In order to represent the bonds between atoms in a molecule, visual aids are needed. To draw three-dimensional structures in two-dimensions, Lewis structures are used. A Lewis structure is a symbol in which an atom's valence electrons are represented by dots placed around the element's symbol.
The Lewis structure of a compound shows how the valence electrons are arranged among the atoms in the molecule to show the connectivity of the atoms.
Instead of using two dots to indicate the two electrons that comprise the covalent bond, a line is substituted for the two dots that represent the two electrons.
To determine the number of electrons to be used to connect the atoms, find the sum of the valence electrons for all atoms in the molecule. This is done by simply adding up the number of valence electrons of the atoms in the molecule. If the species is an ion, add an electron for each negative charge or subtract an electron for each positive charge. Don't worry about keeping track of which electrons come from which atoms, only their total number is important.
Consider carbon dioxide, CO2. C has 4 valence electrons and each O has 6 valence electrons. Therefore, there is a total of 16 electrons to be placed in the Lewis structure.
Write the symbols for the atoms involved to show which atoms are connected together. Atoms are often written in the order in which they are connected in the molecule or ion; in HCN, carbon is the central atom.
Draw a single bond between each pair of atoms bonded together.
Complete the octets of the atoms bonded to the central atom. Remember that hydrogen atoms only need 2 electrons.
Place any remaining electrons on the central atom, even if doing so results in more than an octet.
There are no more electrons available in this example. The structure now has 6 electrons around the first oxygen, 6 electrons around the second oxygen and the two bonds on the carbon atom each have 2 electrons.
Total = 6 + 6 + 2 + 2 = 16 electrons
If the valence shell of the central atom is complete, you have drawn an acceptable Lewis structure. Carbon is electron deficient - it only has four electrons around it. This is not an acceptable Lewis structure.
If there aren't enough electrons to give the central atom an octet, try multiple bonds. Use one of the unshared pairs of electrons on the atoms bonded to the central atom to form double or triple bond.
Continue this process of making multiple bonds between the outer atoms and the central atom until the valence shell of the central atom is complete.
An image is shown of a central C connected to an O on each side with double bonds. Each O still has 4 dots around them.
Double check to make sure that you have used the correct number of electrons in the Lewis structure and that no atom exceeds its valence shell.
molecules with an odd number of electrons
molecules in which an atom has less than an octet (usually in B and Be compounds)
molecules in which an atom has more than an octet. This is the largest class of exceptions and consists of molecules in which there are more than eight electrons in the valence shell of an atom, in elements such as P, S, As, and I.