ACER Week 4 Flipped Lecture #11: Lewis Structures, Resonance, and Formal Charge + 10/2 Friday Problem Solving Set #4

+ Lab D Notes

5 steps for drawing Lewis Dot Structure

1. Determine the total number of electrons pairs (total number of valence electrons divided by 2)

2. Arrange the skeleton structure of the molecule with the atom that is least electronegative and can form the most bonds as the central atom. Connect the other atoms to the central atom with one electron pair each.

3. Distribute the remaining electron pairs to the outer atoms as lone pairs in order to fill their octets.

4. Only if there are additional electron pairs after the octets are filled, add lone pairs to the central atom. The central atom can exceed an octet if it is an element from period 3 or below.

5. If the central atom still does not have an octet, convert the lone pairs of outer atoms into double or triple bonds.

What are resonance structures? What is the resonance hybrid? How do resonance structures contribute to the resonance hybrid?

Resonance structures exist if there are multiple possible forms of a Lewis structure. Each Lewis structure is a resonance structure.

The resonance hybrid is the average of the resonance structures. It can have fractional bond orders due to delocalized electrons.

Equivalent resonance structures that only differ the positions of bonds or lone pairs will contribute to the resonance hybrid equally. Resonance structures that have a lower energy will contribute more.

What is formal charge and what is it used for? How do you calculate it?

It is the charge of each atom in a molecule and the net charge of the molecule (all the formal charges summed). It is used to approximate the energy of a resonance structure. The lower the formal charge, the lower the energy of a molecule.

Formal Charge = Valence Electrons - (Lone pairs + ½ Bonds)

What is a limitation of formal charge?

It assumes that all electrons are shared equally in a bond, which overexaggerates the covalent character of bonds.

How is Rydberg’s constant expressed for hydrogen atoms versus other elements?

R_H for hydrogen atoms and R_∞ for other elements

Break down the units for Newtons (N)

kg * m / s²

What is a version of the formula for potential energy between two particles that uses a Coulomb’s constant with units aJ*pm?

What is Coulomb’s constant?

k = 231 aJ*pm

What electronegativity difference indicates a polar covalent bond?

Between 0.5 and 2

What kind of covalent bonds does Oxygen form?

Polar bonds, except with itself

1 Attojoule aJ is how many Joules?

1 x 10^-18 J

Ionic bonds form between…

A metal and nonmetal

Covalent bonds form between

two nonmetals

What is the Rydberg constant in energy (Joules) sometimes called?

Ry