O
- C2H4
- amide ion NH2–
- oxonium ion H3O+
- carbonate ion CO3 2-
- HNO3
- N2O
- Be2Cl4 (clue: it exists as a dimer)
- a molecule composed of ammonia and boron trifluoride
- PF3
- SF6
- phosphorus hexachloride ion PCl6 –
Answers
