ELEMENTS, ATOMS, AND COMPOUNDS

Transcription

1 Chapter 2 The Chemical Basis of Life Chapter 2: Big Ideas Elements, Atoms, and Compounds Chemical Bonds Water s Life- Supporting Properties Organisms are composed of elements ELEMENTS, ATOMS, AND COMPOUNDS Living organisms are composed of matter: occupies space and has mass (weight). Matter is composed of chemical elements. Element -- cannot be broken down to other substances. 92 elements in nature only a few exist in a pure state. Elements combine into compounds Table 2.1 Compound : substance consisting of two or more different elements in fixed ratio. more common than pure elements. Sodium chloride = NaCl Water = H 2 O

2 Elements in Living Organisms Trace Elements 25 elements = essential to life. Four elements make up ~96% of the weight of most living organisms: oxygen, carbon, hydrogen, nitrogen. Trace elements = essential but only needed in minute quantities. Some trace elements prevent disease. Without iron, your body cannot transport oxygen. Iodine deficiency prevents production of thyroid hormones, resulting in goiter. Trace elements added to food and water debatable value Fluoride added to toothpaste (helpful) and municipal water (highly debated) to reduce tooth decay. Chemicals added to food to help preserve it, make it more nutritious*, and/or make it look better. *(We re learning a lot about how we assimilate minerals these additives not always valuable to nutrition) Atoms Each element has one kind of atom. Atom = smallest unit of matter that still retains the properties of an element. (Greek = indivisible ) Three main subatomic particles: Protons = positively charged Electrons = negatively charged Neutrons = electrically neutral Figure 2.3A Helium Atoms Nucleus 2e - Atomic number = # protons Atomic Mass = protons + neutrons (electrons weigh very little) Protons Neutrons Electrons Electron cloud Mass number = 4

3 Isotopes Variant forms of elements Different isotopes of an element have the same number of protons and electrons, but different numbers of neutrons. 99% Naturally occuring Behave identically in chemical reactions. In radioactive isotopes, nucleus decays spontaneously, giving off particles and energy. 1% Natural, but rare Minute quantities RADIOACTIVE Can be: Harmful (e.g. Chernobyl in 1986) damage DNA and tissues by breaking bonds Helpful (e.g. PET Scans) radioactive compounds act as tracers in metabolic processes Radioactive isotopes can help or harm us Carbon Dating PET (positron-emission tomography) Scan Over last 15,000 yrs, atmospheric ratio of constant Detects location of injected radioactive materials. Useful for diagnosing heart disorders, cancer, and in brain research. 14C:12C = Ratio in living organisms same as atmosphere When something dies (tree charcoal, hair, bones), stops exchanging, and 14C begins degrading. So, ratio of 14C:12C declines Humans preserved in bogs Chemical Properties of Atom determined by Arrangement of Electrons (e-) CHEMICAL BONDS Electrons found in Shells 1, 2, or 3 shells -- inner shell has 2 spaces, outer shells have 8 spaces Unfilled spaces in outermost shell create reactivity atoms join together (w/ chemical bond) to make molecule See drawings on board

4 Figure 2.5 First shell Hydrogen Helium Covalent bonds: Atoms share one or more outershell electrons Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Second shell Strongest type of bond Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Third shell Electronegativity Two Kinds of Covalent Bonds Atoms in covalently bonded molecules compete for shared electrons. The attraction (pull) for shared electrons is called electronegativity. Strength of electronegativity depends on how many electrons atom lacks in outer shell More electronegative atoms pull harder. Nonpolar Covalent If only one element, pull is equal O 2 or H 2 Polar Covalent 2 different elements share electrons unequally (one pulls more) H 2 O Oxygen missing 2 e - while Hydrogens each missing 1 e - Creates a Polar Molecule Water is a polar molecule, with slightly negative and positive ends (slightly -) Ionic bonds give and take electrons (they don t share) Ion = atom with electrical charge resulting from gain or loss of electrons. (slightly +) (slightly +) Two ions with opposite charges attract each other. When the attraction holds the ions together, it is called an ionic bond. e.g. Salt (NaCl) Easily separated not strong like Covalent Bond We ll learn why this is critical to life shortly.

5 Figure 2.7A_s2 Figure 2.7B Transfer of electron Cl - Na + Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl - Chloride ion Sodium chloride (NaCl) Hydrogen bonds = weak covalent bonds Chemical reactions make and break chemical bonds Hydrogen, as part of a polar covalent bond, has a partial positive charge. No more electrons to share, but attracted to + on neighboring molecules Structure = Function Hydrogen and oxygen can react to form water: 2H 2 + O 2 2H 2 O Reactants Product Very weak, but make many structures possible Reactions do not create or destroy only rearrange. Hydrogen bonds and liquid water WATER S LIFE-SUPPORTING PROPERTIES Cohesion = 2 molecules of same kind stick together Plants use cohesion to transport water / nutrients from roots to leaves. Surface tension (water striders, belly flop off high dive!) 1. Polarity 2. H-Bonds Adhesion = 2 different molecules stick together (water to inside of glass)

6 Water s Hydrogen Bonds Stable in solid water (ice) Unstable in liquid water Absent in vapor (steam) *Higher temp = more active molecules Water s H-bonds moderate temperature Example: Ocean Temperature Heat energy absorbed by water & H-bonds break as water is heated by sun (steam) During hot weather, ocean stores heat, but warms only a few degrees Heat energy released into air as H-bonds form when cooling (from vapor to water) During cool/cold weather, ocean lets off heat and so, cools slowly (stabilizes temp for marine life) Water s H-bonds moderate temperature Evaporative cooling = perspiration (sweating) / transpiration (plants sweating ) Molecules with greatest energy ( hottest ) vaporize first, leaving cooler ones behind. Ice = less dense than liquid water When water freezes, each molecule forms a stable hydrogen bond with its neighbors. As ice crystals form, the molecules are less densely packed than in liquid water. Because ice is less dense than water, it floats. If ice didn t float, lakes would freeze solid and kill fish Ice formation cracks rocks & bottles in freezer Water = the Universal Solvent of Life A solution is a liquid consisting of a uniform mixture of two or more substances. Water = the Universal Solvent of Life Ion in solution The dissolving agent is the solvent. The substance that is dissolved is the solute. An aqueous solution is one in which water is the solvent (true in all living cells). Salt crystal Water s polarity = key. Water surrounds ions and separates them.

7 The chemistry of life is sensitive to acidic and basic conditions In aqueous solutions, a small percentage of water molecules break apart into ions. Some are hydrogen ions (H + ). Some are hydroxide ions (OH ). Both types are very reactive. The chemistry of life is sensitive to acidic and basic conditions Acid: compound that donates H + to solution HCl (hydrochloric acid in stomach) Base: compound that accepts H + (and removes it from solution) NaOH (sodium hydroxide soaps, detergents) The chemistry of life is sensitive to acidic and basic conditions The ph scale describes how acidic or basic a solution is. Ranges from 0 to = most acidic, 14 = most basic. Each ph unit represents a tenfold change in the concentration of H +. CONNECTION: Acid Rain & Ocean Acidification Fossil fuel combustion releases polluting gases Sulfur and nitrous oxides react with water in the air to form acids. These acids fall to Earth as acid precipitation, which is rain, snow, or fog with a ph lower than 5.2. ph 1.7 fog recorded in LA! (gastric juices) CO 2 dissolving in seawater lowers ocean ph in a process known as ocean acidification Eats away at coral reefs, killing reef fish diversity Mind Map have positively charged Atoms have negatively have neutral charged (a) (b) (c) Fill in the blanks number present equals atomic number of each element (d) number may differ in number in outer shell determines formation of Chemical Bonds electron transfer between atoms creates electron sharing between atoms creates ions (e) attraction between ions creates unequal equal sharing creates sharing creates (f) (g) nonpolar covalent bonds example is can lead to water has important qualities due to polarity and (h)