G10 Lesson 4.3: Molecular Structures

Molecular Structures

Lesson 3

Focus Question

How are electrons shared in covalent
molecules?

New Vocabulary

structural formula

resonance

coordinate covalent bond

Review Vocabulary

ionic bond: the electrostatic force that holds
oppositely charged particles together in an ionic
compound

Structural Formulas

A structural formula uses letter symbols and
bonds to show relative positions of atoms.

Multiple Choice

What is the molecular formula for structure showed?

• C₈H₁₈

• C₃H₃

• C₈H₃

• C₃H₂(CH₃)₃

Multiple Choice

What is the condensed formula of this structure?

• CH₃CH₂CH₂OH

• CH₃COCH₃

• CH₃CH₂CHO

• CH₃OCH₂CH₃

Multiple Choice

According to the octet rule most elements need _______ valence electrons.

• 2

• 8

• 6

• 18

Multiple Choice

Which of the following is the correct Lewis dot structure for the molecule fluorine (F₂)?

• A

• B

• C

• D

Multiple Choice

What is the correct structure for BF₃?

• Option A.

• Option B. 

• Option C.

• Option D.

Structural Formulas

Drawing Lewis structures
1.

Predict the location of atoms. The atom with the
least attraction for shared electrons will be the
central atom. (It’s usually closest to the left side of
the periodic table). Others become terminal atoms.
Hydrogen is always a terminal atom.

2.

Determine the number of electrons available for
bonding. This is the number of valence electrons.

3.

Determine the number of bonding pairs. Divide the
number of electrons available by two.

4.

Place bonding pairs. Place a single bond between
the central atoms and each terminal atom.

Structural Formulas

5. Determine the number of electron pairs
remaining. Subtract the number of pairs in Step 4
from the number of bonding pairs in Step 3. Place
lone pairs around terminal atoms (except H atoms)
to satisfy the octet rule. Remaining pairs will be
assigned to the central atom.
6. Determine whether the central atom satisfies the
octet rule. If not, convert one or two of the lone
pairs on the terminal atoms into a double or a triple
bond between the terminal atom and the central
atom. Carbon, nitrogen, oxygen, and sulfur often
form double and triple bonds.

Structural Formulas

Lewis Structures for Polyatomic Ions
•

To find the total number of electrons available for
bonding in a polyatomic ion, first find the number
available in the atoms present in the ion.

•

Add the ion charge if the ion is negative, or
subtract the charge if the ion is positive.

Multiple Choice

What is a Lewis structure?

• A 3D structure showing valance electrons

• A 2D structure showing valance electrons

• A 2D structure showing electrons in a covalently bonded molecule

• A 3D structure showing electrons in a covalently bonded molecule

Draw

Draw the Lewis structure for HF.

Multiple Choice

What does a dash represent in a Lewis structure?

• Single electrons

• The bonding pair of electrons between two atoms

• A double bond

• One electron bonding two atoms

Multiple Choice

What do the electrons shared in a covalent bond "count" toward?

• The number of bonds between atoms

• The octet for one of the atoms

• The octet for both atoms they are between

• The number of atoms bonded in a molecule

Resonance Structures

• Resonance is a condition that occurs when

more than one valid Lewis structure can be
written for a molecule or ion.

• Two or more correct Lewis structures that

represent a single ion or molecule are
resonance structures.

• The molecule behaves as though it has only

one structure.

• The bond lengths are identical to each other

and intermediate between single and double
covalent bonds.

Resonance Structures

This figure shows three correct ways to draw the
structure for (NO3)-1.

Multiple Choice

What is the name for when more than one valid Lewis structure can be drawn for a molecule?

• coordinate covalent bond

• resonance

• endothermic

• structural formula

Multiple Choice

What is the picture showing?

• All of the resonance structures for carbonate.

• The formal charge on each atom in carbonate.

• The electronegativity for carbonate.

• The ionization energy for carbonate.

Multiple Choice

For carbonate ions, how many resonance structures can be drawn ?

• 2

• 3

• 4

• 5

Multiple Choice

What is the formal charge on the oxygen atom at the top of this Lewis dot structure?

• 0

• -1

• +1

• -3

Multiple Choice

What is the formal charge on the phosphorus atom in this Lewis dot structure?

• 0

• -1

• +1

• -3

Multiple Choice

What is the formal charge on the oxygen atom at the top of this Lewis dot structure?

• 0

• +1

• -1

• -2

Exceptions to the Octet Rule

Odd Number of Valence Electrons
• A small group of molecules might have an odd

number of valence electrons and be unable to
form an octet around each atom.

Exceptions to the Octet Rule

Suboctets and Coordinate Covalent Bonds
• A few compounds form suboctets—stable configurations with fewer than 8 electrons
around an atom.

• A coordinate covalent bond forms when one

atom donates both of the electrons to be shared
with an atom or ion that needs two electrons. An
example is shown on the next slide.

Multiple Choice

How can coordinate covalent bond form between ammonia, NH₃ and hydrogen ion, H⁺ to form ammonium ion, NH₄⁺?

• Ammonia, NH₃ and hydrogen ion, H⁺ will each share one electron

• The pair of electrons not involved in covalent bond in ammonia, NH₃ will be shared with hydrogen ion, H⁺

• Ammonia, NH₃ and hydrogen ion, H⁺ will each share one electron

Multiple Choice

Forms when one atom donates a pair of electrons to be shared with an atom or ion that needs two electrons to becomes stable.

• covalent bond 

• James Bond

• polar covalent bond

• coordinate covalent bond

Multiple Choice

Which of these statements best describes a coordinate covalent bond?

• A pair of electrons shared between two atoms where each atom has donated one electron.

• A pair of electrons shared between two atoms where one atom has donated two electrons.

• Two pairs of electrons shared between two atoms where each atom has donated one electron.

• Two pairs of electrons shared between two atoms where each atom has donated two electrons.

Exceptions to the Octet Rule

Multiple Choice

According to the Octet Rule, atoms of elements react with each other in order to get ______ electrons in their outermost energy level or shell.

• 2

• 4

• 8

• 10

Multiple Select

Select the 3 exceptions to octet rule.

• Expanded octet

• Even number electron

• Incomplete octet

• Odd number electron

Multiple Choice

Molecule that has less than 8 electrons surrounding the central atom is classified as

• even number electron

• odd number electron

• expanded octet

• incomplete octet

Multiple Choice

Do you think Nitrogen at the central already achieved an octet?

• Yes

• No

Multiple Choice

Which of the following is the correct Lewis dot structure for the molecule fluorine (F₂)?

• A

• B

• C

• D

Multiple Choice

When writing Lewis Structures, only __________ electrons are used.

• Inner shell

• Core 

• Valence

• Stable

Multiple Choice

• achieved octet-aka meets octet rule

• incomplete octet-doesn't satisfy octet rule

• expended octet-aka too many electrons

• odd number electrons

Multiple Choice

How many electrons does each line in covalent bond contain?

• 1

• 2

• 3

• 4

Multiple Choice

How many total valence electrons are participating in bonding in the molecule above?

• 8

• 4

• 2

• 3

Think of the Quiz answers before moving

to the next slide

Quiz

Hydrogen often forms double bonds.

D

Hydrogen is always a terminal atom.

C

The central atom is usually the one closest to the
right side of the periodic table.

B

The atom with the most attraction to shared
electrons will be the central atom in the molecule.

A

Which of the following is true about drawing Lewis
structures?

1.

Quiz

Hydrogen often forms double bonds.

D

Hydrogen is always a terminal atom.

C

The central atom is usually the one closest to the
right side of the periodic table.

B

The atom with the most attraction to shared
electrons will be the central atom in the molecule.

A

Which of the following is true about drawing Lewis
structures?

1.

CORRECT

Quiz

Bond lengths are identical to each other and
intermediate between single and double covalent
bonds.

D

C

Resonance structures are two or more correct Lewis
structures for a single ion or molecule.

B

Resonance occurs when more than one valid Lewis
structure can be written for a molecule or ion.

A

What is not true about resonance and resonance
structures?

2.

The molecule behaves as though it has multiple
structures.

Quiz

Bond lengths are identical to each other and
intermediate between single and double covalent
bonds.

D

C

Resonance structures are two or more correct Lewis
structures for a single ion or molecule.

B

Resonance occurs when more than one valid Lewis
structure can be written for a molecule or ion.

A

What is not true about resonance and resonance
structures?

2.

CORRECT
The molecule behaves as though it has multiple
structures.

Quiz

stable configurations
with less than 8
electrons around the
atom

D

molecules with
double-bonded carbon
atoms

C

coordinate covalent
bonds

B

molecules with odd
numbers of valence
electrons

A

Which of the following is not an exception to
the octet rule?

3.

Quiz

stable configurations
with less than 8
electrons around the
atom

D

molecules with
double-bonded carbon
atoms

C

coordinate covalent
bonds

B

molecules with odd
numbers of valence
electrons

A

Which of the following is not an exception to
the octet rule?

3.

CORRECT

Quiz

a bond in which one
atom donates both
electrons to be shared

D

a double bond

C

a bond in which both
atoms donate electrons
to be shared

B

a bond that has more
than one Lewis structure

A

Which of the following best describes a coordinate
covalent bond?

4.

Quiz

a bond in which one
atom donates both
electrons to be shared

D

a double bond

C

a bond in which both
atoms donate electrons
to be shared

B

a bond that has more
than one Lewis structure

A

Which of the following best describes a coordinate
covalent bond?

4.

CORRECT