© 2009 FIDM/The Fashion Institute of Design & Merchandising

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Chapter 2: The Chemical Basis of Life

ELEMENTS, ATOMS, AND MOLECULES 1. Elements

a. Of the 25 chemical elements essential to life, carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) make up most (about 96%) of living matter.

b. Trace elements, (such as iron, iodine, magnesium, zinc, fluorine) are essential to health and may be added to food and water

c. Compounds are formed when elements combine in specific ratios; vitamins, proteins, fats are examples

2. Atoms a. smallest unit of an element, consist of protons and neutrons (in nucleus) and

electrons (arranged in electron shells around nucleus) b. Atomic number = number of protons (electrons are the same) c. Atomic mass = number of protons and neutrons in the nucleus d. Isotopes = atoms of an element with varying number of neutrons e. Radioactive isotopes – unstable isotopes that may be harmful, but also useful in

medicine and research 3. Chemical bonds and Molecules

a. Electron arrangement in the outer shell of an atom determines chemical properties of atom; an atom with an electron shell that is not full tends to react with other atoms to gain, lose, or share electrons to form chemical bonds.

b. Ionic bonds – when atoms gain or lose electrons to create charged atoms called ions, the oppositely charged ions attract one another to form ionic bonds

c. Covalent bonds – when atoms share electrons to complete their outer shells they form covalent bonds

d. Molecules = when two or more atoms are bonded together (typically covalent bonds)

4. Polarity a. Electronegativity – atom’s attraction for electrons b. Nonpolar molecule– a molecule is nonpolar when the electrons are shared

equally between the atoms (such as O2, H2, CH4) c. Polar molecule - electrons are NOT shared equally; water molecule (H2O) is

an example – oxygen has a stronger attraction to the electrons, so is more electronegative than the hydrogen atoms

d. Hydrogen bonds –the slightly positively-charged H atoms in one water molecule may attract a neighboring O (or N) with a slightly negative charge, forming a weak hydrogen bond

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WATER’S LIFE-SUPPORTING PROPERTIES 1. Hydrogen bonds

a. Cohesion – when molecules “stick” together; hydrogen bonds make water molecules cohesive; this allows water to rise against the force of gravity as it does from roots to leaves in a tree; creates surface tension allowing insects to “stride” across a pond

b. Moderate temperature changes – it takes a lot of energy to disrupt hydrogen bonds, so water can absorb a lot of heat energy before temperature begins to rise; when water is cooled, heat energy is released and hydrogen bonds form to slow the cooling process

c. Evaporation (or vaporization) – • The high boiling point of water (100 degrees Celsius) is due to the hydrogen

bonds because it takes a lot of heat energy to break apart the water molecules.

• Evaporative cooling occurs because the water molecule takes heat energy with it when it evaporates.

d. Ice – hydrogen bonds hold molecules in ice farther apart than in liquid water because the bonds are stable, making ice less dense and able to float in water; this property helps prevent oceans and lakes from freezing solid

2. Solutions – a uniform liquid mixture of two or more substances a. Solvent – the dissolving agent b. Solute – the substance dissolved c. Aqueous solutions – when water is the solvent; water is the solvent of life;

water’s polarity makes it versatile: polar or charged solutes dissolve when water molecules surround them • In solution, some water dissociates into ions it forms hydrogen ions (H+)

and hydroxide ions (OH-) • Acids = a compound that releases or donates hydrogen ions (H+) in solution • Bases = a compound that accepts or removes H+ • pH scale – measurement of H+ ; 0 (most acidic, highest concentration H+) to

14 (most basic, lowest concentration of H+) • most cells close to pH 7 (neutral); maintained by buffers, substances that

resist pH changes by accepting or donating H+ as needed d. Acid precipitation - formed when air pollutants from burning fossil fuels

combine with water vapor in the air to form sulfuric and nitric acids; threatens ecosystems

CHEMICAL REACTIONS 1. Matter is rearranged in chemical reactions as bonds are broken and reformed into

new substances. 2. Reactants = the starting substance in a chemical reaction 3. Products = the resulting substances following the reaction 4. Conservation of Matter – the number of atoms is maintained from the reactants to

the products; the only difference is the arrangement of the atoms