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Lewis Structures, Molecular Geometry, and Polarity of Small Covalent Molecules

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Lewis Structures of Small Covalent Molecules

Introduction to Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the distribution of electrons. They are essential for understanding molecular geometry, bond angles, and polarity in covalent compounds.

  • Lewis structures show how atoms are bonded and how electrons are distributed.

  • They help predict molecular shape and polarity.

Steps to Draw Lewis Structures

Follow these steps to construct accurate Lewis structures for small covalent molecules:

  1. Decide which atom is the central atom.

    • Usually, the least electronegative atom is placed in the center.

    • Hydrogen and halogens (Group 17) cannot be central atoms because they only form one bond.

  2. Attach the remaining atoms to the central atom using single bonds.

  3. Include multiple bonds and lone pairs to satisfy the common bonding pattern for all atoms.

Common Bonding Patterns

Each element tends to form a specific number of bonds and lone pairs, as summarized in the table below:

Element

Preferred Bonds

Lone Pairs

H

1

0

C

4

0

N

3

1

O

2

2

F, Cl, Br, I

1

3

Si

4

0

S

2

2

P

3

1

Ne

0

4

Additional info: This table is inferred from the periodic trends and the provided notes.

Examples of Lewis Structures

NF3 (Nitrogen Trifluoride)

NF3 is a covalent molecule with nitrogen as the central atom.

  • Step 1: Nitrogen is the central atom (least electronegative, not a halogen).

  • Step 2: Attach three fluorine atoms to nitrogen with single bonds.

  • Step 3: Satisfy the bonding pattern: N forms 3 bonds and has 1 lone pair; F forms 1 bond and has 3 lone pairs.

Lewis Structure:

.. .. .. :F: :F: :F: | | | .. .. .. :N:--F:--F:--F: .. .. ..

Example: NF3 is a polar molecule because the central atom (N) has a lone pair.

CSH2 (Thioformaldehyde)

CSH2 contains carbon, sulfur, and hydrogen.

  • Step 1: Carbon is the central atom (H cannot be central; C is less electronegative than S).

  • Step 2: Attach S and two H atoms to C.

  • Step 3: Satisfy bonding patterns: C forms 4 bonds (may include double bonds); S forms 2 bonds and has 2 lone pairs; H forms 1 bond.

Lewis Structure:

H H | | S=C--H--H | | .. ..

Example: CSH2 is a polar molecule because the central atom (S) has a lone pair.

SiH2I2 (Silicon Diiodide Dihydride)

SiH2I2 contains silicon, hydrogen, and iodine.

  • Step 1: Silicon is the central atom (H and I cannot be central).

  • Step 2: Attach two H and two I atoms to Si.

  • Step 3: Satisfy bonding patterns: Si forms 4 bonds; H forms 1 bond; I forms 1 bond and has 3 lone pairs.

Lewis Structure:

H H | | I--Si--I | | .. .. ..

Example: SiH2I2 is a polar molecule because the central atom (Si) is bonded to different atoms.

Molecular Geometry and Bond Angles

Determining Molecular Geometry

The geometry of a molecule depends on the number of electron groups (bonded atoms and lone pairs) around the central atom.

  • Electron Groups (EG): Total number of bonded atoms and lone pairs around the central atom.

  • Bonded Atoms (BA): Number of atoms directly bonded to the central atom.

  • Lone Pairs (LP): Number of lone electron pairs on the central atom.

Formula:

Geometry Table

# of Electron Groups (EG)

# of Bonded Atoms (BA)

# of Lone Pairs (LP)

Molecular Geometry

Bond Angle

Example

2

2

0

Linear

180°

CO2

3

3

0

Trigonal planar

120°

BF3

4

4

0

Tetrahedral

109.5°

CH4

4

3

1

Trigonal pyramidal

109.5°

NH3

4

2

2

Bent (angular)

109.5°

H2O

Steps to Determine Geometry and Bond Angle

  1. Draw the Lewis structure.

  2. Count the number of bonded atoms (BA) and lone pairs (LP) around the central atom.

  3. Determine the molecular geometry using the table above.

  4. Determine the bond angle.

Examples

  • NF3: 3 BA, 1 LP → Trigonal pyramidal, bond angle ≈ 109.5°

  • CSH2: 3 BA, 0 LP → Trigonal planar, bond angle ≈ 120°

  • SiH2I2: 4 BA, 0 LP → Tetrahedral, bond angle ≈ 109.5°

Molecular Polarity

Determining Molecular Polarity

Molecular polarity depends on the distribution of electron density and the types of atoms bonded to the central atom.

  1. Draw the Lewis structure.

  2. Apply the following rules:

  • a) Molecules whose central atom has a lone pair are polar.

  • b) Molecules whose central atom does not have a lone pair and is bonded to different types of atoms are polar.

  • c) Molecules whose central atom does not have a lone pair and is bonded to the same type of atoms are nonpolar.

  • d) Molecules which contain only C and H atoms are nonpolar.

Examples

  • NF3: Polar molecule (central atom has a lone pair).

  • CSH2: Polar molecule (central atom has a lone pair).

  • SiH2I2: Polar molecule (central atom bonded to different atoms).

Summary Table: Lewis Structures, Geometry, and Polarity

Formula

Lewis Structure

Electron Groups (EG)

Bonded Atoms (BA)

Lone Pairs (LP)

Geometry

Bond Angle

Polarity

NF3

N with 3 F atoms, 1 lone pair

4

3

1

Trigonal pyramidal

109.5°

Polar

CSH2

S with C and 2 H atoms, 2 lone pairs

3

3

0

Trigonal planar

120°

Polar

SiH2I2

Si with 2 H and 2 I atoms

4

4

0

Tetrahedral

109.5°

Polar

Additional info: Table entries inferred from the provided notes and standard chemical knowledge.

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