chapter 03

Bio1100 Chapter 3 The Chemistry of Life

An atom is the smallest particle of an .
- • An element has unique chemical properties and is made of one kind of atom.
  The nucleus of an atom contains positively charged particles called protons and particles with no charge called neutrons.
  The number of protons of an atom determines its atomic number, while the number of protons and neutrons make up its atomic mass.
  The atomic number determines chemical properties of elements such as hydrogen and carbon.
  Electrons are negatively charged particles and orbit the nucleus in energy shells.
  Atoms are electrically neutral: they always have an equal number of protons and electrons.
- • Electrons circulate around the nucleus in paths with different energy levels called electron shells. Lower electron shells possess lower energy, and electrons tend to occupy lower shells first, since they are most stable there.
- • The 1st electron shell of an atom can hold up to 2 electrons. Subsequent shells hold up to 8 electrons.
  An atom is most stable when its outermost, or valence, shell is filled.
  A helium atom has 2 protons, 2 neutrons, and 2 electrons. The electrons fill its one electron shell.
  A nitrogen atom has 7 protons, 7 neutrons, and 7 electrons. It has 3 vacancies in its valence shell.
  Review:
are atoms that have gained ( ) or lost ( ) one or more electrons.
- • A sodium atom has 11 protons and 11 electrons. It becomes a positively charged ion (cation) when it loses one electron. The cation is stable because it has a complete complement of electrons (8) in its outermost (valence) shell - shell #2.
Atoms can fulfill their valence shells by forming .
- bonds form when of opposite charge are attracted to each other.
  - • Ionic bond.
    1. A sodium atom (Na) can donate an electron to a chlorine atom (Cl), creating a positively charged sodium cation and a negatively charged chloride anion.
    2. Sodium chloride (NaCl, or table salt) forms a crystal: a regular lattice of alternating sodium and chloride ions held together by ionic bonds.
    Review:
- bonds form when atoms electrons to make a ; each bond involves one of electrons.
  - • Covalent bond.
    In a water molecule, one oxygen and two hydrogen atoms share electrons to fulfill their valence shells.
    The oxygen atom forms a covalent bond with each hydrogen atom by sharing one electron pair.
    Two hydrogen atoms can also form a covalent bond with each other.
    Tutorial:
    A double bond is the sharing of four electrons.
    Continue.
    Review:
  - • Polar covalent bond.
    The oxygen atom in a water molecule is more electronegative and attracts the shared electrons more strongly than the hydrogen atoms.
    Thus water is a polar molecule with a partially positive "pole" at the hydrogen end and a partially negative "pole" at the oxygen end.
    The two covalent bonds in a water molecule are called polar covalent bonds.
    Review:
- bonds form between the partially charged ends of molecules.
- • Hydrogen bond.
  The partial positive charge of the hydrogen atoms of one water molecule (H₂O) is attracted to the partial negative charge of the oxygen atom of another molecule.
  These weak attractions between polar molecules are called hydrogen bonds.
  Review:
The bonds between polar water molecules give water unique properties.
- • Due to its numerous hydrogen bonds between polar molecules, water has many unique properties.
  - Heat storage.
  - Ice formation.
  - High heat of vaporization.
  - High cohesion and surface tension.
  - Good solvent for polar (hydrophilic) molecules.
  Hydrogen bonds in water:
- • Ice formation.
  When water cools below 0°C, it forms a regular crystal structure that is less dense than its liquid form; thus ice floats on top of liquid water.
  The individual water molecules in ice are held apart by more stable, rigid hydrogen bonds.
- • Cohesion.
  The force of hydrogen bonds causes water molecules to stick together and form a strong surface at the boundary with air.
  This surface tension allows this water strider to walk on water.
  Water also adheres to other surfaces, forming a meniscus in glass tubes.
  Tutorial:
- • The partial charges on water molecules are attracted to the charged sodium and chloride ions of salt.
  The water molecules form hydration shells, separating the ions from the crystal and dissolving the salt (solute).
  A solution with water as the solvent is called an aqueous solution, and ionic or polar substances that dissolve easily in water are called hydrophilic.
The bonds in water sometimes dissociate to form negatively charged ions and positively charged ions.
- • A small percent of the molecules in water can dissociate into a negatively charged OH^- (hydroxide ion) and a positively charged H⁺ hydrogen ion ( proton ) in a reversible process.
  Review:
The concentration of ions in a solution determines a chemical property called .
- • pH scale.
  Pure water has 10^-7 moles of hydrogen ions per liter; this is represented as 7 on a negative logarithmic pH scale.
  A substance that increases concentrations of hydrogen ions has a pH lower than 7 and is called an acid.
  A substance that lowers concentrations of hydrogen ions has a pH greater than 7 and is called a base.
  Thus a solution with a high concentration of hydrogen ions has a low pH and is called an acid.
  And a solution with a low concentration of hydrogen ions has a high pH and is called a base.
  Review:
- Changes in pH can be minimized by .
- • A buffer minimizes changes in pH of a solution as H⁺ is added to or removed from the system.
- • Carbon dioxide forms carbonic acid (H₂CO₃) in water (or blood).
  Carbonic acid is in dynamic equilibrium with hydrogen ion (H⁺) and bicarbonate ion (HCO₃^-).
  The HCO₃^- removes excess H⁺ in the blood by forming H₂CO₃.
  If blood is low in H⁺, the H₂CO₃ dissociates to restore H⁺ concentrations.
  Carbonic acid is a buffer that maintains blood pH near neutral.