2.2 Chemical Bonds

Chemical bonds are the glue that holds atoms together, forming molecules and compounds. They're the foundation of all matter, from the air we breathe to the cells in our bodies. Understanding these bonds is key to grasping how substances interact and function.

This section dives into different types of chemical bonds, from ionic to covalent. We'll explore how these bonds affect a substance's properties and behavior. Plus, we'll look at the crucial role of hydrogen bonds in water, which are essential for life as we know it.

Chemical Bonds and Molecules

Molecules, compounds and chemical bonds

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• Atoms are the fundamental units that make up all matter
  • Atoms combine through chemical bonds to create molecules or compounds with distinct properties
• Molecules consist of two or more atoms joined by chemical bonds
  • Molecules can contain atoms of the same element (oxygen gas, O2) or different elements (water, H2O)
• Compounds are substances containing two or more different types of atoms linked by chemical bonds
  • Compounds exhibit properties that are different from those of their constituent atoms (table salt, NaCl)
• Chemical bonds are attractive forces that hold atoms together in molecules and compounds
  • Chemical bonds develop when atoms share or exchange electrons to reach a more stable electronic arrangement (covalent bonding, ionic bonding)
  • The strength of a chemical bond is measured by its chemical bond energy

Ions, cations and anions

• Ions are atoms or molecules that have acquired an electric charge by gaining or losing electrons
  • Ions are created through the process of ionic bonding
• Cations are ions with a positive charge
  • Cations are formed when atoms lose one or more electrons (sodium ion, Na+; calcium ion, Ca2+; iron(III) ion, Fe3+)
• Anions are ions with a negative charge
  • Anions are formed when atoms gain one or more electrons (chloride ion, Cl-; oxide ion, O2-; phosphate ion, PO43-)

Types of Chemical Bonds

Ionic vs covalent bonds

• Ionic bonds occur between a metal and a nonmetal
  1. Electrons are transferred from the metal to the nonmetal
  2. This results in the creation of positively charged metal ions (cations) and negatively charged nonmetal ions (anions)
  3. Ionic compounds have high melting and boiling points, are brittle in solid form, and can conduct electricity when dissolved in water (sodium chloride, NaCl; potassium bromide, KBr)
• Covalent bonds occur between two nonmetals
  1. Electrons are shared between the atoms
  2. This results in the formation of molecules held together by shared electron pairs
  3. Covalent compounds generally have lower melting and boiling points, are often soft or flexible as solids, and do not usually conduct electricity (water, H2O; methane, CH4)
  • The distance between the nuclei of bonded atoms is called the bond length

Spectrum of covalent bonds

• Nonpolar covalent bonds develop when electrons are shared equally between two identical atoms or atoms with similar electronegativity
  • Examples include hydrogen gas (H2), nitrogen gas (N2), and carbon tetrachloride (CCl4)
• Polar covalent bonds develop when electrons are shared unequally between two atoms with different electronegativity
  • The more electronegative atom attracts the shared electrons more strongly, leading to a partial negative charge ($\delta-$) near that atom and a partial positive charge ($\delta+$) near the less electronegative atom (hydrogen fluoride, HF; ammonia, NH3)
• The polarity of molecules significantly influences their interactions and functions in biological systems
  • Polar molecules are attracted to other polar molecules and can dissolve in polar solvents like water (sugars, amino acids)
  • Nonpolar molecules are attracted to other nonpolar molecules and can dissolve in nonpolar solvents like lipids (fats, oils)

Hydrogen bonds in water

• Hydrogen bonds are intermolecular forces that form between a hydrogen atom bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine) and another highly electronegative atom
• In water molecules (H2O), hydrogen bonds develop between the partially positive hydrogen atoms and the partially negative oxygen atoms of adjacent water molecules
• Hydrogen bonding in water results in unique properties essential for life:
  1. High specific heat capacity enables water to absorb substantial heat energy without significantly changing temperature, helping regulate body temperature in organisms
  2. Cohesion allows water molecules to stick together due to hydrogen bonding, facilitating the transport of water and dissolved nutrients in plants through capillary action
  3. Adhesion causes water molecules to be attracted to other polar substances, allowing them to "climb" up narrow tubes like blood vessels in animals or xylem in plants
  4. High surface tension created by hydrogen bonding forms a strong, elastic surface film on water, enabling some organisms to walk on water and preventing small organisms from sinking
• Hydrogen bonding also plays a vital role in the structure and function of biological macromolecules such as proteins, DNA, and RNA

Electronic Structure and Bonding

• Valence electrons are the outermost electrons of an atom and are involved in chemical bonding
• The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight valence electrons
• Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons around them
  • Some molecules can have multiple valid Lewis structures, a phenomenon known as resonance