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Essentials of Biology Sylvia S. Mader Chapter 2 Lecture Outline Prepared by: Dr. Stephen Ebbs Southern Illinois University Carbondale Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2.1 The Nature of Matter • Matter refers to anything that takes up space and has mass. • Matter is composed of elements, substances that cannot be broken down into another substance. – There are 92 natural elements. – Living organisms are made primarily of six elements. 2.1 The Nature of Matter (cont.) Atomic Structure • The atomic theory states that elements are made of tiny particles called atoms. • The name of an atom is represented by the atomic symbol. – Hydrogen = H – Sodium = Na – Chloride = Cl Atomic Structure (cont.) • Atoms are composed of three smaller particles. – Neutrons, which have no electrical charge. – Protons, which have a positive charge. – Electrons, which have a negative charge. • Atoms have a mass number determined by the weight of the neutrons and protons. Atomic Structure (cont.) Atomic Structure (cont.) • All atoms of an element have the same number of protons, the atomic number. • The atomic number and mass number are often included with the chemical symbol. Mass number Atomic number 12 6 C Chemical symbol The Periodic Table • The elements are organized to form the periodic table. – The columns in the table are groups. – The rows in the table are periods. • Elements in groups have similar chemical and physical characteristics. The Periodic Table Isotopes • The atomic number indicates the number of protons in an atom. • The atomic mass is the sum of the number of protons and neutrons. • If an atom has the same number of protons but a different number of neutrons it is an isotope. Isotopes (cont.) • The nucleus of an isotope can be unstable and may disintegrate, or decay. • Decaying isotopes emit radiation. • Radiation can be detected with instruments such as a Geiger counter. Uses of Radioactive Isotopes • Radioactive isotopes can be used as tracers to follow the movement of that element. • Radioactive isotopes can be used to sterilize medical and dental instruments. • Radioactive isotopes can also be used to kill cancer cells. Uses of Radioactive Isotopes (cont.) Uses of Radioactive Isotopes (cont.) Arrangements of Electrons in an Atom • Electrons encircle the nucleus of an atom at discrete energy levels called electron shells. • In atoms with two or more shells, the outer shell follows the octet rule (8 electrons) • The electrons in the outer valence shell determine the chemical reactivity of atoms. Arrangements of Electrons in an Atom (cont.) Types of Chemical Bonds • A group of atoms bonded to one another form a molecule. • If the molecule has more than one type of element present it is a compound. • Different types of bonds hold molecules and compounds together. Types of Chemical Bonds (cont.) • Charged atoms, or ions, can form when atoms lose or gain electrons. • Positive and negative ions are attracted to one another and bond together in ionic bonds. • A salt is a dry solid composed of atoms connected by ionic bonds. Types of Chemical Bonds (cont.) Types of Chemical Bonds (cont.) • A covalent bond results when two atoms share electrons, thereby completing their valence shells. • When molecules contain covalent bonds, the structure of the molecule can be drawn with a formula or model. Types of Chemical Bonds (cont.) Chemical Reactions • When molecules or compounds are chemically changed it is called a chemical reaction. • Photosynthesis is an example of a chemical reaction. Chemical Reactions (cont.) The chemical reaction for photosynthesis 6 CO2 + 6 H2O C6H12O6 + 6 O2 carbon dioxide water glucose oxygen • Molecules that participate in a reaction are reactants. • Molecules formed by a reaction are products. 2.2 Water’s Importance to Life • Water is the single most important molecule of earth. • All organisms are 70-90% water. • Water has unique properties that make it a life-supporting substance. The Structure of Water • Atoms differ in their electronegativity, or their affinity for electrons in a covalent bond. • The unequal sharing of electrons in a molecule such as water makes the molecule polar. • Polar water molecules are attracted to one another and can form hydrogen bonds. The Structure of Water (cont.) Properties of Water • Water is a solvent that can dissolve many substances. – Molecules that attract water are hydrophilic. – Molecules that cannot attract water are hydrophobic. • Water dissolves polar nonionic substances, ions, and some nonpolar gases. Properties of Water (cont.) • Water shows cohesion and adhesion. • When water molecules cling together with hydrogen bonds it is called cohesion. • When water molecules adhere to polar surfaces it is called adhesion. Properties of Water (cont.) Properties of Water (cont.) • Water also has a high surface tension. • The stronger the force between molecules in a liquid, the stronger the surface tension. Properties of Water (cont.) Properties of Water (cont.) • Water has a high heat capacity, protecting organisms from temperature changes. • Water has a high heat of vaporization. • These properties of water keep temperatures compatible with those of living organisms. Properties of Water (cont.) • Frozen water (ice) is less dense than liquid water, so ice floats. • Unlike other substances, water expands as it freezes. Properties of Water (cont.) Acids and Bases • When water dissociates, it releases an equal number of ions. – Hydrogen ions (H+) – Hydroxide ions (OH-) H–O–H H+ + OH- Acids and Bases (cont.) • Acidic solutions have a high H+ concentration. • An acid is a substance that releases H+ when dissolved in water. HCl H+ + Cl- Hydrochloric acid Acids and Bases (cont.) Acids and Bases (cont.) • Basic solutions have a low H+ concentration. • A base is a substance that releases OHwhen dissolved in water. NaOH Na+ + OH- Sodium hydroxide Acids and Bases (cont.) pH and the pH scale • The pH is a mathematical way of indicating the number of H+ ions in a solution. • The pH scale is used to express acidity or basicity (alkalinity). Buffers and pH • Within the body, the pH is kept in a narrow range to maintain health. • A buffer is a chemical or combination of chemicals that keeps a pH within a given range. • Buffers resist changes in pH by taking up + extra H or OH from solution. Buffers and pH (cont.)