A geometrical arrangement of molecular atoms having three branches or atoms connected to a central atom is called trigonal geometry. Now that there are different combinations of this type of geometry, trigonal pyramidal and trigonal planar arise as the most prominent ones.
Trigonal pyramidal molecular geometry is an arrangement of molecular atoms in three dimensional space such that the central atom contains three atoms bonded as the name trigonal suggests, but a lone pair of electrons is also present on the central atom.
Trigonal planar molecular geometry is a similar yet different arrangement of atoms around the central atom. The branched atoms constitute a perfect plane with no lone pair of electrons.
Trigonal pyramidal vs Trigonal planar geometries
| Trigonal pyramidal | Trigonal planar |
| A central atom connecting to three other atoms and a lone pair of electrons seemingly present on pyramidal corners constitute a trigonal pyramidal geometry | A central atom connecting three other atoms seemingly available on triangle corners in a plane constitute a trigonal planar geometry |
| In molecular geometry systems, trigonal pyramidal is referred to as AX3E system | In molecular geometry systems, trigonal planar is referred to as AX3 system |
| There exists a lone pair of electrons essentially on the central atom | There exist no lone pair of electrons on the central atom |
| Trigonal pyramidal geometry is never in a single plane | Trigonal planar geometry is planar |
| The molecules having trigonal pyramidal geometry constitute a bond angle of 107 degrees | The molecules of trigonal planar geometry have a bond angle of 120 degrees |
| The sp3 hybridization is present in trigonal pyramidal molecules | The sp2 hybridization is present among trigonal planar molecules |
| The bond pairs and lone pairs both are present in trigonal pyramidal geometry | Only the bond pairs of electrons exist among planar molecules |
| Bond pair-bond pair and lone pair-bond pair electronic repulsions exist | Only bond pair-bond pair electronic repulsions exist there as there are no lone pairs |
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For example, Ammonia (NH3), Chlorate ion (ClO3-), and sulphite ions (SO32-) |
For example, Boron trifluoride (BF3), Phosgene (COCl2), carbonate (CO32-) etc |
The coordination number for both of these geometries is 3.
Trigonal pyramidal molecules
Molecules containing trigonal pyramidal geometries are chlorate ion, sulfite ion, xenon trioxide, ammonia and other pnictogen hydrides, etc. The details of some of these molecules are given below:
Geometry of ammonia (NH3)
Ammonia is a stable binary hydride and the most simple of pnictogen hydrides. It attains a trigonal pyramidal geometry in order to stabilize a lone pair of electrons on nitrogen.
Geometry of chlorate ion (ClO3–)
Chlorate ion is a white crystalline flammable material. It contains chlorine atom in (+5) oxidation state having a trigonal pyramidal shape even though it has two double and a single bond. It is the only possibility, by which chlorate ion is able to set its geometry with its lone pair.
Geometry of sulfite ion (SO32-)
Sulfite ion is the conjugate base of bi sulfide. Sulfite has a double bond and two negatively charged oxygen atoms. It forms trigonal pyramidal geometry.
Trigonal planar molecules
Planar geometries are the best ones for attaining resonance, a stability phenomenon. This is due to the fact that parallel orbitals should face each other in the best way when it comes to planarity and stability. Molecules having trigonal planar geometry are boron trifluoride, formaldehyde, phosgene, sulfur trioxide, carbonate, nitrate, guanidium, and some organic compounds. The details of some of such geometrical structures are:
Geometry of boron trifluoride (BF3)
BF3 is the simplest of the structures of trigonal planar geometry. Trihalides of all other elements from group 3 of the periodic table form similar geometries.
Geometry of phosgene (COCl2)
Phosgene, an industrial chemical has a planar structure. This is due to the property of carbon to form a pi bond with the oxygen atom.
Geometry of carbonate ion (CO32-)
Carbonate ion, a polyatomic anion is a conjugate base of carbonic acid (hydrogen carbonate). It attains a geometry with three corners and a plane so that the hybridization could take place easily. This gives carbonate ions special stability.
Related topics
- Molecular geometry
- Electronic vs. molecular geometry
- Wedge and dash in geometry
- Ball and stick model
Observing Molecular Geometries
The use of three dimensional models has been a very good approach to observe and predict molecular geometries and their behaviors during interactions in various environments.
The molecular geometries are significantly different from electronic geometries referring to the differences among lone pairs and bond pairs of electrons.
Key Takeaway(s)
Trigonal pyramidal shape
- Hybridization = sp3 (tetrahedral type)
- Bond angle = ~107°
Trigonal planar shape
- Hybridization = sp2 (planar)
- Bond angle = ~120°
Concepts Berg
How do you know if a molecule is a trigonal planar or trigonal pyramidal?
A molecule possesses a lone pair in a trigonal pyramidal state. This can be a significant parameter in differentiating trigonal pyramidal geometry from trigonal planar one, A lone pair containing molecule, i.e. in a trigonal pyramidal shape, acts as a lewis base whereas the one with planar shape will act as a lewis acid.
Is PCL3 trigonal pyramidal?
Phosphorous trichloride (PCl3) is a molecule with trigonal pyramidal geometry. The factors that confirm its pyramidal shape are as:
- sp3 hybridization of P and Cl atoms in PCl3.
- Two extra electrons in the valence shell of the phosphorous atom, which serve as the lone pair.
- An angle of 103° between individual atoms makes it more like pyramidal (~ 107°) than planar (~ 120°).
What is the trigonal planar shape?
A trigonal planar shape means a triangular shape (with three corners) and a central atom bonded to all of the three side atoms. It is a fan-like shape corresponding to an angle of 120° between individual atoms in a molecule. Moreover, an overall plane in geometry means a planar shape.
Why is ammonia pyramidal?
Ammonia is a trigonal pyramidal shaped molecule because of :
- An exact 107° angle between individual atoms of ammonia corresponds to pyramidal shape.
- Nitrogen belonging to the fifth group of the periodic table has five valence electrons. Out of the five valence electrons, three attach hydrogens, and the remaining two functions as a lone pair, making it a trigonal pyramidal.
- The sp3 hybridization of nitrogen in ammonia.
What is the difference between trigonal pyramidal and bent shapes?
A non-collinear arrangement of adjacent atoms in a molecule with a central atom bonded to two further atoms is called a bent shape. On the other hand, a trigonal pyramidal shape corresponds to an overall four atoms. The three side atoms are bonded to the central one having a lone pair of electrons.
Bent shapes correspond to an angle of 109.5° whereas trigonal pyramidals prefer 107°.
Are tetrahedral and trigonal pyramidal the same? or Tetrahedral vs Trigonal pyramidal
A tetrahedron means four heads. The trigonal pyramidal and tetrahedral shapes both have four heads but the difference is in their regularity. A regular tetrahedron is called tetrahedral whereas an irregular or false tetrahedron refers to a pyramidal shape. So in fact, tetrahedral and pyramidal shapes are the same with some differences.
What does trigonal pyramidal look like?
A trigonal pyramidal geometry looks exactly like the pyramids of ancient Egypt.
Is PH3 trigonal pyramidal?
Phosphine molecule (PH3) is a triangular pyramidal molecule. Its geometry is confirmed by the following deductions.
- Two extra electrons in the valence shell of the phosphorous atom (phosphorous belonging to group 5 of the periodic table, corresponding to five valence electrons while only three are used for bonding with hydrogens).
- The hybridization state of phosphorous in phosphine is sp3.
- A bond angle of 93.5° negates its existence as a trigonal planar shape.
What is the difference between trigonal and triangular?
Trigonal is a geometrical term used for the reference of symmetrical triangles with regular medians. Triangular on the other hand is just a simple shape meaning three angles.
Which d orbitals are involved in the hybridization to form square planar and trigonal bipyramidal geometries?
In case of dsp2 hybridized molecules, (square planar shapes), d(x2 – y2) is involved. Whereas in the bipyramidal cases, sp3d hybridized molecules, d(z2) is involved in hybridization. These orbitals are also differentiated as eg and t2g orbitals.
