Chemical Bonds

Learning Objectives

  • By the end of this section, you will be able to:

    • Explain the relationship between molecules and compounds

    • Distinguish between ions, cations, and anions

    • Identify the key difference between ionic and covalent bonds

    • Distinguish between nonpolar and polar covalent bonds

    • Explain how water molecules link via hydrogen bonds

Overview of Atoms and Chemical Bonds

  • Atoms are held together by chemical bonds, which are electrical attractions.

  • Chemical bonds allow atoms to group together, forming molecules or compounds.

  • Atoms do not physically touch due to the repulsion of negatively charged electrons in their valence shells.

Types of Molecules and Compounds

  • Molecule: A stable grouping of two or more atoms. It can consist of:

    • Atoms of the same element (e.g., H2, hydrogen gas)

    • Different elements (e.g., H2O for water, CH4 for methane)

  • Chemical Compounds: Formed by different elements, such as water and methane.

Ions and Ionic Bonds

  • Ion: An atom with an electrical charge resulting from the loss or gain of electrons.

  • Cation: Positively charged ions (e.g., K+ for potassium).

  • Anion: Negatively charged ions (e.g., F– for fluoride).

  • Ions form when atoms lose or gain electrons to achieve full valence shells.

    • Example: Potassium (K) often donates one electron to become K+; Fluorine (F) often accepts one electron to become F–.

Formation of Ionic Bonds

  • Ionic Bond: A bond formed between cations and anions due to their opposite charges.

  • Example: Sodium donates an electron to chlorine to form Na+ and Cl–, creating table salt.

Covalent Bonds

  • Unlike ionic bonds, covalent bonds involve the sharing of electrons.

  • This sharing can be:

    • Single Bond: One pair of electrons shared.

    • Double Bond: Two pairs of electrons shared.

    • Triple Bond: Three pairs of electrons shared.

Nonpolar vs Polar Covalent Bonds

  • Nonpolar Covalent Bonds: Electrons are shared equally, leading to no charge difference across the molecule.

  • Polar Covalent Bonds: Electrons are shared unequally, resulting in partial positive and negative charges.

    • Example: Water (H2O) is a polar molecule due to the unequal sharing of electrons between hydrogen and oxygen.

Water as a Polar Molecule

  • Water molecules exhibit a dipole moment, where:

    • Oxygen is slightly negative due to stronger attraction to electrons.

    • Hydrogen atoms are slightly positive due to weaker attraction.

  • This charge distribution allows water molecules to interact with other polar molecules and ions, leading to essential biological processes.

Hydrogen Bonds

  • Hydrogen Bond: A weak bond formed when a hydrogen atom bonded to an electronegative atom is attracted to another electronegative atom.

  • Commonly occurs between water molecules, resulting in surface tension and cohesion.

  • Hydrogen bonding is vital for the structure and function of biological molecules, allowing for the solubility of ions like salts in water and influencing biological systems.

Properties Resulting from Hydrogen Bonds

  • Water acts as an excellent solvent for ionic compounds (e.g., table salt).

  • Opposite charges in water molecules cause them to attract ions, enabling electrical charges (electrolytes) essential for functions like heart rhythms (EKG) and brain activity (EEG).

  • Water repels nonpolar molecules (e.g., oils), leading to the formation of distinct layers when mixed.