Molecular geometry describes the three-dimensional arrangement of atoms within a molecule. This spatial organization dictates a molecule’s behavior, influencing its polarity, physical state, and reactivity. The shape is determined by the Valence Shell Electron Pair Repulsion (VSEPR) theory, which holds that electron groups around a central atom will position themselves as far apart as possible to minimize repulsion.
Creating the Lewis Structure
Creating the Lewis structure begins by calculating the total number of valence electrons contributed by every atom in the molecule or ion. For a neutral molecule like water (\(\text{H}_2\text{O}\)), the two hydrogen atoms (one valence electron each) and the one oxygen atom (six valence electrons) contribute a total of eight valence electrons.
Next, identify the central atom, which is typically the least electronegative element (excluding hydrogen, since it can only form one bond). Draw a skeleton structure by connecting the central atom to the surrounding atoms using single bonds, with each bond accounting for two valence electrons. In \(\text{H}_2\text{O}\), oxygen is the central atom, and four of the eight total electrons are used to form the two single bonds to the hydrogen atoms.
The remaining valence electrons are then distributed around the terminal atoms to satisfy the octet rule, which states that most atoms need eight electrons in their outer shell for stability. Hydrogen is an exception, requiring only two electrons. Any electrons still remaining after the terminal atoms have their octets are then placed on the central atom as lone pairs. For water, the two hydrogen atoms are already complete with two electrons each, and the final four electrons are placed on the central oxygen atom as two lone pairs, completing its octet.
Counting Electron Domains
The next step is to use the Valence Shell Electron Pair Repulsion theory to count the electron domains around the central atom. An electron domain is defined as any region of high electron density, and this count is also known as the steric number. Each single bond, double bond, triple bond, and lone pair of electrons counts as one electron domain.
In carbon dioxide (\(\text{CO}_2\)), the central carbon atom is double-bonded to two oxygen atoms, meaning it has two electron domains. In contrast, the oxygen atom in water has two single bonds to hydrogen and two lone pairs, resulting in four electron domains. The electron domains, regardless of whether they are bonding or non-bonding, will repel each other to achieve the maximum separation possible in three-dimensional space.
Determining Electron Geometry
The electron geometry is determined by the total number of electron domains. This geometry describes the arrangement of all electron groups—both bonding pairs and lone pairs—around the central atom. The fundamental shapes established by the VSEPR theory are a result of this electron-electron repulsion.
If the central atom has two electron domains, the electron geometry is linear, with the domains separating to an angle of \(180^\circ\). Three electron domains result in a trigonal planar geometry, where the electron groups lie in a flat plane with \(120^\circ\) angles between them. Four electron domains create a tetrahedral geometry, which features bond angles of approximately \(109.5^\circ\).
Five electron domains arrange themselves in a trigonal bipyramidal geometry, which involves two distinct types of positions, axial and equatorial. Six electron domains form an octahedral geometry, where all positions are equivalent and the bond angles are \(90^\circ\). This electron geometry establishes the fundamental spatial framework for the atoms.
Finalizing the Molecular Shape
Molecular geometry describes only the arrangement of the atoms themselves, while electron geometry accounts for all electron domains, including lone pairs. The molecular shape will only match the electron geometry when there are no lone pairs on the central atom, as seen in methane (\(\text{CH}_4\)), which is tetrahedral for both.
When lone pairs are present, they exert a stronger repulsive force than bonding pairs because they are held closer to the central atom’s nucleus. This increased repulsion pushes the bonded atoms closer together, effectively reducing the bond angles from the ideal geometry and distorting the molecular shape.
For molecules with a tetrahedral electron geometry (four domains), one lone pair creates a trigonal pyramidal molecular shape, as seen in ammonia (\(\text{NH}_3\)), where the bond angles are reduced to about \(107^\circ\). Two lone pairs result in a bent or V-shaped molecular geometry, such as in water (\(\text{H}_2\text{O}\)), where the angle is squeezed even further to about \(104.5^\circ\).
Lone pairs also apply to geometries with five or six electron domains, leading to various derived shapes.
Derived Shapes
- Within the trigonal bipyramidal family, one lone pair results in a seesaw shape.
- Two lone pairs create a T-shaped molecule.
- Three lone pairs produce a linear shape.
- For the octahedral family, one lone pair changes the shape to square pyramidal.
- Two lone pairs result in a square planar shape.
These derived shapes are the final description of the molecule’s three-dimensional structure.