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Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It describes how the atoms are distributed in space and how they are connected by chemical bonds.
- It includes the general shape of the molecule, which can be linear, bent, trigonal, tetrahedral, etc.
- It also includes specific values of bond lengths, bond angles, torsional angles and any other geometrical parameters that determine the exact position of each atom.
- Molecular geometry is an important factor that affects the chemical and physical properties of a molecule, such as its colour, reactivity, polarity and biological activity.
- For example, the molecular geometry of water determines its ability to form hydrogen bonds and its unique properties as a solvent.
| Table of Content |
Key Terms: Molecular Geometry, Atom, Molecule, VSEPR theory, Bond Lengths, Lewis electron dot structure
What is Molecular Geometry?
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Molecular geometry is the study of the three-dimensional shape of a molecule. The central atom and the surrounding atoms and electron pairs determine it.
- The shape of a molecule is influenced by the repulsion and attraction forces between the electron pairs
- It tend to arrange themselves as far apart as possible to minimise repulsion.
- The shape of a molecule also depends on the number and type of bonds that the central atom forms with the surrounding atoms.
Determine Molecular Geometry
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There are various methods to determine the molecular geometry of a molecule, such as spectroscopic methods, diffraction methods and computational methods. However, one of the simplest and most widely used methods is the Lewis electron dot structure, which shows the valence electrons of the atoms in a molecule.
- The Lewis structure helps to identify the bond pairs and the lone pairs on the central atom.
- Then, one can apply the valence-shell electron-pair repulsion (VSEPR) theory to predict the molecular geometry and the electron-group geometry.
Types of Molecular Geometry
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There are various types of Molecular Geometry. A few are mentioned below:
Linear Molecular Geometry
When the central atom has two electron groups, this molecular geometry results. The electron groups point away from each other, making a linear shape.
- The bond angle is 180°.
- The molecular geometry is also linear since there are no lone pairs on the central atom.
- Examples of molecules with linear geometry are CO2, BeCl2, and HCN.
Trigonal Planar Molecular Geometry
This molecular geometry results when the central atom has three electron groups. The electron groups form a shape with three equal sides on a flat surface.
- The bond angle is 120°.
- Examples of molecules with trigonal planar geometry are BF3, SO3, and NO3-.
Tetrahedral Molecular Geometry
A regular tetrahedron is a three-dimensional shape that forms when the central atom has four electron groups. The electron groups are positioned in this shape.
- The bond angle is 109.5°.
- Examples of molecules with tetrahedral geometry are CH4, SiCl4, and NH4.
Trigonal bipyramidal Molecular Geometry
Two triangular pyramids that have a common base are the three-dimensional shape that forms when the central atom has five electron groups. The electron groups are positioned in this shape.
- The bond angle between the equatorial groups (the ones on the base) is 120°
- The bond angle between the axial groups (the ones on the tips) and the equatorial groups is 90°.
- Examples of molecules with trigonal bipyramidal geometry are PCl5, PF5, and AsF5.
Octahedral Molecular Geometry
A regular octahedron is a three-dimensional shape that forms when the central atom has six electron groups. The electron groups are positioned in this shape.
- The bond angle between any two groups is 90°.
- Examples of molecules with octahedral geometry are SF6, SeF6, etc.
VSEPR Theory
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It is a model used in chemistry to predict the geometry of molecules based on the number and arrangement of electron pairs around the central atoms.
The main features of VSEPR theory are:
- It assumes that the electron pairs in the valence shell of a central atom repel each other and try to minimise this repulsion by occupying positions as far apart as possible.
- It classifies the electron pairs into bonding pairs and lone pairs and treats double and triple bonds as single electron groups.
- It determines the electron-group geometry and the molecular geometry based on the number of electron groups and the number of lone pairs around the central atom.
- It predicts the bond angles and the shape of the molecule based on the molecular geometry.
VSEPR Structure
Regular and Irregular Geometry of a Molecule
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According to the VSEPR theory, the shape of a covalent molecule can be either regular or irregular, depending on the arrangement of the electron pairs around the central atom.
Regular Geometry
A regular geometry of a molecule has all the bond pairs and lone pairs equally spaced around the central atom, resulting in a symmetrical shape.
- For example, methane (CH4) has a regular tetrahedral geometry, with four bond pairs and no lone pairs on the carbon atom.
- A regular geometry of a molecule also implies that all the bond angles and bond lengths are equal.
Irregular Geometry
An irregular geometry of a molecule has some bond pairs and lone pairs closer or farther apart than others, resulting in an asymmetrical shape.
- For example, ammonia (NH3) has an irregular trigonal pyramidal geometry, with three bond pairs and one lone pair on the nitrogen atom.
- The lone pair occupies more space than the bond pairs, causing the bond angle to be less than 109.5°.
- An irregular geometry of a molecule also implies that some bond angles and bond lengths are different from others.
Geometry of Covalent Molecules Based on VSEPR Theory
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The VSEPR theory is a simple and useful tool to predict the geometry of some covalent molecules.
| Molecular Type | No. of bond pairs | No. of lone pairs | Geometry of molecule | Shape of the molecule | Examples |
|---|---|---|---|---|---|
| AX2 | 2 | 0 |  |
Linear | CO2, BeCl2 |
| AX3 | 3 | 0 |  |
Trigonal planar | BF3, SO3 |
| AX3E | 3 | 1 |  |
Trigonal pyramidal | NH3, PCl3 |
| AX4 | 4 | 0 |  |
Tetrahedral | CH4, SiCl4 |
| AX4E | 4 | 1 |  |
Seesaw | SF4, TeCl4 |
| AX4E2 | 4 | 2 |  |
Square planar | XeF4, ICl4- |
| AX5 | 5 | 0 |  |
Trigonal bipyramidal | PCl5, PF5 |
| AX5E | 5 | 1 |
Octahedral |
Square pyramidal | BrF5, IF5 |
| AX6 | 6 | 0 |
Octahedral |
Octahedral | SF6, SeF6 |
Things to Remember
- Molecular geometry is the three-dimensional arrangement of the atoms and bond pairs in a molecule.
- The VSEPR theory predicts the molecular geometry based on the repulsion and attraction forces between the electron groups.
- There are five basic electron-group geometries: linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral.
- A regular geometry of a molecule has all the bond pairs and lone pairs equally spaced around the central atom, resulting in a symmetrical shape.
- An irregular geometry of a molecule has some bond pairs and lone pairs closer or farther apart than others, resulting in an asymmetrical shape.
Also Read:
| Related Articles | ||
|---|---|---|
| Chemical Tranquilizers | Analgesics | Thermochemistry |
| Hydrogen Spectrum | Positron | Tetravalency |
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Sample Questions
Ques. What is molecular geometry and why is it important? (2 marks)
Ans. Molecular geometry is three dimensions, based on how the atoms are arranged. three-dimensional arrangement of atoms in a molecule. It is important because it determines the physical and chemical properties of the molecule, such as polarity, reactivity, shape, and intermolecular forces.
Ques. How can we determine the molecular geometry of a molecule using VSEPR theory? (3 marks)
Ans. VSEPR (Valence Shell Electron Pair Repulsion) theory is a simple model that predicts the molecular geometry of a molecule based on the number and arrangement of electron pairs around the central atom. The basic steps are:
- Draw the Lewis structure of the molecule and identify the central atom.
- Count the number of electron pairs (bonding and non-bonding) around the central atom. This is the steric number.
- Arrange the electron pairs as far apart as possible to minimize repulsion. This is the electron domain geometry.
- Determine the molecular geometry by considering only the positions of the atoms, not the lone pairs.
Ques. What are the types of molecular structure and how are they different from each other? (2 marks)
Ans. There are two types of molecular structure: linear and non-linear. Linear molecules have two atoms or groups of atoms attached to the central atom, forming a straight line. Non-linear molecules have more than two atoms or groups of atoms attached to the central atom, forming various shapes such as bent, trigonal planar, tetrahedral, etc.
Ques. What are the factors that affect the bond angles in a molecule? (3 marks)
Ans. The bond angles in a molecule depend on the following factors:
- The steric number of the central atom: higher steric numbers usually result in larger bond angles.
- The presence and number of lone pairs on the central atom: lone pairs tend to repel more than bonding pairs, causing the bond angles to decrease.
- The electronegativity difference between the central atom and the surrounding atoms: higher electronegativity difference causes more bond polarity, which can distort the bond angles.
Ques. What is the molecular geometry of CO32- ion and why? (2 marks)
Ans. The molecular geometry of CO32- ion is trigonal planar. This is because the central carbon atom has three sigma bonds with three oxygen atoms and no lone pairs, giving it a steric number of 3. The electron domain geometry and the molecular geometry are both trigonal planar, with bond angles of 120°.
Ques. What is the hybridization of the central atom in each of the following molecules? Explain your reasoning. (5 marks)
a) CH4
b) NH3
c) H2O
d) CO2
Ans. The hybridization of the central atom in each molecule is as follows:
- CH4: four sigma bonds and no lone pairs, giving it a steric number of 4. To form four equivalent orbitals, it needs to hybridize one s orbital and three p orbitals, resulting in sp3 hybridization.
- NH3: three sigma bonds and one lone pair, giving it a steric number of 4. To form four equivalent orbitals, it needs to hybridize one s orbital and three p orbitals, resulting in sp3 hybridization.
- H2O: two sigma bonds and two lone pairs, giving it a steric number of 4. To form four equivalent orbitals, it needs to hybridize one s orbital and three p orbitals, resulting in sp3 hybridization.
- CO2: two double bonds and no lone pairs, giving it a steric number of 2. To form two equivalent orbitals, it needs to hybridize one s orbital and one p orbital, resulting in sp hybridization.
Ques. What is the difference between electron domain geometry and molecular geometry? (2 marks)
Ans. Electron domain geometry is the arrangement of all the electron pairs (bonding and non-bonding) around the central atom, while molecular geometry is the arrangement of only the atoms around the central atom. Electron domain geometry affects molecular geometry, but they are not always the same.
Ques. What are the molecular geometries of the following molecules? Draw their Lewis structures and indicate the bond angles. CH4, NH3, H2O, CO2, SO2 (5 marks)
Ans. The molecular geometries of the following molecules are:
- CH4: Tetrahedral, with bond angles of 109.5°. The Lewis structure is:
- NH3: Trigonal pyramidal, with bond angles of 107°. The Lewis structure is:
- H2O: Bent, with bond angles of 104.5°. The Lewis structure is:
- CO2: Linear, with bond angles of 180°. The Lewis structure is:
- SO2: Bent, with bond angles of 120°. The Lewis structure is:
Ques. What is the effect of multiple bonds on the molecular geometry of a molecule? (3 marks)
Ans. Multiple bonds (double or triple) count as one electron group to determine the molecular geometry of a molecule. However, multiple bonds have more electron density and repulsion than single bonds, which can affect the bond angles and the polarity of the molecule. For example, CO2 has a linear geometry with bond angles of 180°, while SO2 has a bent geometry with bond angles of 120°.
Ques. What is the molecular geometry of a molecule with five electron groups and one lone pair around the central atom? (2 marks)
Ans. The molecular geometry of a molecule with five electron groups and one lone pair around the central atom is square pyramidal. This is because the lone pair occupies one of the axial positions and pushes the other four bonds into a square plane.
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