The Biology Project: Biochemistry

Chemistry Tutorial

Chemical bonds and attractive forces

A molecule is two or more atoms linked by a chemical bond. Molecules can contain different types of bonds. If atoms are sharing electrons, then the bond between them is covalent. If an atom gives up an electron to another atom, then they have an ionic bond.
Covalent bonds
Methane has four covalent bonds between carbon (C) and hydrogen (H). The figure below shows the methane molecule in four different views. Notice how these different views represent the atoms and their bonds differently. Electronegativity refers to the tendency for atoms to bind electrons. Oxygen (0) with an electronegativity of 3.5 has a strong affinity. Hydrogen (H)(2.1) and carbon (C)(2.5) each have lower affinities. A bond between C and H will have nearly equal sharing of electrons. Oxygen and hydrogen form a highly polar bond because of the much stronger affinity for electrons by O.

4 different modes of representation of covalent bonds:structural formula, diagram, ball & stick model, and space-filling model

Insulin molecule C254H377N65O76S6 Insulin is a complicated molecule called a protein. We will later consider simpler ways to consider the complicated molecules of life.
Ionic bonds
Ionic bonding in sodium chloride (NaCl) Ions are produced when atoms can obtain a stable number of electrons by giving up or gaining electrons. For example Na (sodium) can donate an electron to Cl (chlorine) generating Na+ and Cl-. The ion pair is held together by strong electrostatic attractions.
Non-covalent bonds and other weak forces
Linus Pauling, 1946
  • Chemical reactivity of molecules- tendency to break and form chemical bonds.
  • Biology of molecules- size and shape of molecules, and the nature of weak interactions with other molecules.
Non-covalent bonds and other weak forces are important in biological structures.
  • Electrostatic bonds(ionic)-result from the electrostatic attraction between two ionized groups of opposite charge, such as carboxyl (-COO-) and amino (-NH3+). In water, these bonds are very weak.
  • Hydrogen bonds-result from electrostatic attraction between an electronegative atom (O or N) and a hydrogen atom that is bonded covalently to a second electronegative atom.
  • N-H ----- O=C- -O-H----- O=C-
  • Van der Waals bonds-are short range attractive forces between chemical groups in contact. Caused by slight charge displacements.
  • Hydrophobic attractions-cause non-polar groups such as hydrocarbon chains to associate with each other in an aqueous environment.
  • Multiple weak bonds or forces can cause strong interactions
  • Biological recognition results from a three dimensional structure that allows multiple weak forces between molecules

Illustration of biological energies in Kcal/mol

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