Undergraduate
  1. General chemistry  1.1. Introduction to Chemistry  1.2. Atomic Structure  1.2.1. Subatomic particles  1.2.2. Atomic Model  1.2.3. Quantum numbers  1.2.4. Electron Configuration  1.2.5. Periodic trends  1.2.6. Isotopes and their applications  1.3. Chemical bond  1.3.1. Ionic bond  1.3.2. Covalent bonds  1.3.3. Metal Bond  1.3.4. Hybridization  1.3.5. Molecular geometry and VSEPR theory  1.3.6. Bond polarity and dipole moment  1.4. Stoichiometry  1.4.1. Mole concept  1.4.2. Empirical and molecular formula  1.4.3. Limiting reactant and excess reactant  1.4.4. Percent Yield and Purity  1.4.5. Dilution calculation  1.5. States of matter  1.5.1. Gas Laws  1.5.2. Matter and Intermolecular Forces  1.5.3. Solid and Crystal Structures  1.5.4. Phase diagram  1.5.5. Plasma and supercritical fluid  1.6. Chemical reactions  1.6.1. Types of Chemical Reactions  1.6.2. Balancing Chemical Equations  1.6.3. Thermochemical reactions  1.6.4. Reaction energy profiles  1.7. Solutions and Mixtures  1.7.1. Concentration units  1.7.2. Syndrome properties  1.7.3. Solubility and Solubility Rules  1.7.4. Henry's Law and Raoult's Law  1.7.5. Partition coefficient  1.8. Chemical equilibrium  1.8.1. Law of mass action  1.8.2. Le Chatelier's principle  1.8.3. Reaction quotient  1.8.4. To use this online calculator for Equilibrium Constant, enter Equilibrium Constant (Eq.) and hit the calculate button. Here is how the Equilibrium Constant calculation can be explained with given input values -> 0.05 = 0.05/0.  1.8.5. Acid-base balance  1.9. Acids and bases  1.9.1. Arrhenius, Brønsted–Lowry, and Lewis definitions  1.9.2. pH and pOH  1.9.3. Acid-base titration  1.9.4. Buffer Solution  1.9.5. Acid-base indicator  1.9.6. Polyprotic Acids and Bases  1.10. Electrochemistry  1.10.1. redox reactions  1.10.2. Galvanic and Electrolytic Cells  1.10.3. Nernst equation  1.10.4. Electrolysis  1.10.5. Faraday's laws of electrolysis  1.11. Thermodynamics  1.11.1. Laws of Thermodynamics  1.11.2. Enthalpy, entropy and Gibbs free energy  1.11.3. Spontaneity of reactions  1.11.4. Thermodynamic cycles (Hess's law, Carnot cycle)  1.12. Kinetics  1.12.1. Rate law and reaction order  1.12.2. Activation Energy and Catalyst  1.12.3. Collision theory  1.12.4. Reaction mechanism  1.12.5. Transition state theory
  2. Organic chemistry  2.1. Structure and relationships  2.1.1. Hybridisation in Carbon Compounds  2.1.2. Resonance and aromatization  2.1.3. Molecular orbitals  2.1.4. Inductive and mesomeric effects  2.2. Hydrocarbons  2.2.1. Hydrocarbons  2.2.2. Alkene  2.2.3. Alkynes  2.2.4. Aromatic compounds  2.2.5. Cycloalkanes and Conformation  2.3. Functional Group  2.3.1. Alcohols and phenols  2.3.2. Ethers and epoxides  2.3.3. Aldehyde and ketone  2.3.4. Carboxylic acids and derivatives  2.3.5. Amin  2.3.6. Esters and amides  2.3.7. Thiols and sulfides  2.4. Stereoscopic  2.4.1. Isomerism in Stereochemistry  2.4.2. Chirality and optical activity  2.4.3. Structural analysis  2.4.4. Diastereomers and Enantiomers  2.5.1. Substitution reactions  2.5.2. Elimination reactions  2.5.3. Addition reactions  2.5.4. Radical reactions  2.5.5. Rearrangement reactions  2.5.6. Pericyclic Reactions  2.6. Spectroscopy and structural analysis  2.6.1. Infrared Spectroscopy  2.6.2. Nuclear magnetic resonance spectroscopy  2.6.3. Mass Spectrometry  2.6.4. UV-visible spectroscopy  2.6.5. X-ray crystallography  2.7. Polymer chemistry  2.7.1. Addition and Condensation Polymers  2.7.2. Polymerization mechanism  2.7.3. Biodegradable Polymer  2.7.4. Conducting and smart polymers
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Ligands and coordination compounds  3.1.2. Crystal field theory  3.1.3. Molecular Orbital Theory in Coordination Compounds  3.1.4. Jahn–Taylor distortion  3.2. Main Group Chemistry  3.2.1. Alkali and alkaline earth metals  3.2.2. Halogens and Noble Gases  3.2.3. Transition metals and their complexes  3.2.4. Lanthanides and Actinides  3.3. Solid state chemistry  3.3.1. Crystal lattices and unit cells  3.3.2. Defects in solids  3.3.3. Electrical and magnetic properties  3.3.4. Superconductivity  3.4. Bio-inorganic chemistry  3.4.1. Metal ions in biology  3.4.2. Enzymatic reactions with metals  3.4.3. Metal poisoning and detoxification  3.4.4. Metalloproteins and Metalloenzymes
  4. Physical chemistry  4.1. Quantum Chemistry  4.1.1. Wave–particle duality  4.1.2. Schrödinger Equation  4.1.3. Quantum Numbers and Orbitals  4.1.4. Atomic and molecular spectroscopy  4.2. Statistical mechanics  4.2.1. Maxwell–Boltzmann distribution  4.2.2. Partitioning functions  4.2.3. Bose–Einstein and Fermi–Dirac statistics  4.3. Chemical thermodynamics  4.3.1. Gibbs Free Energy  4.3.2. Phase transition  4.3.3. Thermodynamic efficiency  4.4. Surface chemistry  4.4.1. Absorption and Catalysis  4.4.2. Colloids and Emulsions
  5. Analytical Chemistry  5.1. Classical methods  5.1.1. Gravimetric Analysis  5.1.2. titrimeter  5.2. Instrumental methods  5.2.1. Chromatography  5.2.2. Spectroscopy  5.2.3. Electroanalytical methods  5.2.4. X-ray diffraction
  6. Industrial Chemistry  6.1. Petrochemistry  6.2. Chemical engineering principles  6.3. Green Chemistry  6.4. Pharmaceuticals and Drug Synthesis
  7. Biochemistry  7.1. Carbohydrates  7.2. Proteins and enzymes  7.3. Lipids and membranes  7.4. Nucleic acids  7.5. Metabolism and Bioenergetics  7.6. Enzyme kinetics and mechanism

Undergraduate → General chemistry → Chemical bond ↓


Molecular geometry and VSEPR theory


Molecular geometry is an important concept in chemistry that describes the three-dimensional arrangement of atoms in a molecule. It is important because it affects many of the molecule's physical properties, reaction pathways, and interactions with other molecules. One widely used model to predict molecular geometry is the valence shell electron pair repulsion (VSEPR) theory. This theory helps chemists understand the spatial arrangement of the atomic components of compounds based on the repulsion between electron pairs in the valence shells of atoms.

Understanding molecular geometry

Before delving deeper into VSEPR theory, let's understand why molecular geometry is important. When atoms join together to form molecules, their electron clouds overlap, and this determines how the atoms are located in space relative to each other. The spatial arrangement, or geometry, of a molecule affects its physical and chemical properties, such as boiling and melting point, polarity, and interactions with other molecules. For example, the difference in boiling point between water (H2O) and hydrogen sulfide (H2S) can be attributed to their molecular geometry.

VSEPR theory: The basics

VSEPR stands for Valence Shell Electron Pair Repulsion Theory. It holds that electron pairs around a central atom will arrange themselves to be as far apart as possible to minimize repulsion between these pairs. The theory considers both bond electrons, which are shared between atoms, and lone pairs, which are localized to a single atom. The presence of lone pairs can significantly affect the geometry of the molecule because they occupy space and repel other electron groups more strongly than bond pairs.

Application of VSEPR theory

When predicting the geometry of a molecule using VSEPR theory, you take the following steps:

  1. Count the total number of valence electrons in the molecule.
  2. Determine the arrangement of electron pairs (both bonding and lone pairs) such that there is minimum repulsion between them.
  3. Identify the resulting molecular shape based on the positions of the bonded atoms.

Let's explore some common molecular geometries and the associated VSEPR models:

Linear geometry

In linear geometry, the central atom is surrounded by two pairs of electrons at an angle of 180 degrees. This is typical for molecules with the formula AX2, where A is the central atom and X represents the surrounding atoms. An example of this is carbon dioxide (CO2).

      OCO

In this example, the molecule is linear with a bond angle of 180 degrees.

Trigonal planar geometry

Trigonal planar molecules have a central atom surrounded by three electron pairs at an angle of 120 degrees. A typical example of this is boron trifluoride (BF3).

                F
               ,
          F - B
               ,
                F

Tetrahedral geometry

In tetrahedral geometry, the central atom is surrounded by four bond pairs. An example of this is methane (CH4). The bond angles are approximately 109.5 degrees.

          H
          ,
      H – C – H
          ,
          H

This arrangement minimises repulsion and achieves symmetry.

Trigonal bipyramidal geometry

This geometry occurs in molecules with five regions of electron density. Phosphorus pentachloride (PCl5) is an example. This structure consists of three equatorial bonds at 120 degrees and two axial bonds at 90 degrees.

             Chlorine
             ,
      Cl – P – Cl
             ,
         cl cl

Octahedral geometry

In octahedral geometry, the central atom is surrounded by six bond pairs with 90 degree angles. An example of this would be sulfur hexafluoride (SF6).

            F
            ,
        F - S - F
            ,
            F
         FF

Lone pair effect

Lone pairs have a significant effect on molecular geometry. They occupy more space than bonding pairs, which can reduce the bond angle between adjacent atoms. This often results in deviations from ideal angles. Let's illustrate this with a water molecule (H2O).

                H
               ,
          Hey
               ,
                H

Water is not a linear molecule, even though its formula is similar to CO2. Instead, the arrangement is "bent" with a bond angle of about 104.5 degrees due to the two lone pairs on the oxygen.

Distorted geometry: examples and explanations

Let us look at molecules with distorted geometries due to lone pairs:

  • Ammonia (NH3): Its geometry is trigonal pyramidal. The lone pair makes the bond angle less than the ideal 109.5 degrees, to about 107 degrees.
                H
               ,
          H–N
               ,
                H
  • Sulfur tetrafluoride (SF4): The geometry is fluctuating due to a lone pair. The axial and equatorial bond angles are less than ideal.
                 F
                 ,
          F - S - F
                 ,
                F
                  F

Role of electronegativities and dipole moment

Molecular shape affects the electronegativities distribution and dipole moment. Molecules such as CO2 are nonpolar because their symmetrical arrangement cancels the individual bond dipoles. However, a bent shape such as H2O results in a net dipole moment, making it polar.

Conclusion

Understanding molecular geometry through VSEPR theory is fundamental in chemistry. Predicting shapes helps chemists predict reactivity, physical properties, and interaction behavior with other chemical species. By considering bonds and lone pairs as well as their repulsion, one can understand a clear picture of the molecular world.


Undergraduate → 1.3.5


U
username
0%
completed in Undergraduate

← Prev (1.3.4)
Hybridization
Next (1.3.6) →
Bond polarity and dipole moment

Comments 



Molecular geometry and VSEPR theory

https://www.chemistryelite.com/ug/1.3.5?ceal=en