Lecture Six

Question 1

What are the five steps to making a Lewis structure.

Answer 1

1) Determine the total number of valance electrons in the molecule or ion.
2) Arrange the atoms to show how they are connected - atoms are grouped around a central atom, which is usually the lease electronegative. Place a bonding pair of electrons between each pair of adjacent atoms to give a single bond.
3) Begin with the terminal atoms and add enough electrons to each atom to give all the atoms an octet.
4) Place any electrons let over on the central atom.
5) Minimise the formal charges: if necessary use lone pairs from terminal atoms to form multiple bonds to the central atom in order to achieve an octet.

Question 2

What is formal charge and how do you work it out?

Answer 2

1) Depends on the difference between the number of valance electrons in the free atom and the number assigned to it in the Lewis electron structure.
2) Way of computing the charge distribution within a Lewis structure; the sum of the formal charges on the atoms within a molecule or ion, must equal the overall charge on the molecule.
3) Does not represent a true charge on an atom, simple used to predict the most likely structure when a compound has more than one Lewis structure.

Formal charge = valance e- - (nonbonding e- + (bonding e-)/2)

Question 3

What is resonance?

Answer 3

It is possible to write more than one Lewis structure for a substance that does not violate the octet rule, but not all of the Lewis structure mat be equally reasonable.
Choose the most stable Lewis structure by considering the formal charge on the atoms.
Structure with the smallest formal charges are the most stable.
Structures that have more negative formal charge on the more electronegative atom are more stable.
Adjacent charges should have opposite sign.

Question 4

What is a coordination number of an atom?

Answer 4

In molecular compounds, the coordination number of an atom defines the number of atoms or groups attached to it.

Question 5

Define isoelectric.

Answer 5

Molecules that contain the same number of valance electrons.

Question 6

Define isostructural.

Answer 6

Molecules with the same molecular geometry.