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 + H2O

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 (HCO3

-)

OH- + H2CO3 HCO3- + H2O

HCO3- + H+ H2CO3