The Triangular Shape of Molecules in Chemistry

Introduction

The five main shapes of molecules according to the VSEPR theory are linear (example: CO2), Trigonal Planar (example: BCl3 ), Tetrahedral (example: CH4), Trigonal bipyramidal (example: PCl5 ), and Octahedral (example: SF6). A few questions arise. Are there molecules in ‘triangular shapes’ with atoms/molecules as vertices and bonds as the sides of the triangular shape? Are the ‘trigonal planar’ and ‘triangular’ terms referring to the same molecular structure?.

The molecules which exist in triangular shapes are highly reactive (example: Cyclopropane). This is because the ring strain in such molecules is high, and they tend to react to any other chemical species and form more stable compounds. Every chemical compound tends to lower its energy state; the lower the energy state of the molecule, the more stable the chemical compound is. High ring strain increases the energy state of the molecule and thus these triangular chemical compounds tend to be highly reactive to achieving a lower energy state and be stable.

VSEPR Theory Shapes of Molecules

The shape of a molecule plays a vital role in determining the chemical and physical properties of molecules. Chemical bonding determines the shape, reactivity, polarity, colour, and magnetic property of a molecule.

The electron pair repulsion forms the theoretical basis of the VSEPR theory (Valence Shell Electron Pair Repulsion Theory). The repulsive nature of electron pairs increases in this order:

Bond pair(bp)-Bond pair(bp) < lone pair(lp)-Bond pair(bp) < lone pair(lp)-lone pair(lp)

VSEPR Theory Postulates

• Number of valence shell electron pairs (bonded or non-bonded) around the central atom determine the shape of a molecule.
• As the electron clouds are negatively charged, and like charges repel each other, the pair of electrons in the valence shell repel each other.
• More repulsion means more instability; therefore, these electron pairs tend to occupy such positions in space that could maximise the distance between them and thus minimise the repulsion between them.
• The VSEPR theory deals with the valence shell, and the valence electrons are taken as electron pairs localising on the valence shell at maximum distance from one another. The valence shell is taken as a sphere.
• Multiple bonds are treated as if it is a single electron pair and the two or three electron pairs of multiple bonds are treated as a single super pair.
• A molecule may have two or more resonance structures. In that case, the VSEPR model applies to any such structure.

The presence of lone pairs of electrons results in deviations of molecules from their ideal shapes because the lone pair of electrons experience greater repulsion as compared to bond pair electrons. The reason behind greater repulsive forces is that the lone pairs are localised on the central atoms.

The geometrical shapes of molecules are predicted with the help of the VSEPR Theory. For this purpose, the molecules are divided into two categories:

• Molecules in which the central atom has no lone pair.
• Molecules in which the central atom has one or more lone pairs.

We can now dive deeper into the various shapes that molecules might acquire due to the nature of hybridisation and/or other factors which often need to be studied on an individual basis.

Linear Molecule

An example of Linear molecular structure is CO2, wherein the carbon (C) atom is present in the middle and the two oxygen (O) atoms are 1800 apart. The hybridisation of a linear molecule is sp.

Trigonal Planar 

An example of a molecule possessing trigonal planar molecular geometry is BCl3. In this molecule, the boron (B) atom is present at the centre, and three chlorine (Cl) atoms are present at the corners of an equilateral triangle which is trigonal planar molecular geometry. 

An ideal trigonal planar shape is available in a molecule wherein all the three ligands are identical; the bond angle in such a molecule is 1200. Deviation in the ideal shape of a molecule occurs when different ligands are present as electronegativity difference, and electron pair interactions affect the shape of molecules. The hybridisation possessed by trigonal planar is sp2 hybridisation.

Tetrahedral

An example of Tetrahedral molecular structure is CH4, wherein the carbon(C) atom is present in the centre, and all other hydrogen atoms are arranged in a three-dimensional arrangement with bond angles of 109.50. The hybridisation of a tetrahedral molecule is sp3.

Trigonal bipyramidal 

An example of a molecule possessing trigonal bipyramidal structure is PCl5, wherein a Phosphorous(P) atom is present in the centre and five chlorine (Cl) atoms surround it, out of which three atoms are present in a plane forming a bond angle of 1200 and two are arranged in a three-dimensional arrangement on the opposite ends of the molecule. The hybridisation possessed by trigonal bipyramidal is sp3d.

Octahedral

An example of a molecule possessing an octahedral molecular structure is SF6, wherein the sulphur (S) atom is present in the centre and all of the six fluorine atoms surround the central atom in a three-dimensional structure with bond angles of 900. The hybridisation possessed by octahedral is sp3d2. 

Conclusion

A huge section of chemistry deals with the study of molecular shape and geometry because this ultimately decides the energy state of the molecules and defines their reactivity, polarity, etc. To a large extent, the VSEPR theory is successful in predicting the molecular geometry of chemical compounds, whereas the concepts of electron pair repulsions are still a point of discussion.

We have covered the VSEPR theory, which explains and justifies the molecular geometry to a large section. We have also discussed the trigonal planar shape of a molecule wherein we understood that a  trigonal planar structure is the one wherein an atom is present in the centre and three atoms are present at the vertex of an equilateral triangle. Molecules in a triangular shape, that is where atoms/molecules form the vertex of the triangle and bonds form the sides, are highly unstable because the bond angles in these molecules are about 600. Such a small bond angle leads to strain and the bond’s instability, and due to less stability, these molecules are not easily found as they tend to react as soon as possible and form some stable compounds.