Grade 9
  1. Matter and its nature  1.1. Introduction to matter  1.2. Physical and chemical properties of matter  1.3. States of matter  1.3.1. Solid state  1.3.2. Fluid state  1.3.3. Gaseous state  1.3.4. Plasma and Bose–Einstein condensate  1.4. Changes in the states of matter  1.4.1. Melting and freezing  1.4.2. Evaporation and Condensation  1.4.3. Sublimation and deposition  1.5. Classification of matter  1.6. Elements, Compounds, and Mixtures  1.7. Pure substances and impurities  1.8. Separation Techniques  1.8.1. Filtration  1.8.2. Distillation  1.8.3. Crystallization  1.8.4. Chromatography  1.8.5. Centrifugation  1.8.6. Sublimation  1.8.7. Decantation and sedimentation
  2. Atomic Structure  2.1. Discovery of atoms  2.2. Dalton's atomic theory  2.3. Structure of the atom  2.4. Subatomic particles  2.5. Atomic number and mass number  2.6. Isotopes and Isobars  2.7. Electronic configuration  2.8. Bohr's atomic model  2.9. Modern Atomic Model (Quantum Mechanical Model - Introduction)  2.10. Valency and chemical bonding
  3. C hemical reactions and equations  3.1. Introduction to Chemical Reactions  3.2. Chemical Equation Formulation  3.3. Balancing Chemical Equations  3.4. Types of Chemical Reactions  3.4.1. Combination Reactions  3.4.2. Decomposition reactions  3.4.3. Displacement reactions  3.4.4. Double displacement reactions  3.4.5. Redox reactions  3.4.6. Endothermic and Exothermic Reactions  3.5. Effects of chemical reactions  3.6. Energy Changes in Chemical Reactions  3.7. Catalysts and their role in reactions
  4. Periodic table and periodicity  4.1. History of Periodic Classification  4.2. Mendeleev's periodic table  4.3. Modern Periodic Table  4.4. Periods and groups in the periodic table and periodicity  4.5. Trends in the Periodic Table  4.5.1. Atomic size  4.5.2. Ionization Energy  4.5.3. Electron affinity  4.5.4. Electronegativity  4.5.5. Metallic and Non-Metallic Properties  4.6. Noble gases and their properties
  5. Chemical bond  5.1. Introduction to Chemical Bonding  5.2. Types of chemical bonds  5.2.1. Ionic bond  5.2.2. Covalent bond  5.2.3. Metallic bond  5.2.4. Hydrogen bonding  5.2.5. Van der Waals force  5.3. Properties of Ionic and Covalent Compounds  5.4. Molecular Structure and Geometry (VSEPR Theory - Basics)
  6. Acids, Bases and Salts  6.1. Introduction to Acids and Bases  6.2. Properties of Acids  6.3. Properties of bases  6.4. pH scale and its importance  6.5. Indicators and their uses  6.6. Neutralization reactions  6.7. Strength of Acids and Bases (Strong vs. Weak)  6.8. Preparation of salts  6.9. Uses of acids, bases and salts in daily life
  7. Metals and Nonmetals  7.1. Physical Properties of Metals  7.2. Physical Properties of Nonmetals  7.3. Chemical properties of metals  7.4. Chemical Properties of Nonmetals  7.5. Reactivity Series of Metals  7.6. Extraction of metals  7.7. Corrosion and its prevention  7.8. uses of metals and nonmetals
  8. Carbon and its compounds  8.1. Introduction to Carbon  8.2. Allotropes of carbon (diamond, graphite, fullerene)  8.3. Hydrocarbons  8.3.1. Hydrocarbons  8.3.2. Alkene  8.3.3. Alkynes  8.4. Functional groups in organic chemistry  8.5. Isomerism in organic compounds  8.6. Alcohols and Carboxylic Acids  8.7. Soaps and detergents  8.8. Polymers (Introduction to Natural and Synthetic Polymers)
  9. Solutions and Mixtures  9.1. Types of mixtures  9.2. Homogeneous and Heterogeneous Mixtures  9.3. Concentration of solutions  9.4. Solubility and factors affecting solubility  9.5. Suspensions and Colloids  9.6. Saturated, unsaturated and supersaturated solutions
  10. Air and atmosphere  10.1. Composition of air  10.2. Role of gases in the atmosphere  10.4. Acid rain and its effects  10.5. Greenhouse effect and global warming  10.6. Ozone layer and its depletion  10.7. Air pollution control measures
  11. Water and its importance  11.1. Composition and properties of water  11.2. Hardness of water (temporary and permanent hardness)  11.3. Causes of water pollution  11.4. Effects of Water Pollution  11.5. Water purification methods (filtration, boiling, chlorination)
  12. Industrial Chemistry  12.1. Introduction to Industrial Chemistry  12.2. Important industrial chemicals (sulfuric acid, ammonia, sodium hydroxide)  12.3. Chemical processes in industry  12.4. Uses of Industrial Chemistry in Daily Life  12.5. Environmental impact of industrial chemical processes

Grade 9 → Chemical bond ↓


Molecular Structure and Geometry (VSEPR Theory - Basics)


Understanding the molecular structure and geometry of molecules is fundamental to the study of chemistry. The molecular geometry of a molecule determines many properties, such as polarity, reactivity, state of matter, color, magnetism, biological activity, and more. To predict the shape of molecules, we rely on the valence shell electron pair repulsion (VSEPR) theory. This theory is helpful in predicting the geometry of a molecule on the basis that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion.

Basics of VSEPR theory

VSEPR theory is based on the observation of electron pairs around a central atom. These electron pairs can be bond pairs, which are involved in chemical bonds, or lone pairs, which are not shared with other atoms. The shape of any given molecule can be predicted by considering these pairs.

Key concepts of VSEPR theory

  • The electron pairs around the central atom will keep themselves as far away from each other as possible.
  • The lone pair of electrons occupies more space than the bonding pair, because lone pairs are confined to a single atom.
  • The shape of a molecule is determined by the number of bond pairs and lone pairs around the central atom.

General molecular geometry

Several basic shapes are commonly described by VSEPR theory:

Linear geometry

A molecule will have linear geometry when there are two bond pairs and no lone pairs on the central atom. The bond angle is 180 degrees. A classic example of this is carbon dioxide, CO 2.

        Linear:
        O=C=O
Hey C Hey

Trigonal planar geometry

A molecule assumes a trigonal planar shape when it has three bond pairs and no lone pairs on the central atom. The bond angle is typically 120°. A common example is boron trifluoride, BF 3.

B F F F

Tetrahedral geometry

For tetrahedral geometry, there are four bond pairs and no lone pairs on the central atom. This leads to a bond angle of 109.5°. A well-known example is methane, CH 4.

C H H H H

Bent geometry

Molecules such as water, H 2 O have a bent structure. This is mainly because there are two bond pairs and two lone pairs on the central atom, leading to a bond angle of about 104.5 degrees.

H H Hey

Discovery of other general geometries

Triangular pyramid geometry

In the case of ammonia, NH 3, the structure is trigonal pyramidal, since there are three bond pairs and one lone pair around the nitrogen atom, resulting in a bond angle of approximately 107°.

N H H H

Octahedral geometry

Octahedral geometry is characterized by six bond pairs and no lone pairs on the central atom. Compounds such as sulfur hexafluoride, SF 6 exhibit this pattern with a bond angle of 90 degrees.

S F F F

Understanding the effect of lone pairs

An important factor in determining molecular shape is the effect of lone pairs. Lone pairs occupy more space than bond pairs, partly because they are located around only one nucleus. This additional repulsion can lead to smaller bond angles relative to the optimal geometry without lone pairs.

Examples and exercises

To deepen your understanding of VSEPR and molecular geometry, let's take a look at some examples. We'll examine several molecules, determine their Lewis structures, and predict their geometry based on the number of bonding and lone pairs.

Example 1: Carbon tetrachloride (CCl 4)

Carbon tetrachloride is a molecule with four chlorine atoms bonded to a central carbon atom. Drawing the Lewis structure for CCl 4, we see that the carbon atom satisfies the octet rule with four single bonds. With no lone pair on the central atom, it assumes a tetrahedral arrangement with a bond angle close to 109.5 degrees.

Example 2: Nitrite ion (NO 2 -)

The nitrite ion has a resonance structure with two nitrogen-oxygen single bonds and one nitrogen-oxygen double bond. Nitrogen has a lone pair, which makes the electron-domain geometry of the molecule trigonal in a plane, with the molecular geometry bent. This is an example of the effect of resonance on chemistry that draws conclusions from electron domain statistics to predict molecular shapes.

Exercise 1

Consider the Lewis structure for formaldehyde, CH 2 O Identify and determine the ideal molecular geometry and the presence of any lone pairs on the central atom.

        yes
         ,
          C
         ,
        H

Solution: The structure shows that oxygen has two lone pairs and form a trigonal planar group around the formaldehyde carbon.

Conclusion

VSEPR theory provides an important basis for predicting molecular geometry based on electron pair repulsion. Although it simplifies complex quantum chemical interactions, it serves as a valuable tool in high school and undergraduate chemistry.

Remember, the basic idea is that electron pairs attempt to minimize repulsion and thus adopt specific predicted structures or geometries. By becoming familiar with the molecular geometries determined by VSEPR theory, you can better understand the molecular world!


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Molecular Structure and Geometry (VSEPR Theory - Basics)

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