Lecture 2 Notes 1 23 15 Office hours Bryan Phillips W Th 9 00 10 30 Learning Objectives Reactivity of atoms and molecule formation Types of chemical bonds and molecular interactions in living organisms Properties of water and the significance of those properties for life Atoms in the same column have the same number of electrons in the valence shell These two bonds give stability to atoms by giving full outer electron shells Covalent bond Sharing of electron pairs Strong Ionic bond Involves the transfer of an electron from one interacting atom to the other atom and the subsequent electrical attraction between ions of opposite charge Sharing of electrons in a covalent bond is not always equal Some atoms have greater electronegativity Electronegativity is the attractive force that an atomic nucleus exerts on shared electrons It depends on the number of protons and the distance between the nucleus and electrons When sharing is UNEQUAL a Polar Covalent Bond is formed EX H2O which can interact with both positively and negatively charged molecules When one atom is much more electronegative than the other a complete transfer donation of electrons may occur This results in two oppositely charged ions with full outer electron shells Cations Positive Anions Negative Ionic bonds are formed by the electrical attraction of cations and anions ionic compound This bond can form between or within molecules Hydrogen bond Electrical attraction between a positively charged hydrogen atom and a negative atom N or O Polar molecules are attracted to water Non polar molecules are more attracted to one another than to water Water The liquid of life It s structure gives it unique and versatile properties Polar molecules Bonds with other water molecules The polar covalent bonds of water make it the best solvent around Polar and ionic compounds dissolve easily Most of the chemical reactions of living organisms occur in the water filled cells Water ionizes in cells pH Negative log of molar concentration of H ions Pure water has a pH of 7 neutral Acids and Bases Acid When dissolved releases hydrogen ions H ions can attach to other molecules and change their properties Strong acid dissociates completely in water Weak acid incomplete dissociation Base Accept hydrogen ions Strong base dissociates completely Weak base dissociates incompletely Life can only exist in a small pH range Buffer Help maintain constant pH by absorbing or releasing H ions Is a weak acid and its corresponding base Clicker Questions Most common molecule found in living organisms water If a molecule is hydrophilic it is both polar and has a partially positive and partially negative region