Chapter 4: Chemical Bonding in Molecules P R E S ENTED B Y : I S H AQ T I MP A C Lesson 1: Ionic Bonding • Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, and is the primary interaction occurring in ionic compounds. • Ions are atoms that have gained one or more electrons (known as anions, which are negatively charged) and atoms that have lost one or more electrons (known as cations, which are positively charged). This transfer of electrons is known as electrovalence in contrast to covalence. Lesson 1: Ionic Bonding • Example: Lesson 2: Covalent Bonding • A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. • For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration. Lesson 2: Covalent Bonding • Example: Lesson 2: Covalent Bonding Types of Covalent Bonds: 1. Single Bond - a chemical bond in which one pair of electrons is shared between two atoms. 2. Double Bond - a chemical bond in which two pairs of electrons are shared between two atoms. 3. Triple Bond - a chemical bond in which three pairs of electrons are shared between two atoms. Lesson 2: Covalent Bonding Lesson 3: Metallic Bonding • Metallic bonding is a type of chemical bonding that rises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions. • It may be described as the sharing of free electrons among a structure of positively charged ions (cations). • Metallic bonding accounts for many physical properties of metals, such as strength, ductility, thermal and electrical resistivity and conductivity, opacity, and luster. Lesson 3: Metallic Bonding • Example: Lesson 4: Bond Properties Bond Length • Bond length or Bond distance is the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of the molecule. • Bond length is related to bond order: when more electrons participate in bond formation the bond is shorter. Bond length is also inversely related to bond strength and the bond dissociation energy: all other factors being equal, a stronger bond will be shorter. In a bond between two identical atoms, half the bond distance is equal to the covalent radius. Lesson 4: Bond Properties Bond Energy • Bond energy or Bond enthalpy is the measure of bond strength in a chemical bond. • Bond energy is the average of all the bond-dissociation energies in a molecule, and will show a different value for a given bond than the bond-dissociation energy would. • This is because the energy required to break a single bond in a specific molecule differs for each bond in that molecule. Lesson 5: Polarity • Polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. • The polarity of a bond is the distribution of electrical charge over the atoms joined by the bond. • Polar molecules must contain polar bonds due to a difference in electronegativity between the bonded atoms. • There are three main properties of chemical bonds that must be considered—namely, their strength, length, and polarity. Lesson 5: Polarity Bond polarity is typically divided into three groups that are loosely based on the difference in electronegativity between the two bonded atoms. According to the Pauling scale: • Nonpolar bonds generally occur when the difference in electronegativity between the two atoms is less than 0.5 • Polar bonds generally occur when the difference in electronegativity between the two atoms is roughly between 0.5 and 2.0 • Ionic bonds generally occur when the difference in electronegativity between the two atoms is greater than 2.0 Lesson 5: Polarity Polarity of molecules: • Polar molecules - a polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically. • Water (H2O) is an example of a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other. Lesson 5: Polarity Polarity of molecules: • Nonpolar molecules - a molecule may be nonpolar either when there is an equal sharing of electrons between the two atoms of a diatomic molecule or because of the symmetrical arrangement of polar bonds in a more complex molecule. • For example, boron trifluoride (BF3) has a trigonal planar arrangement of three polar bonds at 120°. This results in no overall dipole in the molecule. Not every molecule with polar bonds is a polar molecule. Lesson 5: Polarity Polarity of molecules: • Amphiphilic molecules - large molecules that have one end with polar groups attached and another end with nonpolar groups are described as amphiphiles or amphiphilic molecules. • They are good surfactants and can aid in the formation of stable emulsions, or blends, of water and fats. Surfactants reduce the interfacial tension between oil and water by adsorbing at the liquid–liquid interface. Lesson 6 – Formal Charge • A Formal Charge (FC) is the charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity. • When determining the best Lewis structure for a molecule, the structure is chosen such that the formal charge on each of the atoms is as close to zero as possible. Lesson 6 – Formal Charge Formula of Formal Charge: Formal Charge = V – N - 𝑩 𝟐 V is the number of valence electrons of an atom. N is the number of non-bonding valence electrons of an atom and B is the total number of electrons shared in bonds with other atoms in the molecule. Example: BH4 • The number of valence electrons for boron is 3. • The number of non-bonded electrons is zero. • The number of bonds around boron is 4. Lesson 6 – Formal Charge Solution: Formal Charge = V – N 3–0- 𝑩 𝟐 8 2 3–0–4 3 – 4 = -1 The formal charge of B in BH4 is negative 1. Lesson 7 – Types of Intermolecular Forces of Attraction There are 3 forces that hold molecules together. They are the Van der Waals Force, the hydrogen bonds (H-Bonds), and the dipole – dipole forces. • Van der Waals Force - are distance-dependent interactions between atoms or molecules. Unlike ionic or covalent bonds, these attractions are not a result of any chemical electronic bond, and they are comparatively weak and more susceptible to being disturbed. Van der Waals forces quickly vanish at longer distances between interacting molecules. Lesson 7 – Types of Intermolecular Forces of Attraction • Hydrogen Bond - A hydrogen bond is a partially electrostatic attraction between a hydrogen (H) atom which is bound to a more electronegative atom or group, such as nitrogen (N), oxygen (O), or fluorine (F)— the hydrogen bond donor—and another adjacent atom bearing a lone pair of electrons—the hydrogen bond acceptor. Lesson 7 – Types of Intermolecular Forces of Attraction • Dipole – dipole - are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Dipole-dipole forces have strengths that range from 5 kJ to 20 kJ per mole. They are much weaker than ionic or covalent bonds and have a significant effect only when the molecules involved are close together (touching or almost touching). Lesson 8 – Application of Intermolecular Forces of Attraction • Intermolecular forces (IMF) – are the forces which mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles like atoms or ions. Intermolecular forces are weak relative to intramolecular forces. • Intermolecular forces of attraction lead to properties like boiling point, melting point, toughness, durability and a lot more. And these set of properties must be considered in the manufacture of different materials and equipment. Lesson 8 – Application of Intermolecular Forces of Attraction They are applied in the following: • Medical implants are devices that are used to replace missing body parts. It can be made from skin, body tissues, metal, plastics, or ceramics. • Simple plasters are adhesives that are extensively used in the medical world. Adhesives allows tablets to be protected from the effects of moisture and allows wound to be dressed. Lesson 9 – Biological Macromolecules • Biological macromolecules – are important cellular components and perform a wide array of functions necessary for the survival and growth of living organisms. • The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Lesson 9 – Biological Macromolecules 4 major classes of Biological Macromolecules: 1. Carbohydrates - Carbohydrates comprise the largest number of organic molecules in organisms. Basically, carbohydrates are sugars; their origin can be traced to photosynthesis, the process by which organisms such as plants use sunlight to transform carbon dioxide and water into food. The simplest sugar is glucose. Lesson 9 – Biological Macromolecules 2. Proteins – are probably the most versatile of all the organic molecules, making up many structures and executing various functions within organisms. Building blocks called amino acids make up proteins. 3. Lipids - perhaps better known as fats, come in different forms in your body and contain the most energy of all the organic compounds. 4. Nucleic Acids - are DNA and RNA, or deoxyribonucleic acid and ribonucleic acid, respectively. They make the proteins that are present in almost every structure and perform almost every function in your body.
