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Chapter 2 The Chemical Context of Life PowerPoint Lectures for Biology, Ninth Edition Neil Campbell and Jane Reece Elements and Compounds • An element is a substance that cannot be broken down to other substances by chemical reactions • A compound is a substance consisting of two or more elements in a fixed ratio Sodium Chlorine Sodium chloride Essential Elements of Life • Carbon, hydrogen, oxygen, and nitrogen make up 96% of living matter • Trace elements are those required by an organism in minute quantities Atomic Number and Atomic Mass • Atoms of the various elements differ in number of subatomic particles – Atomic number – Mass number – Atomic mass Isotopes • Atoms of an element have the same number of protons but may differ in number of neutrons • some are radioactive • Some applications of radioactive isotopes in biological research: – Dating fossils – Tracing atoms through metabolic processes – Diagnosing medical disorders The Energy Levels of Electrons • Energy is the capacity to cause change • Potential energy is the energy that matter has because of its location or structure • electron shell A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons. Third energy level (shell) Second energy level (shell) Energy absorbed First energy level (shell) Energy lost Atomic nucleus Electron Orbitals • An orbital is the three-dimensional space where an electron is found 90% of the time Covalent Bonds Hydrogen atoms (2 H) • A covalent bond is the sharing of a pair of valence electrons by two atoms Hydrogen molecule (H2) • A molecule consists of two or more atoms held together by covalent bonds – single covalent bond – double covalent bond Oxygen (O2) Name (molecular formula) Water (H2O) Electronshell diagram Structural formula Spacefilling model Name (molecular formula) Methane (CH4) Electronshell diagram Structural formula Spacefilling model δ– O H δ+ H H2O δ+ Ionic Bonds • Atoms sometimes strip electrons from their bonding partners – Example: sodium to chlorine Na Cl Na+ Cl– Sodium atom (an uncharged atom) Chlorine atom (an uncharged atom) Sodium ion (a cation) Chlorine ion (an anion) Sodium chloride (NaCl) • An anion is a negatively charged ion • A cation is a positively charged ion • An ionic bond is an attraction between an anion and a cation Na+ Cl– Weak Chemical Bonds • Weak chemical bonds reinforce shapes of large molecules and help molecules adhere to each other – Ionic bonds – Hydrogen bonds – Van der Waals Interactions Hydrogen Bonds δ– δ+ Water (H2O) δ+ Hydrogen bond δ– Ammonia (NH3) δ+ δ+ δ+ Van der Waals Interactions • Molecules or atoms that are very close together can be attracted by fleeting charge differences Molecular Shape and Function • A molecule’s shape is usually very important to its function – determined by the positions of its atoms’ valence orbitals s orbital z Three p orbitals Four hybrid orbitals x y Tetrahedron Hybridization of orbitals Chemical reactions make and break chemical bonds • The starting molecules of a chemical reaction are called reactants • The final molecules of a chemical reaction are called products 2 H2 O2 Reactants 2 H2O Reaction Products