Chapter 2 0 Essential Chemistry for Biology
Matter: Elements and Compounds • Matter is anything that occupies space and has mass. • Matter is found on the Earth in three physical states: • Solid • Liquid • Gas • Matter is composed of chemical elements. • Elements are substances that cannot be broken down into other substances. • There are 92 naturally occurring elements on Earth. • All of the elements are listed in the periodic table.
6 C 12 H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Rf Db Sg Bh Hs Mt Uun Uuu Uub Uuq Uuh Uuo Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Figure 2.1
Twenty-five elements are essential to life. • Four elements make up about 96% of the weight of the human body: • Oxygen • Carbon • Hydrogen • Nitrogen
Carbon C: 18.5% Oxygen O: 65.0% Calcium Ca: 1.5% Phosphorus P: 1.0% Potassium K: 0.4% Sulfur S: 0.3% Sodium Na: 0.2% Chlorine Cl: 0.2% Magnesium Mg: 0.1% Hydrogen H: 9.5% Trace elements: less than 0.01% Manganese Mn Boron B Chromium Cr Molybdenum Mo Nitrogen N: 3.3% Cobalt Co Selenium Se Silicon Si Copper Cu Tin Sn Fluorine F Vanadium V Iodine I Zinc Zn Iron Fe Figure 2.2
Lack of iodine can cause goiter Figure 2.3
Elements can combine to form compounds. • Compounds are substances that contain two or more elements in a fixed ratio. • Common compounds include • NaCl (table salt) • H2O (water)
Atoms • Each element consists of one kind of atom. • An atom is the smallest unit of matter that still retains the properties of an element. • Atoms are composed of subatomic particles. • A proton is positively charged. • An electron is negatively charged. • A neutron is electrically neutral.
Proton • Positive charge • Determines element Electron • Negative charge • Participates in chemical reactions • Outer-shell electrons determine chemical behavior Neutron • No charge • Determines isotope Nucleus • Consists of neutrons and protons Atom Figure UN2-5
Elements differ in the number of subatomic particles in their atoms. • The number of protons, the atomic number, determines which element it is. • An atom’s mass number is the sum of the number of protons and neutrons. • Mass is a measure of the amount of matter in an object.
First electron shell can hold 2 electrons Outer electron shell can hold 8 electrons Electron Hydrogen H Atomic number = 1 Carbon C Atomic number = 6 Nitrogen N Atomic number = 7 Oxygen O Atomic number = 8 Figure 2.5
Chemical Bonding and Molecules • Chemical reactions enable atoms to give up or acquire electrons to complete their outer shells. • Chemical reactions usually result in atoms • Staying close together • Being held together by chemical bonds • When an atom loses or gains electrons, it becomes electrically charged. • Charged atoms are called ions. • Ionic bonds are formed between oppositely charged ions.
The outer electron is stripped from sodium and completes the chlorine atom’s outer shell Outer shell has 1 electron Complete outer shells Outer shell has 7 electrons The attraction between the ions—an ionic bond—holds them together Na Sodium atom Cl Chlorine atom Na Sodium ion Cl Chlorine ion Sodium chloride (NaCl) Figure 2.6-2
Covalent Bonds • A covalent bond forms when two atoms share one or more pairs of outer-shell electrons. • Atoms held together by covalent bonds form a molecule. • The number of covalent bonds an atom can form is equal to the number of additional electrons needed to fill its outer shell.
Name molecular formula Electron configuration Structural formula Space-filling model Ball-and-stick model Hydrogen gas H2 Single bond a pair of shared electrons Oxygen gas O2 Double bond two pairs of shared electrons Methane CH4 Figure 2.7
Hydrogen Bonds • Water is a compound in which the electrons in its covalent bonds are shared unequally. • This causes water to be a polar molecule, one with opposite charges on opposite ends.
slightly slightly H H O slightly – Figure UN2-2
The polarity of water results in weak electrical attractions between neighboring water molecules. • These interactions are called hydrogen bonds.
Hydrogen bond Figure 2.8
Water’s Life-Supporting Properties • The polarity of water molecules and the hydrogen bonding that results explain most of water’s life-supporting properties. • Water molecules stick together. • Water has a strong resistance to change in temperature. • Frozen water floats. • Water is a common solvent for life.
Evaporation from the leaves Microscopic tubes Cohesion due to hydrogen bonds between water molecules Flow of water SEM Figure 2.10
Heat and temperature are related, but different. • Heat is the amount of energy associated with the movement of the atoms and molecules in a body of matter. • Temperature measures the intensity of heat. • Water can absorb and store large amounts of heat while only changing a few degrees in temperature. • Water can moderate temperatures. • Earth’s giant water supply causes temperatures to stay within limits that permit life. • Evaporative cooling removes heat from the Earth and from organisms.
The Biological Significance of Ice Floating • When water molecules get cold enough, they move apart, forming ice. • A chunk of ice has fewer molecules than an equal volume of liquid water. • Ice floats because it is less dense than the liquid water around it. • If ice did not float, ponds, lakes, and even the oceans would freeze solid. • Life in water could not survive if bodies of water froze solid.
Hydrogen bond Ice Liquid water Figure 2.13
Water as the Solvent of Life • A solution is a liquid consisting of a homogeneous mixture of two or more substances. • The dissolving agent is the solvent. • The dissolved substance is the solute. • When water is the solvent, the result is an aqueoussolution.
Acids, Bases, and pH • A chemical compound that releases H+ to solution is an acid. • A compound that accepts H+ and removes it from solution is a base. • Buffers are substances that resist pH change. • Buffers • Accept H+ ions when they are in excess • Donate H+ ions when they are depleted • Increases in global CO2 concentrations may lead to the acidification of the oceans. • To describe the acidity of a solution, chemists use the pH scale.
14 Oven cleaner 13 Household bleach 12 Household ammonia 11 Increasingly basic lower Hconcentration Basic solution Milk of magnesia 10 9 Seawater 8 Human blood Pure water 7 Neutral [H+] [OH–] 6 Urine Neutral solution 5 Tomato juice 4 Increasingly acidic greater Hconcentration Grapefruit juice, soft drink 3 2 Lemon juice, gastric juice 1 Acidic solution 0 pH scale Figure 2.16