por The molecular geometry either molecular structure is the spatial distribution of atoms around a central atom. Atoms represent regions where there is a high electronic density, and are therefore considered electronic groups, regardless of the bonds they form (single, double or triple).
The molecular geometry of an element can characterize some of its physical or chemical properties (boiling point, viscosity, density, etc.). For example, the molecular structure of water determines its solubility.
This concept is born from the combination and experimental data of two theories: the valence bond (VTE) and the valence shell electron pair repulsion (VSEPR). While the first defines the bonds and their angles, the second establishes the geometry and, consequently, the molecular structure.
What geometric shapes are molecules capable of adopting? The two previous theories provide the answers. According to VSEPR, the atoms and lone pairs of electrons must be arranged in space in such a way as to minimize the electrostatic repulsion between them.
So, the geometric shapes are not arbitrary, but seek the most stable design. For example, in the image above, a triangle can be seen on the left, and an octahedron on the right. The green dots represent the atoms and the orange stripes the bonds.
In the triangle, the three green points are oriented at a 120º separation. This angle, which is equal to that of the bond, allows the atoms to repel each other as little as possible. Therefore, a molecule with a central atom attached to three others will adopt a trigonal planar geometry.
However, VSEPR predicts that a lone pair of electrons on the central atom will distort the geometry. For the case of the trigonal plane, this torque will push down the three green points, resulting in a trigonal pyramid geometry.
The same can also happen with the octahedron in the image. In it all the atoms are separated in the most stable way possible.
[toc]
How to know in advance the molecular geometry of an atom X?
For this, it is necessary to consider also the lone pairs of electrons as electronic groups. These, together with the atoms, will define what is known as the electronic Geometry, which is an inseparable companion of molecular geometry.
Starting from the electronic geometry, and having detected the free pairs of electrons by means of the Lewis structure, it can be established what the molecular geometry will be. The sum of all molecular geometries will provide a sketch of the global structure.
Types of molecular geometry
As seen in the main image, the molecular geometry depends on how many atoms surround the central atom. However, if an unshared pair of electrons is present, it will change the geometry because it occupies a lot of volume. Therefore, it exerts a steric effect.
According to this, the geometry can present a series of characteristic shapes for many molecules. And it is here where the different types of molecular geometry or molecular structure arise.
When is geometry equal to structure? Both denote the same thing only in cases where the structure does not have more than one type of geometry; otherwise, you must consider all types present and give the structure a global name (linear, branched, globular, flat, etc.).
The geometries are especially useful to explain the structure of a solid from its structural units.
Linear
All covalent bonds are directional, so the AB bond is linear. But will the AB2 molecule be linear? If so, the geometry is simply represented as: BAB. The two B atoms are separated by an angle of 180º, and according to TEV, A must have sp hybrid orbitals.
Angular
A linear geometry can be assumed in the first instance for the AB2 molecule; however, it is essential to draw the Lewis structure before reaching a conclusion. Once the Lewis structure has been drawn, the number of lone pairs of electrons (:) on the A atom can be identified.
When this is so, above A the pairs of electrons push the two atoms of B downwards, changing their angles. As a result, the linear BAB molecule ends up becoming a V, a boomerang, or an angular geometry (image above).
The water molecule, HOH, is the ideal example for this type of geometry. In the oxygen atom there are two lone pairs of electrons which are oriented at an approximate angle of 109º.
Why this angle? Because the electronic geometry is tetrahedral, which has four vertices: two for the H atoms, and two for the electrons. In the image above, note that the green dots and the two «lobes with eyes» draw a tetrahedron with the bluish dot in its center.
If O did not have free pairs of electrons, water would form a linear molecule, its polarity would decrease, and oceans, seas, lakes, etc., would probably not exist as known.
tetrahedral
The top image represents tetrahedral geometry. For the water molecule, its electronic geometry is tetrahedral, but by eliminating the lone pairs of electrons it can be seen that it becomes an angular geometry. This is also seen simply by removing two green dots; the remaining two will draw the V with the blue dot.
What if instead of two pairs of free electrons there were only one? Then there would be a trigonal plane (main image). However, by removing an electronic group, the steric effect produced by the lone pair of electrons is not avoided. Therefore, it distorts the trigonal plane to a triangular-based pyramid:
Although the trigonal pyramid and tetrahedral molecular geometry are different, the electronic geometry is the same: tetrahedral. So the trigonal pyramid doesn’t count as electronic geometry?
The answer is no, since it is the product of the distortion caused by the «lobe with eyes» and its steric effect, and said geometry does not take subsequent distortions into account.
For this reason, it is always important to first determine the electronic geometry with the help of Lewis structures before defining the molecular geometry. The ammonia molecule, NH3, is an example of trigonal pyramid molecular geometry, but with tetrahedral electronic geometry.
trigonal dipyramid
Until now, with the exception of linear geometry, in the tetrahedral, the angular and the trigonal pyramid their central atoms have sp3 hybridization, according to the TEV. This means that if their bond angles were determined experimentally, they should be around 109º.
From the trigonal dipyramidal geometry, there are five electronic groups around the central atom. In the image above it can be seen with the five green dots; three in the triangular base, and two in axial positions, which are the upper and lower vertices of the pyramid.
What hybridization does the blue dot have then? It needs five hybrid orbitals to form the single bonds (in orange). It achieves this through the five sp3d orbitals (product of the mixture of one s, three p and one d orbital).
When considering five electronic groups, the geometry is the one already exposed, but since there are pairs of electrons without sharing, it again suffers distortions generated by other geometries. Likewise, the following question arises: can these pairs occupy any position in the pyramid? These are: the axial or the equatorial.
axial and equatorial positions
The green dots that make up the triangular base are in equatorial positions, while the two at the upper and lower ends are in axial positions. Where preferentially will the unshared pair of electrons be located? In that position that minimizes electrostatic repulsion and steric effect.
In an axial position, the pair of electrons would «press» perpendicularly (90º) on the triangular base, while if it were in an equatorial position, the two remaining electronic groups of the base would be separated by 120º and would press the two ends at 90º (instead of three, as with the base).
Therefore, the central atom will seek to orient its lone pairs of electrons at the equatorial positions to generate more stable molecular geometries.
Oscillating and T-shaped
If in the trigonal dipyramid geometry one or more of its atoms were replaced by lone pairs of electrons, one would also have different molecular geometries.
On the left of the top image, the geometry changes to the wobble shape. In it, the lone pair of electrons pushes the rest of the four atoms in the same direction, bending their bonds to the left. Notice that this pair and two of the atoms lie in the same triangular plane as the original dipyramid.
And to the right of the image, the T-shaped geometry. This molecular geometry is the result of substituting two atoms for two pairs of electrons, resulting in the three remaining atoms aligning in the same plane that draws exactly one letter T.
So, for a molecule of the AB5 type, it adopts the trigonal dipyramid geometry. However, AB4, with the same electronic geometry, will adopt the oscillating geometry; and AB3, the T-shaped geometry. In all of them A will (generally) have sp3d hybridization.
To determine the molecular geometry it is necessary to draw the Lewis structure and therefore its electronic geometry. If this is a trigonal dipyramid, then lone pairs of electrons will be ruled out, but not their steric effects on the rest of the atoms. Thus, it is possible to perfectly discern between the three possible molecular geometries.
octahedral
To the right of the main image the octahedral molecular geometry is represented. This type of geometry corresponds to AB6 compounds. AB4 form the square base, while the two remaining B are positioned in axial positions. Thus, several equilateral triangles are formed, which are the faces of the octahedron.
Here, again, there can be (as in all electronic geometries) lone pairs of electrons, and therefore other molecular geometries derive from this fact. For example, AB5 with octahedral electronic geometry consists of a pyramid with a square base, and AB4 of a square plane:
For the case of octahedral electronic geometry, these two molecular geometries are the most stable in terms of electrostatic repulsion. In square plane geometry, the two pairs of electrons are 180º apart.
What is the hybridization for atom A in said geometries (or structures, if it is the only one)? Again, the TEV establishes that it is sp3d2, six hybrid orbitals, which allows A to orient the electronic groups at the vertices of an octahedron.
Other molecular geometries
By modifying the bases of the pyramids mentioned so far, they can…