Unit 5 Notes Guide – Part 3 Name # Molecular Structure Block: Chemistry

I. Drawing Lewis Dot Structuresa. The Octet Rule

i. All elements are seeking to achieve __________valence electrons.

ii. ________________ and helium follow the ________ rule since the lowest energy level is full

with ______ valence electrons rather than eight

b. Steps to drawing Lewis dot structures:

1. Find the __________ number of valence electrons

2. Write the symbols of the atoms to show which atoms are attached and connect them with a

__________ bond.

a. ____________ is always in the middle

b. Otherwise, the __________ electronegative atom is center

i. NEVER ________________

3. Complete the octets (or duets) of the atoms with ________ __________

4. Place any leftover electrons on the _______________ atom

5. If the number of electrons exceeds the value found in Step 1, move lone pair electrons from the

______________ atom and an outside atom to form ____________ or ____________ bonds

6. ______________ to make sure the number of electrons is equal to Step 1

Examples:

CCl4 CH3Cl

1

electrons electrons

Bond

Lone pair

CN-1 C2H4

Practice #3Draw Lewis dot structures for the following molecules

H2O NH3 O2

c. Exceptions to the Octet Rule

i. ________________ and ____________ are satisfied with only 2 valence electrons

(only s orbital)

ii. __________ is satisfied with 6 valence electrons

iii. ____________________ is satisfied with up to 10 valence electrons

iv. ____________ is satisfied with up to 12 valence electrons

v. ______________ elements and beyond will fill octet but may exceed due to empty d orbitals

Practice #4Draw Lewis dot structures for the following molecules

SF6 BH3 PCl5

2

electrons electrons

II. Resonancea. Resonance is the use of two or more Lewis structures to represent the covalent bonding in a

molecule

b. Rather than having distinctly different bonds, the true structure is an ______________ or ____________ of all the resonance structures

III. VSEPR Theorya. The VSEPR model uses the Coulombic __________________ between electrons as a basis for

predicting the 3D ______________________ of electron pairs around a central atom

VSEPR

b. The structure around a given atom is determined mainly by ____________________ electron-pair

repulsions.

c. There are two types of electron groups

i. ________ pairs

ii. Bonds

Multiple bonds count as _______ group towards repulsion

*Table with the different molecular geometries on next page*

d. Limitations of VSEPR Theory3

Bond

Lone pair

i. VSEPR theory can only provide a limited explanation when considering the bond angles of

exceptionally large atoms

Ex: phosphine vs. ammonia

e. Molecules with no Central Atom

i. In molecules containing no single central atom, each atom is assigned an arrangement of its own

Number of

Electron Groups

Number of

Lone PairsMolecular Geometry Bond Angles Example

1 or 2 0 Name:CO2

3

0

Name:

BF3

1Name:

SO2

4

0

Name:d

CH4

1

Name:

NH3

2Name:

H2O

4

5 0

Name:f

PCl5

6 0

Name:df

SF6

IV. Polaritya. An ionic bond is a transfer of electrons, and a covalent bond is a sharing of electrons

b. Remember, we measure electronegativity on a scale of 1-4 (Pauling Electronegativity scale)

c. The degree to which a bond is ionic or covalent is determined by calculating the

____________________ in electronegativity

i. Ionic: ≥ _______

ii. Polar covalent: _______ - _______

iii. Nonpolar covalent: ≤ _______

d. Nonpolar Covalent Bonds

i. A covalent bond in which the bonding electrons are shared ______________ between the two

atoms.

Electronegativity is less than 1.7

The distribution of shared electrons is ________________________

e. Polar Covalent Bonds

i. A covalent bond in which the atoms have an ______________ attraction for electrons, so the

sharing is ______________.

The distribution of shared electrons is ______ symmetrical

f. Molecular Polarity

i. Polar molecule – a molecule in which one end of the molecule is ________________ positive,

while the other end is slightly _________________.

(lowercase delta = = slightly)

ii. The two charges ends are similar to the poles of a magnet

iii. A molecule with two poles is called a ____________.

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Represented by an arrow pointing towards the negative end and a cross on the positive

end

g. Covalent molecules are either __________ or ________________

i. Molecules with all nonpolar covalent bonds are ________________

ii. Molecules with polar covalent bonds (dipoles) can be ____________ polar or nonpolar

depending on the ________________ of the molecular geometry

All the dipoles in a molecule ______________ to give

one overall dipole moment

Dipoles that oppose each other ____________ ______

Molecules with all of the ________ polar covalent bond can be overall nonpolar if the

geometry is linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral

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