Ch. 4 Chemical Basis of Life

4.1 Life requires about 25 chemical elements.

I. Elements A. matter: anything that occupies space and has mass B. element: pure substance that cannot be broken down into other substances by chemical or physical means Ex: gold, helium, mercury, etc.

Elements C. oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up about 96 percent of the living matter in your body. Calcium (Ca), phosphorus (P), potassium (K), sulfur (S), and a few other elements account for most of the remaining 4 percent D. trace element: element critical to health that makes up less than 0.01 percent of body mass, Ex: iodine (I), iron (Fe), copper (Cu), Flourine (F), manganese (Mn), selenium (Se)

Elements in the body

II. Compounds A. compound: substance containing two or more elements chemically combined in a fixed ratio, Ex: water (H2O) is a compound that always contains the same ratio of hydrogen combined with oxygen B. Compound's properties may differ greatly from those of its component elements. The white crystals of table salt, or sodium chloride (NaCl), on a pretzel look very different from the silvery gray sodium metal and yellowish-green chlorine gas.

4.2 Chemical properties are based on the structure of atoms.

I. Atoms A. atom: smallest particle of an element B. subatomic particles 1. proton: + charge, atomic weight of 1 2. electron: - charge, atomic weight considered zero 3. neutron: no charge, atomic weight of 1 4. nucleus: protons and neutrons packed together at core, + charge keeps the – charged electrons orbiting, Ex: Helium atom

Helium atom

Atoms C. atomic number: number of protons in an atom's nucleus; is unique for each element D. atoms often loses or gain electrons, but do not usually change the number of protons or neutrons

II. Isotopes A. isotope: one of several forms of an element, each containing the same number of protons in their atoms but a different number of neutrons, Ex: 12C, 13C, 14C B. radioactive isotope: isotope in which the nucleus decays (breaks down) over time, giving off radiation in the form of matter and energy, used in medicine and radiometric dating, can also damage cells C. Atomic mass or atomic weight is found by averaging the percentages of each isotope for an element

Isotopes

III. Electrons and Reactivity A. Electrons determine how atoms react with each other, usually electrons in the highest energy levels (outer layers) are responsible for reactions B. 1st energy level – holds 2 electrons, 2nd – 8, 3rd – 18, 4th – 32, 5th – 32, 6th – 32, 7th – 32 C. atoms want to fill their energy levels by losing or gaining electrons to make them more stable

Electrons

4.3 Chemical bonds join atoms to one another.

I. Ionic Bonds A. ionic bond: chemical bond that occurs when an atom transfers an electron to another atom B. atoms give or take electrons to giving each an individual charge, the positive ion is called a cation, the negative an anion, see Ex: NaCL tablesalt C. ion: atom that has become electrically charged as a result of gaining or losing an electron, Ex: Na+1, Cl-1

Ionic Bonding

II. Covalent Bonds A. covalent bond – two or more atoms share electrons B. valence electrons (outer layer) are represented in dot diagrams, where to shared dots are equivalent to a covalent bond, Ex: H2 C. The number of bonds an atom can form usually equals the number of additional electrons needed to fill is highest energy level. Ex: Hydrogen need 1 electron, so one covalent bond

Covalent Bonds D. molecule: two or more atoms held together by covalent bonds E. chemical formula tells the specific amount of each atom in the molecule, structural formula tells how the atom are arranged

Covalent Bonding

III. Chemical Reactions A. atoms in molecules are constantly rearranged when bonds broken (releasing energy) and bonds created (absorbing energy) B. chemical reaction: breaking of old and formation of new chemical bonds that result in new substances, Ex: see water reaction C. Chemical equation shows the staring materials (reactants) and ending materials (products) in the appropriate quantities

Modeling Molecules

Chemical Reaction

4.4 Life depends on the unique properties of water.

I. The Structure of Water A. cells are 70-95% water, and are suspended in solutions mostly containing water B. Water’s special properties are because the electrons in the covalent bonds of the molecule are not shared equally, oxygen pulls them more than hydrogen, also the shape of the molecule is bend, not linear making it polar (slight charge) C. polar molecule: molecule in which opposite ends have opposite electric charges

The Structure of Water D. this polarity makes water molecules attracted to each other forming a weak bond between molecules, hydrogen bond E. hydrogen bond: bond created by the weak attraction of a slightly positive hydrogen atom to a slightly negative portion of another molecule

Water Molecule

II. Water’s Life Supporting Properties A. cohesion: tendency of molecules of the same kind to stick to one another Ex: surface tension, and adhesion: attraction between unlike molecules, Ex: capillary action, both in plants B. Temperature moderation – caused by hydrogen bonds in water either absorbing or releasing energy 1. thermal energy: total amount of energy associated with the random movement of atoms and molecules in a sample of matter 2. temperature: measure of the average energy of random motion of particles in a substance

Cohesion & Adhesion

Water’s Life Supporting Properties C. low density of ice a result of water’s hydrogen bonds forming a group of four waters in a square D. Waters ability to dissolve other substances 1. solution: uniform mixture of two or more substances 2. solvent: substance in a solution that dissolves the other substance and is present in the greater amount 3. solute: substance in a solution that is dissolved and is present in a lesser amount 4. aqueous solution: solution in which water is the solvent 5. water dissolves ionic solids, salts, also nonionic compounds like sugar

Ice & Water

Water as a Solvent

III. Acids, Bases, and pH A. in aqueous solutions a small amount of the water molecules break apart into positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH-) B. acid: compound that donates H+ ions to an aqueous solution and measures less than 7 on the pH scale, Ex: HCl C. base: compound that removes H+ ions from an aqueous solution and that measures more than 7 on the pH scale Ex: NaOH

Acids, Bases, and pH D. pH scale: a range of numbers used to describe how acidic or basic a solution is; ranges from 0 (most acidic) to 14 (most basic) E. buffer: substance that maintains a fairly constant pH in a solution by accepting H+ ions when their levels rise and donating H+ ions when their levels fall, resist change in pH

pH

IV. An Environment Fit for Life A. Earth is unique in its many properties for supporting life