SCB203 - Lecture 3 Chemical Bonds

Lecture 3: Chemical Bonds

Overview

• Chemical Bonds: Interactions between elements that result in the formation of molecules and compounds.

Key Concepts

• Molecules: Comprised of two or more atoms (same or different elements) chemically bonded together.

  • Example: Oxygen (O2), Water (H2O), Carbon Dioxide (CO2).

• Compounds: Specifically refer to molecules with different elements that are chemically bonded.

  • Example: Water (H2O) and Carbon Dioxide (CO2).

• Molecular/Biomolecules: Very large molecules (macromolecules) found in living organisms.

Chemical Bond Formation

• Valence Electrons and Valence Shells: Outermost electron shell of an atom, containing valence electrons that participate in chemical bonding.

• Types of Chemical Bonds:

  • Ionic Bonds: Formed when electrons are transferred between atoms, resulting in the formation of ions.

    • Example: Sodium chloride (NaCl).

    • Cations and Anions: Ions with positive and negative charges, respectively.

  • Covalent Bonds: Formed when two or more non-metals share electrons.

    • Polar and Non-Polar Covalent Bonds: Determined by the electronegativity of the atoms involved.

  • Hydrogen Bonds: Special weak bonds formed between hydrogen (partially positive) and other electronegative atoms (partially negative).

• Octet Rule: Atoms are most stable when they have eight electrons in their valence shell.

  • Atoms may share or transfer electrons to achieve a filled valence shell.

Types of Bonds

Ionic Bonds

• Definition: Result from the transfer of electrons between metal and non-metal.

• Examples Calculation:

  • Sodium (Na): Atomic number 11, typically has 1 valence electron. Loses one to form a cation (Na+).

  • Chlorine (Cl): Atomic number 17, has 7 valence electrons and accepts 1 electron to form an anion (Cl-).

• Formation of Salt (NaCl): The electrostatic attraction between Na+ and Cl- forms an ionic compound.

Covalent Bonds

• Definition: Result from sharing of electrons between two or more non-metals.

• Types:

  • Single Covalent Bond: Sharing of one pair of electrons (e.g., H2).

  • Double Covalent Bond: Sharing of two pairs of electrons (e.g., O2).

  • Triple Covalent Bond: Sharing of three pairs of electrons (e.g., N2).

• Carbon and its Bonds:

  • Carbon has 4 valence electrons and forms 4 bonds in compounds like methane (CH4).

Molecular Structure

Molecular Formulas

• Purpose: Represent the kinds and numbers of atoms in a molecule (e.g., H2O).

• Types of Atoms in Bonds:

  • Non-Polar Covalent Bonds: Occur in molecules with identical or similar electronegativities (e.g., H2, O2).

  • Polar Covalent Bonds: Occur when atoms have different electronegativities (e.g., H2O).

  • Properties: Water is polar, allowing for unique interactions and properties such as surface tension due to hydrogen bonds.

Summary of Key Bond Types

• Ionic Bond: Result from electron transfer. Example: NaCl.

• Non-Polar Covalent Bond: Equal sharing of electrons. Example: H2.

• Polar Covalent Bond: Unequal sharing of electrons. Example: H2O.

Assessments and Applications

• Understanding Charges: Recognize that Na+ is a cation and that the formation of a cation and an anion indicates an ionic bond.

• Comparative Strengths of Bonds: Ionic bonds are generally stronger than covalent bonds, except in certain cases.

• Molecular Behavior: Different bonding types (ionic, covalent) determine the physical and chemical properties of substances.

Study Guide Topics

• Define molecules, compounds, and macromolecules.

• Describe valence shells and how electrons contribute to bond formation.

• Discuss ionic bond formation, specify examples.

• Explain concepts of anions, cations, and their roles in ionic compounds.

• Define covalent bonds, and differentiate between polar and non-polar bonds.

• Understand hydrogen bonding in water and its biological significance.