PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE TAYLOR SIMON DICKEY HOGAN Chapter 2 Lecture by Edward J. Zalisko The Chemical Basis of Life © 2015 Pearson Education, Inc.

C HEMICAL B ONDS © 2015 Pearson Education, Inc.

2.5 The distribution of electrons determines an atom’s chemical properties Of the three subatomic particles—protons, neutrons, and electrons—only electrons are directly involved in the chemical activity of an atom. Electrons can be located in different electron shells, each with a characteristic distance from the nucleus. An atom may have one, two, or more electron shells. Information about the distribution of electrons is found in the periodic table of the elements. © 2015 Pearson Education, Inc.

Figure 2.5b-0 HydrogenHelium Lithium Beryllium Boron Carbon Nitrogen OxygenFluorineNeon First shell Second shell Third shell Sodium MagnesiumAluminum Silicon Phosphorus Sulfur Chlorine Argon

2.5 The distribution of electrons determines an atom’s chemical properties The number of electrons in the outermost shell, called the valence shell, determines the chemical properties of the atom. Atoms whose outer shells are not full tend to interact with other atoms in ways that enable them to complete or fill their valence shells. © 2015 Pearson Education, Inc.

2.5 The distribution of electrons determines an atom’s chemical properties When two atoms with incomplete outer shells react, each atom will share, donate, or receive electrons, so that both partners end up with completed outer shells. These interactions usually result in atoms staying close together, held by attractions called chemical bonds. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing In a covalent bond, two atoms, each with an unpaired electron in its outer shell, actually share a pair of electrons. Two or more atoms held together by covalent bonds form a molecule. A covalent bond connects two hydrogen atoms in a molecule of the gas H 2. © 2015 Pearson Education, Inc.

Animation: Covalent Bonds © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing Atoms in a covalently bonded molecule continually compete for shared electrons. The attraction (pull) for shared electrons is called electronegativity (aka, cuteness factor). More electronegative atoms pull harder. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing In molecules of only one element, the pull toward each atom is equal, because each atom has the same electronegativity. The bonds formed are called nonpolar covalent bonds. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing Water has atoms with different electronegativities. Oxygen attracts the shared electrons more strongly than hydrogen. So the shared electrons spend more time near oxygen. The oxygen atom has a slightly negative charge and the hydrogen atoms have a slightly positive charge. The result is a polar covalent bond. © 2015 Pearson Education, Inc.

Figure Molecular Formula Electron Distribution Diagram Structural Formula Space-Filling Model O 2 Oxygen CH 4 Methane H 2 O Water Polar covalent bonds in a water molecule Single bond Double bond Nonpolar covalent bonds Polar covalent bonds (slightly −) (slightly +) H H H H H H H H O O O C O H H H 2 Hydrogen

© 2015 Pearson Education, Inc. Figure Polar covalent bonds in a water molecule (slightly +) O H H (slightly −)

2.7 Ionic bonds are attractions between ions of opposite charge An ion is an atom or molecule with an electrical charge resulting from gain or loss of one or more electrons. When an electron is lost, a positive charge results. When an electron is gained, a negative charge results. Two ions with opposite charges attract each other. When the attraction holds the ions together, it is called an ionic bond. Salt is a synonym for an ionic compound. © 2015 Pearson Education, Inc.

Animation: Ionic Bonds © 2015 Pearson Education, Inc.

Figure 2.7a-2 Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl − Chloride ion Na + Na Cl − Cl − + Sodium chloride (NaCl)

© 2015 Pearson Education, Inc. Figure 2.7b-0 Na + Cl −

2.8 Hydrogen bonds are weak bonds important in the chemistry of life In living organisms, most of the strong chemical bonds are covalent, linking atoms to form a cell’s molecules. Crucial to the functioning of a cell are weaker bonds within and between molecules. One of the most important types of weak bonds is the hydrogen bond, which is best illustrated with water molecules. © 2015 Pearson Education, Inc.

2.8 Hydrogen bonds are weak bonds important in the chemistry of life The hydrogen atoms of a water molecule are attached to oxygen by polar covalent bonds. Because of these polar bonds and the wide V shape of the molecule, water is a polar molecule—that is, it has an unequal distribution of charges. This partial positive charge allows each hydrogen to be attracted to a nearby atom that has a partial negative charge. © 2015 Pearson Education, Inc.

2.8 Hydrogen bonds are weak bonds important in the chemistry of life Weak hydrogen bonds form between water molecules. Each hydrogen atom of a water molecule can form a hydrogen bond with a nearby partially negative oxygen atom of another water molecule. The negative (oxygen) pole of a water molecule can form hydrogen bonds to two hydrogen atoms. Thus, each H 2 O molecule can hydrogen-bond to as many as four partners. © 2015 Pearson Education, Inc.

Animation: Water Structure © 2015 Pearson Education, Inc.

Figure 2.8 (−)(−) (+)(+) (−)(−) (+)(+) (−)(−) (+)(+) (−)(−) (+)(+) Hydrogen bond Polar covalent bonds

2.9 Chemical reactions make and break chemical bonds Remember that the structure of atoms and molecules determines the way they behave. Atoms combine to form molecules. Hydrogen and oxygen can react to form water: 2 H 2 + O 2 2 H 2 O © 2015 Pearson Education, Inc.

2.9 Chemical reactions make and break chemical bonds The formation of water from hydrogen and oxygen is an example of a chemical reaction. The reactants (H 2 and O 2 ) are converted to H 2 O, the product. Chemical reactions do not create or destroy matter. Chemical reactions only rearrange matter. © 2015 Pearson Education, Inc.

Figure Reactants Products 2 H 2 O2O2 2 H 2 O

2.9 Chemical reactions make and break chemical bonds Photosynthesis is a chemical reaction that is essential to life on Earth. Carbon dioxide (from the air) reacts with water. Sunlight powers the conversion of these reactants to produce the products glucose and oxygen. © 2015 Pearson Education, Inc.