
Chemistry and Life • All organisms share fundamental similarities in their chemical composition and basic metabolic processes • The structure of atoms determines the way they form chemical bonds to produce complex compounds • molecular biology • Chemistry and physics of the molecules that constitute living things
Inorganic and Organic Compounds • inorganic compounds • Small, simple substances • Biologically important groups include water, simple acids and bases, and simple salts • organic compounds • Generally large, complex carbon-containing compounds • Typically, two or more carbon atoms are bonded to each other to form the backbone, or skeleton, of the molecule
2.1 ELEMENTS AND ATOMS LEARNING OBJECTIVES: • Name the principal chemical elements in living things and provide an important function of each • Compare the physical properties (mass and charge) and locations of electrons, protons, and neutrons. Distinguish between the atomic number and the mass number of an atom • Define the terms orbital and electron shell; relate electron shells to principal energy levels
Elements • elements • Substances that can’t be broken down into simpler substances by ordinary chemical reactions • Each element has a chemical symbol (Example: C for carbon) • Four elements (oxygen, carbon, hydrogen, and nitrogen) make up more than 96% of the mass of most organisms • Calcium, phosphorus, potassium, and magnesium, are present in smaller quantities • Iodine and copper are trace elements
Functions of Elements in Organisms Table 2-1, p. 27
Atoms and Matter • atom • Smallest unit of an element that retains that element’s chemical properties • Made up of tiny subatomic particles of matter • matter • Anything that has mass and takes up space
Subatomic Particles • There are three basic types of subatomic particles: • Anelectroncarries a unit of negative electric charge • Aprotoncarries a unit of positive charge • Aneutronis an uncharged particle • Protons and neutrons compose the atomic nucleus • Electronsmove rapidly around the atomic nucleus • In an electrically neutral atom, the number of electrons equals the number of protons
Atomic Number and the Periodic Table • Every element has a fixed number of protons in the atomic nucleus (atomic number) which determines an atom’s identity and defines the element • The periodic tableis a chart of the elements arranged in order by atomic number and chemical behavior • Bohr models represent the electron configurations of elements as a series of concentric rings
Chemical symbol Atomic number Chemical name Number of e– in each energy level Fig. 2-1, p. 28
Atomic Mass • The mass of a subatomic particle is expressed in terms of the atomic mass unit (amu)ordalton • One amu equals the approximate mass of a single proton or a single neutron; an electron is about 1/1800 amu • The atomic mass of an atom equals the total number of protons and neutrons, expressed in amus or daltons
Characteristics of Subatomic Particles Particle Charge ~Mass Location Proton Positive 1 amu Nucleus Neutron Neutral 1 amu Nucleus Electron Negative ~1/1800 amu Outside nucleus
Isotopes • Most elements consist of a mixture of atoms with different numbers of neutrons and different masses • isotopes • Atoms of the same element (having the same number of protons and electrons) with varying numbers of neutrons • The mass of an element is expressed as an average of the masses of its isotopes
Radioisotopes • Some isotopes are unstable and tend to break down (decay) to a more stable isotope (usually a different element) • radioisotope • Unstable isotope that emits radiation as it decays • Example:14C decays to 14N when a neutron decomposes to form a proton and a fast-moving electron • Radioactive decay can be detected by autoradiography, on photographic film
Radioisotopes in Biology • Radioisotopes such as 3H (tritium), 14C, and 32P can replace normal molecules and are used as tracers in research • In medicine, radioisotopes are used for both diagnosis (such as thyroid function or blood flow) and treatment (such as cancer)
Autoradiography • Tritium (3H) incorporated into the DNA of a fruit fly
Atomic Orbitals and Energy • Electrons move through characteristic regions of 3-D space (orbitals), each containing a maximum of 2 electrons • The energy of an electron depends on the orbital it occupies • Electrons in orbitals with similar energies (the same principal energy level) make up an electron shell • Electrons farther from the nucleus generally have greater energy than those closer to the nucleus
Valence Electrons • The most energetic electrons (valence electrons) occupy the valence shell, represented as the outermost concentric ring in a Bohr model • Valence electrons participate in chemical reactions • An electron can move to a higher orbital by receiving more energy, or give up energy and sink to a lower orbital • Changes in electron energy levels are important in energy conversions in organisms
Nucleus (a) The first principal energy level contains a maximum of 2 electrons, occupying a single spherical orbital (designated 1s). The electrons depicted in the diagram could be present anywhere in the blue area. 1s Fig. 2-4a, p. 30
2s 2py 2px 2pz (b) The second principal energy level includes four orbitals, each with a maximum of 2 electrons: one spherical (2s) and three dumbbell-shaped (2p) orbitals at right angles to one another. Fig. 2-4b, p. 30
z 1s 2s y 2py 2px x 2pz (c) Orbitals of the first and second principal energy levels of a neon atom are shown superimposed. Note that the single 2s orbital plus three 2p orbitals make up neon's full valence shell of 8 electrons. Compare this more realistic view of the atomic orbitals with the Bohr model of a neon atom at right. Fig. 2-4c, p. 30
(d) Neon atom (Bohr model) Fig. 2-4d, p. 30
ANIMATION: The shell model of electron distribution To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE
Key Concepts 2.1 • Carbon, hydrogen, oxygen, and nitrogen are the most abundant elements in living things
ANIMATION: Atomic number, mass number To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE
ANIMATION: Electron arrangement in atoms To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE
ANIMATION: Subatomic particles To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE
2.2 CHEMICAL REACTIONS LEARNING OBJECTIVES: • Explain how the number of valence electrons of an atom is related to its chemical properties • Distinguish among simplest, molecular, and structural chemical formulas • Explain why the mole concept is so useful to chemists
Valence Electrons • Chemical behavior of an atom is determined by the number and arrangement of its valence electrons • Atoms with full valence shells are unreactive • When the valence shell is not full, an atom tends to lose, gain, or share electrons to achieve a full outer shell • Elements in the same vertical column (group) of the periodic table have similar chemical properties
Compounds and Molecules • Two or more atoms may combine chemically • A chemical compound consists of atoms of two or more different elements combined in a fixed ratio • Two or more atoms joined very strongly form a stable molecule • Example: H20 (water) is a molecular compound
Chemical Formulas • A chemical formula is a shorthand expression that describes the chemical composition of a substance • In a simplest formula (empirical formula), subscripts give the smallest ratios for atoms in a compound (e.g. NH2) • A molecular formulagives the actual numbers of each type of atom per molecule (e.g. N2H4) • A structural formula shows the arrangement of atoms in a molecule (e.g. water, H—O—H)
The Mole • The molecular mass of a compound equals the sum of the atomic masses of the component atoms of a single molecule • The amount of a compound whose mass in grams is equivalent to its molecular mass is 1 mole (mol) • Example: • Molecular mass of water (H2O) is (hydrogen: 2 × 1 amu) + (oxygen: 1 × 16 amu) = 18 amu • 1 mol of water is 18 grams (g)
The Mole (cont.) • 1 mol of any substance has exactly the same number of atoms or molecules: 6.02 × 1023 (Avogadro’s number) • Avogadro’s number allows scientists to calculate the number of atoms or molecules in sample simply by weighing it • A 1 molar solution (1 M) contains 1 mol of a substance dissolved in a total volume of 1 liter (L)
Chemical Reactions • Chemical reactions, such as the reaction between glucose and oxygen, are described by chemical equations: C6H12O6 + 6 O2 →6 CO2 + 6 H2O + energy • Substances that participate in the reaction (reactants)are written on the left side of the arrow • Substances formed by the reaction (products)are written on the right side
Chemical Reactions (cont.) • Many reactions proceed forward and reverse simultaneously • At dynamic equilibrium, the rates of forward and reverse reactions are equal CO2 + H2O ↔ H2CO3 • When this reaction reaches equilibrium, there will be more reactants (CO2 and H2O) than product (H2CO3)
Key Concepts 2.2 • The chemical properties of an atom are determined by its highest-energy electrons, known as valence electrons
ANIMATION: Chemical bookkeeping To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE
2.3 CHEMICAL BONDS LEARNING OBJECTIVE: • Distinguish among covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interactions • Compare them in terms of the mechanisms by which they form and their relative strengths
Chemical Bonds • Atoms can be held together by chemical bonds • Valence electrons dictate how many bonds an atom can form • bond energy • Energy necessary to break a chemical bond • Two types of strong chemical bonds: covalent and ionic
Covalent Bonds • Covalent bonds involve sharing electrons between atoms in a way that fills each atom’s valence shell • A molecule consists of atoms joined by covalent bonds • Example: hydrogen gas (H2) • Unlike atoms linked by covalent bonds form a covalent compound
Lewis Structure • A simple way of representing valence electrons is to use dots placed around the chemical symbol of the element: • Oxygen (6 valence electrons) shares electrons with two hydrogen atoms to complete its valence shell of 8 – each hydrogen atom completes a valence shell of 2
Carbon Bonds • Carbon has 4 electrons in its valence shell, all of which are available for covalent bonding (e.g. methane, CH4) • Each orbital holds a maximum of 2 electrons
Single, Double, and Triple Covalent Bonds • When one pair of electrons is shared between two atoms, the covalent bond is called a single covalent bond • A double covalent bondis formed when two pairs of electrons are shared(represented by two parallel solid lines) • A triple covalent bond is formed when three pairs of electrons are shared (represented by three parallel solid lines)
Hydrogen (H) Hydrogen (H) Molecular hydrogen (H2) or H H (a) Single covalent bond formation. Two hydrogen atoms achieve stability by sharing a pair of electrons, thereby forming a molecule of hydrogen. In the structural formula on the right, the straight line between the hydrogen atoms represents a single covalent bond. Fig. 2-5a, p. 33