Lewis Structures & Chemical Bonding
Lewis structures are a simple way of showing the valence electrons and bonding tendencies of covalent compounds.
Drawing Lewis Structures of Atoms and Ions
1. Write the symbol of the element.
2. Count its number of valence electrons, taking into account its charge (extra electrons for anions, fewer for
cations). Remember that valence electrons do not include filled d-orbitals.
3. Draw the electrons around the element symbol as dots.
4. Write the charge, if any, on the top right corner of the element.
E.g. draw the Lewis structure for Cl-.
Drawing Lewis Structures of Covalent Compounds
1. Count the total valence electrons for all atoms or ions in the given compound.
2. Use valence e- to draw bonds connecting the atoms. Remember that single bonds contain 2 e- which count
towards the octets of both atoms they connect.
3. The rest of the valence e- are used to complete octets.
4. Make the molecule as symmetrical as possible. The general bonding will be provided for you.
5. If you are short on e-, try making double or triple bonds.
6. If there is more than one possible structure, draw the one with lower formal charges on its atoms.
Formal charge of an atom = valence e- - ½(bonding e-) - unpaired eE.g. draw the Lewis structures for O2, N2, H2O and CO2
Practice: draw the Lewis structures for NH4+, CHO2-, HOPO
Covalent Bonding
Recall that electrons are shared in covalent compounds between atoms but they are not necessarily shared equally. We
talked about electronegativity or electron affinity as the attraction an atom has for electrons. Atoms that have a higher
electronegativity will have a higher chance of having more electrons around it.
An uneven distribution of electrons between atoms in a compound causes
a dipole moment such as in HF. The electronegativity of fluorine is 4.0 and
the electronegativity of hydrogen is 2.1 so fluorine will attract more electrons
than hydrogen, resulting in an electron density that looks like the following.
If a molecule has a net dipole moment, then we say that it is polar. We show a
dipole moment by drawing an arrow pointing to the more electronegative atom.
On the other hand, some molecules share electrons evenly such as homonuclear diatomic molecules such as H2 and N2.
Their electron densities will look like the diagram below on the left.
In molecules with more than one atom such as H2O and CO2, we draw the same arrows for dipole moments.
CO2 is nonpolar while H2O is polar. This is because the dipoles of CO2 are equal and in opposite directions so they cancel
and result in no net dipole. The dipoles of H2O are not in opposite directions so they don’t cancel completely and result
in a net dipole, making H2O a polar molecule. The shapes of molecules have to the with the nonbonding electrons atoms
have and are beyond the scope of this course.
Practice: draw arrows representing the dipole moments of the following molecules. State the polarity of the molecule.
a) O2
b) CH4 c) NH3 d) CHFCl