CHAPTER 2 The Chemistry of Life

Atoms, Ions and Molecules Concept 2.1 Atoms, Ions and Molecules

ELEMENTS Element: pure substance Cannot be broken down into other substances by chemical means 25 elements are essential to life

ELEMENTS 4 elements make up 96% of all living matter Hydrogen (H) Oxygen (O) Nitrogen (N) Carbon (C)

ELEMENTS Calcium (Ca), phosphorus (P), potassium (K), sulfur (S)  most of remaining 4% Trace elements: less than 0.01 % of body mass are essential to life E.g.: iodine (I), copper (Cu), fluorine (F), manganese (Mn), and selenium (Se)

Iodine is a trace element that prevents goiter

COMPOUNDS COMPOUNDS: substance containing two or more elements combined in a fixed ratio Compound properties may differ greatly from the elements they contain H2O – liquid at room temperature, while hydrogen and oxygen are both gases NaCl – white crystal (table salt), while sodium is a silver-gray metal and chlorine is a yellowish-green gas

Na+ + Cl-  NaCl

ATOMS Atom: smallest possible particle of an element Made up of 3 subatomic particles: proton: single unit of (+) charge electron: single unit of (-) charge neutron: electrically neutral—no charge

ATOMIC NUCLEUS Forms central core of atom Made up of tightly packed protons and neutrons Nucleus has a net (+) charge due to the protons it contains

ATOMIC NUMBER All the atoms of a particular element have the same number of protons Atomic number: the number of protons in the nucleus of an element Examples: Carbon has 6 protons  atomic number is 6 Helium has atomic number of 2  helium has 2 protons

ELECTRONS Small negatively charged particles that travel around nucleus Pictured as traveling around nucleus in a “cloud”

ELECTRONS & REACTIVITY Electrons are said to belong to different energy levels First or lowest level contains 2 electrons and is closest to nucleus Each of the other energy levels contains 8 electrons.

ELECTRON ENERGY LEVELS If the outer most shell is not filled, the atom is reactive. The electrons in the outermost shell are called valence electrons.

Valence Electrons (Bohr Model)

Valence Electrons (Lewis Structure)

CHEMICAL BONDS JOIN ATOMS TO ONE ANOTHER Concept 4.3 CHEMICAL BONDS JOIN ATOMS TO ONE ANOTHER

CHEMICAL BONDS Atoms react with one another to fill their outer energy levels One atom may transfer an electron to another atom or two atoms may share electrons Transferring or sharing electrons creates an attraction called a chemical bond that holds atoms together

IONIC BONDS Ionic bond: one atom transfers an electron to another atom Ions: atoms that become electrically charged as a result of gaining or losing an electron Oppositely charged ions are attracted to one another forming a chemical bond  an ionic bond

COVALENT BONDS Formed when atoms share electrons Electrons may be shared equally or unequally Molecule: two or more atoms held together by covalent bonds

CHEMICAL REACTIONS Breaking of old chemical bonds and formation of new chemical bonds resulting in the formation of new substances Reactants: starting material for a chemical reaction Products: material created as a result of a chemical reaction

CHEMICAL EQUATION

LIFE DEPENDS ON THE UNIQUE PROPERTIES OF WATER Concept 4.4 LIFE DEPENDS ON THE UNIQUE PROPERTIES OF WATER

WATER MOLECULE Water is a polar molecule Electrons in bond are not shared equally between hydrogen and oxygen Electrons attracted to oxygen more strongly than to hydrogen Results in an internal separation of charge Note: Hydrogen atoms in the water molecule are slightly positive and the oxygen atom slightly negative

POLARITY & HYDROGEN BONDING

WATER’S LIFE-SUPPORTING PROPERTIES Polarity of water and effects of hydrogen bonding give water with unique properties: Cohesion and adhesion Temperature moderation Low density of ice compared to water Ability to dissolve substances

COHESION & ADHESION Cohesion: an attraction between like molecules Water’s polarity results in hydrogen bonds forming between water molecules Adhesion: an attraction between unlike molecules Both cohesion and adhesion are important forces in the living world

COHESION & ADHESION Keep large molecules organized so they function properly in cells Help transport water through roots and leaves in plants Provides surface tension on surfaces of ponds and lakes

TEMPERATURE MODERATION Hydrogen bonds in water molecules allow water temperatures to change more slowly Causes oceans and lakes to moderate temperatures of nearby land areas  less extreme temperatures in coastal areas Through evaporation (sweating), water moderates temperature  absorbs heat energy from skin cooling the body

LOW DENSITY OF ICE Density: amount of matter/given volume For most substances, solids are more dense than liquid state of matter Due to hydrogen bonding, water is the opposite Liquid water more dense than solid water Since less dense substances float in more dense substances, ice floats

LOW DENSITY OF ICE Why is this important to living things? If ice sank, bodies of water would freeze from the bottom up Organisms could NOT get to nutrients on the bottom and would be trapped Ice insulates the water below, allowing life to persist below the frozen surface

WATER DISSOLVES OTHER SUBSTANCES Solution: uniform mixture of two or more substances Solvent: the substance that dissolves the other substance; present in the greater amount Solute: the substance that is dissolved; present in the lesser amount Aqueous solution: solution where water is the solvent

SOLUTIONS

ACIDS, BASES, & pH Acid: a compound that donates H+ ions to aqueous solutions Base: a compound that removes H+ ions from an aqueous solution pH scale: range of numbers that describes how acidic or how basic a solution is pH scale ranges from 0 (most acidic) to 14 (most basic); pH of 7 is neutral

ACIDS, BASES, & pH

BUFFERS Buffers: substances that cause a solution to resist changes in pH Molecules in cells are very sensitive to even small changes in pH Many biological fluids such as blood contain buffers that maintain its pH at or very near 7.4

THE END

BUFFERS

COVALENT BONDS

STABLE ISOTOPES Most isotopes are stable—their nuclei do not change over time Other isotopes contain nuclei that will change or decay over time