Life’s Chemical Basis

Start With Atoms  Atoms Fundamental building blocks of matter  Nucleus Positively charged protons Uncharged neutrons (except for hydrogen)  Electrons move around the nucleus Negatively charged

electron nucleus

Elements  Element A pure substance consisting of atoms with the same number of protons (atomic number)  Isotopes Atoms of the same element that differ in number of neutrons (atomic weight)

Periodic Table of Elements

Putting Radioisotopes to Use  Radioisotopes are radioactive isotopes  They are not stable Emit particles and energy as they decay spontaneously into other elements

Why Electrons Matter  Electrons occupy orbitals (volumes of space) around the nucleus  Up to two electrons occupy each orbital  Shell model represents orbital energy levels as successively larger circles, or shells Used to view an atom’s electron structure

Shell Models

Electron Interactions  Atoms with unpaired electrons in their outermost shell tend to interact with other atoms They donate, accept, or share electrons to eliminate vacancies vacancy no vacancy

Electrical Charge  An atom with equal numbers of protons and electrons has no net charge  Ions (positive or negative) Atoms that have gained or lost electrons

Ion Formation

What Happens When Atoms Interact?  Common interactions in biological molecules: Ionic bond Covalent bond Hydrogen bond

Ionic Bonds  Strong association between a positive ion and a negative ion (attraction of opposite charges)

Covalent Bonds  Two atoms share a pair of electrons  Nonpolar covalent bond Atoms share electrons equally  Polar covalent bond Electrons are shared unequally One end slightly negative, other slightly positive Polar molecule has a separation of charge

Covalent Bonds

Hydrogen Bonds  Form between a hydrogen atom and an electronegative atom Each with separate polar covalent bonds  Are not chemical bonds Do not make atoms into molecules Individually weak Collectively stabilize structures of large molecules

Hydrogen Bonds

Water Molecules  Water molecules are polar Form hydrogen bonds with other polar molecules Hydrophilic substances (water-loving) Hydrophobic substances (water-dreading)

Liquid Water: Hydrogen Bonds

Water’s Life-Giving Properties  Polarity gives liquid water unique properties that make life possible: Resistance to temperature changes Internal cohesion Dissolves polar and ionic substances

Water Temperature: From Ice to Evaporation

Water’s Solvent Properties  Solvents dissolve solutes (spheres of hydration)

Water’s Cohesion

Key Concepts: NO WATER, NO LIFE  Life originated in water and is adapted to its properties  Water has temperature-stabilizing effects, cohesion, and a capacity to act as a solvent for many other substances  These properties make life possible on Earth

Acids and Bases  pH scale Indicates hydrogen ion (H + ) concentration of a solution Ranges from 0 (most acidic) to 14 (most basic or alkaline)  At pH 7 (neutral) H + and OH – concentrations are equal

A pH Scale

Acids and Bases  Acids donate H + in water More H + than OH -  Bases accept H + in water More OH - than H +

Salts  Compounds that dissolve easily in water, and release ions other than H + and OH - Form when an acid interacts with a base Example: NaCl HCl + NaOH  NaCl + H 2 O

Buffer System  A set of chemicals (a weak acid or base and its salt) that keeps the pH of a solution stable One donates ions, the other accepts them Example: bicarbonate (HCO 3 - ) OH - + H 2 CO 3  HCO H 2 O HCO H +  H 2 CO 3