Chemical Bonding: Molecular and Lewis Structures (Chapter 6.5–6.6 Study Notes)

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Chemical Bonding: Molecular and Lewis Structures

Introduction to Molecular Compounds

Molecular compounds are formed when atoms of two or more nonmetals share electrons, resulting in covalent bonds. These compounds are distinct from ionic compounds, which involve the transfer of electrons between metals and nonmetals. The sharing of valence electrons allows nonmetal atoms to achieve stable electron configurations, often resembling those of noble gases.

Covalent bond: A chemical bond formed by the sharing of one or more pairs of electrons between atoms.
Molecule: A group of two or more atoms held together by covalent bonds.
Molecular compounds are more numerous than ionic compounds due to the variety of possible combinations among nonmetals.

Table of common molecular compounds and their uses

Naming Molecular Compounds

Naming molecular compounds follows a systematic approach to indicate the types and numbers of atoms present. Prefixes are used to denote the number of each type of atom in the molecule.

Step 1: Name the first nonmetal by its element name.
Step 2: Name the second nonmetal by using the first syllable of its element name followed by "-ide".
Step 3: Add prefixes to indicate the number of atoms (subscripts) for both elements.

Common Prefixes for Molecular Compounds:

Number	Prefix
1	mono
2	di
3	tri
4	tetra
5	penta
6	hexa
7	hepta
8	octa
9	nona
10	deca

Table of prefixes for naming molecular compounds

Examples:

CO: carbon monoxide
N2O: dinitrogen monoxide
PF3: phosphorus trifluoride
CCl4: carbon tetrachloride

Writing Formulas for Molecular Compounds

To write the formula for a molecular compound, use the following steps:

Step 1: Write the chemical symbols in the order of the elements in the name.
Step 2: Use the prefixes to determine the subscripts for each element.

Examples:

Phosphorus pentachloride: PCl5
Dinitrogen trioxide: N2O3
Sulfur hexafluoride: SF6

Distinguishing Ionic and Molecular Compounds

It is important to distinguish between ionic and molecular compounds based on their composition:

Ionic Compounds: The first element is a metal or the polyatomic ion NH4+.
Molecular Compounds: The first element is a nonmetal.

Example: K2O is ionic (potassium is a metal); N2O is molecular (nitrogen is a nonmetal).

Flowchart for naming chemical compounds

Lewis Structures for Molecules

Introduction to Lewis Structures

Lewis structures are diagrams that represent the arrangement of valence electrons among atoms in a molecule. They help visualize how atoms share electrons to achieve stable electron configurations (octets), except for hydrogen, which achieves a duet.

Bonding pairs (shared electrons) are shown as two dots or a single line between atoms.
Non-bonding pairs (lone pairs) are placed on the outside of atoms.

Lewis structure of F2 showing bonding and lone pairs

Steps for Drawing Lewis Structures

Follow these steps to draw Lewis structures for molecules:

Step 1: Determine the arrangement of atoms (central atom is usually the least electronegative).
Step 2: Calculate the total number of valence electrons for all atoms.
Step 3: Attach each bonded atom to the central atom with a pair of electrons (single bond).
Step 4: Use remaining electrons to complete the octets of the outer atoms, then the central atom.

Example: Drawing the Lewis structure for PCl3:

P is the central atom (5 valence electrons).
Each Cl atom has 7 valence electrons.
Attach Cl atoms to P and complete octets.

Lewis structure for PCl3 with bonding pairs only

Examples of Lewis Structures and Molecular Models

Lewis structures can be used to represent simple molecules such as methane (CH4), ammonia (NH3), and water (H2O). These diagrams show both bonding and non-bonding pairs of electrons.

Lewis structures and molecular models for CH4, NH3, and H2O

Double and Triple Bonds

Some molecules require double or triple bonds to satisfy the octet rule for all atoms. Double bonds involve the sharing of two pairs of electrons, while triple bonds involve three pairs. Atoms such as carbon, nitrogen, oxygen, and sulfur commonly form multiple bonds.

Double bond: Two pairs of electrons are shared between two atoms.
Triple bond: Three pairs of electrons are shared between two atoms.
Multiple bonds form when there are not enough valence electrons to complete octets with single bonds alone.

Example: Drawing the Lewis structure for carbon dioxide (CO2):

Carbon is the central atom (4 valence electrons); each oxygen has 6 valence electrons.
Attach each O to C with a single bond, then complete octets.
If the central atom's octet is incomplete, convert lone pairs on outer atoms to bonding pairs, forming double bonds.

Lewis structure for CO2 showing double bonds

Summary Table: Common Molecular Compounds and Their Uses

Formula	Name	Commercial Uses
CO2	Carbon dioxide	Fire extinguishers, dry ice, propellant in aerosols, carbonation of beverages
CS2	Carbon disulfide	Manufacture of rayon
N2O	Dinitrogen oxide	Inhalation anesthetic, "laughing gas"
NO	Nitrogen oxide	Stabilizer, biochemical messenger in cells
SO2	Sulfur dioxide	Preserving fruits, vegetables; disinfectant in breweries; bleaching textiles
SF6	Sulfur hexafluoride	Electrical circuits
SO3	Sulfur trioxide	Manufacture of explosives

References

Timberlake, K. (2018). Chemistry: An Introduction to General, Organic, and Biological Chemistry (13th ed.). New York, NY: Pearson Education.