Difference Between Trigonal Planar and Trigonal Pyramidal

Trigonal planar and trigonal pyramidal are the two most confusing geometries terminologies. The two describe the 3-dimensional arrangement of molecule atoms in space. However, the two aren’t the only terms that describe the order of atoms. Others include bent, tetrahedral, linear, and octahedral.

Therefore, what’s the difference between these two terms? The trigonal planer doesn’t have any lone electron pair in the central atom. On the other hand, trigonal pyramidal has a single lone electron pair in the central atom. So, in this article, we endeavor to explore all the differential properties of these geometries.

About Trigonal Planar

Trigonal planar geometry consists of three periphery atoms connected to the central atom forming triangular corners. As the name suggests, the central atom and the group of atoms repel each other. So, the atoms remain on a single plane hence the name planar.

Trigonal Planar
Trigonal Planar

The peripheral atoms are inclined at an angle of 1200. The molecules also have an equal number of atoms. Good examples of trigonal planar molecules are Boron Trifluoride, carbonates, and sulfates.

Besides the peripheral atoms, there can also be ligands and complex groups around the central atom. C (NH2)3 is a good example of these complex compounds.

Characteristics of trigonal planar

  • The central atom doesn’t have a lone pair of electrons
  • Trigonal planar molecules are triangular in shape and lie on a single plane
  • They consist of molecules with four atoms. The central atom connects to three peripheral ones forming corners of a triangular
  • The peripheral atoms are inclined at an angle of 1200
  • They exhibit bond-bond repulsion

About Trigonal Pyramidal

Molecules with trigonal pyramidal geometry can also be described as SP3. They consist of three bonds and a lone pair of electrons on the middle atom.

Good examples of trigonal pyramidal molecules are Ammonia (NH3). It consists of a single nitrogen atom and three hydrogen atoms. The three hydrogens attach themselves to the nitrogen.

Trigonal pyramidal
Trigonal Pyramidal

The lone pair of electron repels the three hydrogen atoms to form a trigonal pyramid. This leads to increase in angle between the electrons and the atoms. This angle extends as far as 1070.

Characteristics of trigonal pyramidal

  • The central atom appears on the apex and the other three on the base
  • The central atom has a single lone electrons pair
  • The atoms are not on the same plane
  • The angle between the lone pair of electrons and the three atoms are about 1070 apart.

Comparisons Between Trigonal Planar And Trigonal Pyramidal

FeatureTrigonal planarTrigonal pyramidal
DefinitionConsists of molecules with four atoms. One central and three peripherals. They connect to the central atom to form triangular corners.  Consists of four atoms also known as ligands. The central atoms are at the apex and the other three are at the base. They form in three corners of a rectangle.
Lone pair of electronsDoesn’t have a lone pair of electrons in its central atom.Their central atom has a lone pair of electrons in its central atom.
PlaneTrigonal planar molecules are triangular in shape. They also lie on the same plane.Trigonal pyramidal atoms are on different surface.
Angle between atomsPeripheral atoms are at an angle of 1200 between each other.  The bond angle between atoms is about 1070.
RepulsionExhibits bond-bond repulsionExhibits bond-bond and bond electron pair repulsion
BondingFeatures multiple atoms bondingHas only three bonded atoms

FAQs – Frequently Ask Questions

State 5 basic shapes of molecules

According to VSEPR theory, there are five main molecule shapes. These include;

  • Tetrahedral. Linear,
  • trigonal planar
  • Trigonal bipyramidal
  • Octahedral

What differentiates bent and linear shapes?

Linear molecules have lines of atoms at an angle of 1800. They can have 2 or 3 atoms in total. On the other hand, bend is a linear molecule with lone pairs that cause it to bend. The more loan pairs of electrons it has the more the bend.

How can I determine polarity?

Consider every bond to be either non-polar or polar. If the electronegativity of the atoms in the bond is above 0.4, then the bond is polar.

However, otherwise, the bond is nonpolar. Additionally, if there don’t exist polar bonds, the molecule is nonpolar.

Is PH3 trigonal planar or trigonal pyramidal?

PH3 depicts a trigonal pyramidal geometry. Using the Lewis structure, this molecule has eight valence electrons.

What do you use differentiate planar molecules and nonplanar?

The best method of determining whether a molecule is planar or not is determining its hybridization. If you get 2, then the said molecule is planar.

Alternatively, you can determine the dipole moment. If you get zero, then the molecule is polar. The dipole moment is zero when the surrounding atoms are the same.

Final Recap

Geometry is a complicated and vast mathematics topic. There are multiple terminologies used all across the topic, some of which can be a bit challenging to understand.

Trigonal planar and trigonal pyramidal are just but a few. These two being one of the common ones, understanding them will make geometry easier for you.