Lewis Structures

Lewis structures, also known as Lewis dot structures, are diagrams that show the bonding between atoms in a molecule and the lone pairs of electrons that may exist in the molecule.

• Purpose: They provide a simple way to understand how atoms are connected in a molecule and predict the basic shape of the molecule.

Steps to Draw:

1. Count Valence Electrons: Determine the total number of valence electrons for all atoms in the molecule.
2. Connect Atoms: Use single bonds (pairs of electrons) to connect atoms, satisfying the octet rule (except for hydrogen, which follows the duet rule).
3. Place Remaining Electrons: Distribute remaining electrons as lone pairs on outer atoms to satisfy their octets, and then on the central atom.
4. Formal Charge Check: Check formal charges to ensure they are minimized or zeroed out, if possible.

Examples:

a. Water (H₂O):

• Central atom: Oxygen (6 valence electrons), Hydrogen (1 each).
• Structure: H-O-H with 2 lone pairs on oxygen.

b. Carbon dioxide (CO₂):

Structure: O=C=O, with double bonds between carbon and oxygen.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule and the spatial relationships between these atoms.

Determining Factors:

• VSEPR Theory: Predicts the geometry based on minimizing electron pair repulsion around the central atom.
• Electron Pair Arrangement: Bonding pairs and lone pairs around the central atom determine the geometry.

Common Geometries:

• Linear: 180° bond angle, e.g., CO₂.
• Trigonal Planar: 120° bond angle, e.g., BF₃.
• Tetrahedral: 109.5° bond angle, e.g., CH₄.
• Trigonal Bipyramidal: 90° and 120° bond angles, e.g., PCl₅.
• Octahedral: 90° bond angles, e.g., SF₆.

VSEPR Theory (Valence Shell Electron Pair Repulsion Theory)

VSEPR theory predicts the molecular geometry based on the idea that electron pairs (bonding and lone pairs) repel each other and position themselves as far apart as possible to minimize repulsion.

Steps to Predict Geometry:

1. Identify Central Atom: Determine the central atom in the molecule.
2. Count Electron Domains: Count the number of bonding pairs and lone pairs around the central atom (electron domains).
3. Predict Geometry: Choose the molecular geometry based on the arrangement of electron domains and lone pairs, considering their repulsive forces.

Example:

• Ammonia (NH₃):
  • Central atom: Nitrogen (5 valence electrons), Hydrogen (1 each).
  • Electron domains: 4 (3 bonding pairs, 1 lone pair).
  • Predicted geometry: Trigonal pyramidal.

Intermolecular Forces(IMFs)

Intermolecular forces (IMFs) are attractive or repulsive forces between molecules or atoms that determine the physical properties of substances.

Types of Intermolecular forces (IMF):

1. London Dispersion Forces: Weakest IMF due to temporary dipoles.
2. Dipole-Dipole Interactions: Between polar molecules with permanent dipoles.
3. Hydrogen Bonding: Strong IMF between a hydrogen atom and a highly electronegative atom (N, O, F).
4. Ion-Dipole Interactions: Between an ion and a polar molecule.

Effects:

• Influence boiling points, melting points, and phase changes.
• Determine solubility in different solvents.
• Play a role in biological processes and molecular recognition.

Example:

• Water (H₂O):
  • Exhibits hydrogen bonding between water molecules, which contributes to its high boiling point and surface tension.
• Lewis electron-dot formulas are simple representations of the valence-shell electrons of atoms in molecules and ions.
• An ionic bond is a strong attractive force holding ions together. An ionic bond can form between two atoms by the transfer of electrons from the valence shell of one atom to the valence shell of the other.
• Acovalent bond is a strong attractive force that holds two atoms together by their sharing of electrons. These bonding electrons are attracted simultaneously to both atomic nuclei, and they spend part of the time near one atom and part of the time near the other.
• In some cases of covalent bonding, one atom appears to provide both electrons in the bonding pair; the bond is known as coordinate-covalent bond or dative bond.
• Molecular Geometry refers to the spatial arrangements gement of atoms in amolecule.The valence-shell electron-pair repulsion (VSEPR) model is a simple model for predicting molecular geometries.