Chapter 2 lecture

• Element


  Fundamental substance containing only one kind of atom
• Atomic number


  The number of protons that identifies an element
• Mass number


  Protons + neutrons
• Isotopes


  • Forms of an element with different # neutrons so different mass numbers
• Valence electrons


  Electrons present in the valence shell
• Valence shell


  Outermost electron shell that determines how the atom behaves
• Stable atom


  If valence shell is full
• Reactive atom


  If valence shell has unpaired electrons
• Chemical bond


  Attractive force linking atoms to form molecules
• Compound


  Pure substance made up of two or more different elements bonded together in a fixed ratio
• H2O: two atoms of hydrogen (H) bonded to one atom of oxygen atom (O)
• Molecular weight


  Sum of atomic weights of all atoms in the molecule
• Hydrogen has an atomic weight of 1. Oxygen has an atomic weight of 16. What is the molecular weight of water (H2O)?
• Covalent bonds


  Form by sharing one or more electron pairs to fill valence shells
• Covalent bonds


  • Very strong - a lot of energy is required to break them
  • Length, angle, and direction of bonds between any two elements are always the same
  • This orientation gives molecules their shapes, which contributes to their biological functions
• Covalent bonds


  • Single, sharing 1 pair of electrons
  • Double, sharing 2 pairs of electrons
  • Triple, sharing 3 pairs of electrons
• Covalent Bonding Capabilities of Some Biologically Important Elements


  • C H
  • C C
  • N N
• Electronegativity


  Attractive force that an atomic nucleus exerts on electrons
• Polar covalent bond


  Unequal sharing of electrons leads to a partial positive charge (+) and partial negative (-) across the covalent bond
• Nonpolar covalent bond


  Equal sharing of electrons, as atoms have similar electronegativity
• Electronegativities


  • Decreases
  • Decreases
• Ionic bonds


  Form when one atom is MUCH more electronegative. Complete electron transfer results in ions with full outer shells.
• Water molecules surround ions
• Hydrogen bonds


  Attraction between the δ- end of one molecule and the δ+ hydrogen end of another molecule
• Hydrogen bonds


  • Weaker than ionic and covalent bonds
  • But many hydrogen bonds in a large molecule can be strong and play an important role in the structure of DNA and proteins
• Hydrophilic and Hydrophobic


  • Nonpolar molecules, such as hydrocarbons, are hydrophobic ("water-hating")
  • Polar molecules that form hydrogen bonds with water are hydrophilic ("water-loving")
• Hydrophobic Interaction


  Interaction of non-polar substances in a presence of polar substances (especially water)
• Van der Waals interaction

  Interaction of electrons of non-polar substances
• Types of Chemical Bonds

  • Covalent Bonds
  • Ionic Bonds
  • Hydrogen Bonds
• Chemical reactions


  Occur when atoms collide with enough energy to combine or change their bonding partners
• Chemical reactions


  • Math must be balanced, count atoms before/after
  • Either absorb or release energy (often as heat)
• Combustion


  An oxidation–reduction (redox) reaction - electrons are transferred between two molecules
• Oxidizing agent


  Gains electrons and becomes reduced (oxygen)
• Reducing agent


  Loses electrons and becomes oxidized (propane)
• Water


  • Polar
  • Hydrogen bonds
  • Tetrahedral shape
• Water's structure and interactions make it essential to life
• Ice floats because: Water (solid): each water molecule is hydrogen-bonded to four (4) other water molecules in a rigid, crystalline structure. When liquid water freezes, it forms the maximum number of H-bonds with other molecules in the crystal (because its the most stable/lower energy configuration), which creates more space between molecules makes it less dense than water.
• zed (propane)


  OIL RIG
• Key Concept 2.3, Question 1


  Which statement about this reaction is true?
• Statements about the reaction

  • O2 is a product
  • Chemical bonds are conserved during the reaction
  • The same atoms are present before and after the reaction
  • A net input of energy is needed for this reaction
  • The products are similar to the reactants