Shapes of Molecules

Step-by-Step Prediction of Molecular Shape

• Step 1: Identify the Central Atom: Determine which atom is the central focus and find its group number to identify the number of valence electrons.
• Step 2: Count Bonding Pairs: Determine how many atoms are bonded to the central atom. Each bonded atom represents one bonding domain, regardless of bond order (single, double, or triple).
• Step 3: Calculate Lone Pairs: Subtract the electrons used in bonding from the total valence electrons of the central atom. Divide the remaining electrons by 2 to find the number of lone pairs.
• Step 4: Determine Total Electron Domains: Add the number of bonding pairs and lone pairs to find the total number of domains, which determines the 'electron geometry'.
• Step 5: Assign Molecular Shape: Use the combination of bonding and lone pairs to identify the specific molecular geometry and predict the bond angles based on repulsion rules.

• Electron Geometry vs. Molecular Geometry: Electron geometry considers all electron domains (including lone pairs), while molecular geometry describes only the positions of the atoms. For example, a molecule with 4 domains has a tetrahedral electron geometry, but if one domain is a lone pair, its molecular geometry is pyramidal.

• Axial vs. Equatorial Positions: In trigonal bipyramidal structures, there are two distinct types of positions. Equatorial positions (in the horizontal plane) have $120^{\circ}$ angles between them, while axial positions (vertical) are at $90^{\circ}$ to the equatorial plane.

• The 'Minus 2.5' Rule: For every lone pair added to a tetrahedral electron geometry (4 domains), the bond angle typically decreases by approximately $2.5^{\circ}$ (e.g., $109.5 \rightarrow 107 \rightarrow 104.5$).

• Handling Ions: When predicting the shape of an ion, adjust the valence electron count of the central atom. For a negative charge, add electrons; for a positive charge, subtract electrons before calculating lone pairs.

• Drawing Conventions: Use solid lines for bonds in the plane of the paper, wedges for bonds coming toward the viewer, and dashed lines for bonds going away to represent 3D structures accurately.

• Check for Symmetry: Symmetrical molecules (like $CH_4$ or $SF_6$) often have zero net dipole moment if the outer atoms are identical, even if individual bonds are polar.