Covalent molecules often are depicted with Lewis structures,
which are interpreted to discern the shape of the molecule.
(bent shape)
FUNDAMENTAL STEPS FOR DRAWING LEWIS STRUCTURES
1. Find the sum of valence electrons of all atoms in the molecule.
• If it is an anion, add one e- for each negative charge.
• If it is a cation, subtract one e-for each positive charge.
2. Arrange the atoms and connect them with pairs of electrons.
• The central atom is the least electronegative element that isn’t hydrogen. It’s usually first in the formula. It
usually has the highest bonding capacity.
3. Keeping track of electrons, put pairs of electrons on the outer atoms until they have octets or until you run out of e-.
(Remember, H requires only 2 electrons and makes 1 bond.)
4. Place extra electrons on the central atom.
5. If you run out of electrons before everything has an octet, make multiple bonds. Do not do this unless you have to!
6. For ions, put the symbol in brackets and put the charge outside.
7. If needed, draw resonance structures and use formal charge to evaluate for major structures.
Following these steps is the reliable way to obtain correct answers – learn them!
MOLECULAR SHAPES ARE DISCERNED FROM LEWIS STRUCTURES (or other modeling)
A MOLECULE’S SHAPE AND ITS ATOMS’ ELECTRONEGATIVITY DETERMINES POLARITY
bond polarity
• Some covalent bonds have an unequal sharing of electrons.
selected
electronegativity
values
•
Electronegativity: a measure of the relative ability of an atom to attract shared
electrons in a bond
§ Bonds between atoms with an approximate electronegativity difference
> 0.5 and < 2 are polar.
§ Polar (covalent) bond: a bond with a partially positive end and a
partially negative end due to unequal sharing of electrons
§ Nonpolar (covalent) bond: a bond with equal sharing of electrons
between atoms
molecular polarity
• Polar molecule: a molecule with a partially positive end and a partially negative end
• Polar molecules have at least one polar bond.
• Nonpolar molecule: a molecule that isn’t polar.
nonpolar molecule examples
polar molecule examples