Covalent Bonds and Structures

Chemistry 11Ms. McGrath

Covalent Bonds

Because covalent bonds range from nearly ionic to non-polar covalent, the shapes of molecular compounds have agreat variety.

We already know that ionic compoundshave a crystal like shape created by theorderliness of the ions.

Covalent Bonds – Lewis Structures

We will be analyzing and predicting the structures and properties of molecules. Lewisstructures are going to be very important in determining these.

Covalent Bonds – Lewis Structures

Draw the Lewis Structure for methanol, CH2O

Covalent Bonds – Lewis Structures

Step 1Determine the total number of valence electrons in all of the atoms in the molecule

Covalent Bonds – Lewis Structures

Step 1(1 C atom x 4e-) + (1 O atom x 6e-) + (2 H atoms x 1e-)= 4e- + 1e- + 2e-

= 7e-

Covalent Bonds – Lewis Structures

Step 2Draw a skeleton structure for the molecule. The atom with the largest number of unpaired electrons will be the central atom.Join the atoms with one pair of bonding electrons.

Covalent Bonds – Lewis Structures

Step 2O..

H:C:H

Covalent Bonds – Lewis Structures

Step 3Place lone pairs of electrons around all atoms, except the central atom, to form an octet of electrons. Hydrogen, of course, has only two valence electrons.

Covalent Bonds – Lewis Structures

Step 3..

:O:..

H:C:H

Covalent Bonds – Lewis Structures

Step 4(a) If all the valence electrons determined in Step 1 have not been used, add one or more lone pairs around the central atom to complete an octet of electrons(b) If all of the valence electrons have been used up but the central atom does not have an octet, move one or more of the lone pairs to form double or triple bonds between the central atom and an adjacent atom

Covalent Bonds – Lewis Structures

Step 4 ..:O:..

H:C:HThere are 12 electrons in the structure, which is the same as the total number in Step 1. Carbon, however, does not have an octet. Therefore, move one of the lone pairs around the oxygen atom to the position between the oxygen and carbon atoms to form a double bond.

Step 4

:O: .. .. H:C:H

Coordinate Covalent Bonds

A coordinate covalent bond is anothertype of covalent bond where a pair ofelectrons is shared between two atomsbut both electrons were originally partof one of the two atoms (i.e. bothelectrons of the shared pair come fromone of the bonded atoms).

Coordinate Covalent Bonds

Consider the formation of borontrifluoride (BF3):

Coordinate Covalent Bonds

One of the boron’s orbitals is empty andthe BF3 is attracted to molecules withlone pairs of electrons that can be donated.

Coordinate Covalent Bonds

Consider the reaction of BF3 with NH3

to form NH3BF3:

Coordinate Covalent Bonds

The bond between N and B is acoordinate covalent bond – it can’t beexperimentally distinguished from othershared pair covalent bonds.

Resonance Structures

More than one possible Lewis Structure

Draw the Lewis Structure for sulfur dioxide, SO2

Resonance Structures

The Lewis Structure suggests that SO2 contains a single and a double bond. In fact, experimental measurements determine that the bond lengths in SO2

are identical. SO2 contains two “one and a half” bonds.

Resonance Structures

These are models that give the same relative position of atoms as in Lewis Structures, but show different placing of their bonding pairs and lone pairs.It is important to know that resonance structures do not exist in reality.

Resonance Structures

It is important to know that resonance structures do not exist in reality. SO2 does not shift back and forth from one structure to another. An actual SO2 molecule is a combination of its two resonance structure. You can imagine one pair of electrons as resonating across the entire molecule from one oxygen atom to the other.