1. Animation: Ionic Bonds
A cation is a positively charged ion
An anion is a negatively charged ion
An ionic bond is an attraction between an anion and a cation
Animation: Ionic Bonds
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

2. Compounds formed by ionic bonds are called ionic compounds, or salts
Salts, such as sodium chloride (table salt), are often found in nature as crystals
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

3. Weak Chemical Bonds
Most of the strongest bonds in organisms are covalent bonds that form a cell’s molecules
Weak chemical bonds, such as ionic bonds and hydrogen bonds, are also important
Weak chemical bonds reinforce shapes of large molecules and help molecules adhere to each other
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

4. Hydrogen Bonds
A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
In living cells, the electronegative partners are usually oxygen or nitrogen atoms
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

5.   + Water (H2O) + Hydrogen bond   Ammonia (NH3) + + +
Fig. 2-16
  +
Water (H2O) +
Hydrogen bond
 
Ammonia (NH3)
Figure 2.16 A hydrogen bond + + +

6. Van der Waals Interactions
If electrons are distributed asymmetrically in molecules or atoms, they can result in “hot spots” of positive or negative charge
Van der Waals interactions are attractions between molecules that are close together as a result of these charges
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

7. Collectively, such interactions can be strong, as between molecules of a gecko’s toe hairs and a wall surface
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

8. Molecular Shape and Function
A molecule’s shape is usually very important to its function
A molecule’s shape is determined by the positions of its atoms’ valence orbitals
In a covalent bond, the s and p orbitals may hybridize, creating specific molecular shapes
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

9. (a) Hybridization of orbitals
Fig. 2-17 z
Four hybrid orbitals
s orbital
Three p
orbitals x y
Tetrahedron
(a) Hybridization of orbitals
Space-filling
Model
Ball-and-stick
Model
Hybrid-orbital Model
(with ball-and-stick
model superimposed)
Unbonded
electron
pair
104.5º
Water (H2O)
Figure 2.17 Molecular shapes due to hybrid orbitals
Methane (CH4)
(b) Molecular-shape models

10. Molecules with similar shapes can have similar biological effects
Biological molecules recognize and interact with each other with a specificity based on molecular shape
Molecules with similar shapes can have similar biological effects
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

11. (a) Structures of endorphin and morphine
Fig. 2-18
Key
Carbon
Nitrogen
Hydrogen
Sulfur
Natural endorphin
Oxygen
Morphine
(a) Structures of endorphin and morphine
Natural
endorphin
Figure 2.18 A molecular mimic
Morphine
Endorphin
receptors
Brain cell
(b) Binding to endorphin receptors

12. Concept 2.4: Chemical reactions make and break chemical bonds
Chemical reactions are the making and breaking of chemical bonds
The starting molecules of a chemical reaction are called reactants
The final molecules of a chemical reaction are called products
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

13. Fig. 2-UN2
2 H2 O2
2 H2O
Reactants
Reaction
Products

14. Photosynthesis is an important chemical reaction
Sunlight powers the conversion of carbon dioxide and water to glucose and oxygen
6 CO2 + 6 H20 → C6H12O6 + 6 O2
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

15. Fig. 2-19
Figure 2.19 Photosynthesis: a solar-powered rearrangement of matter

16. Some chemical reactions go to completion: all reactants are converted to products
All chemical reactions are reversible: products of the forward reaction become reactants for the reverse reaction
Chemical equilibrium is reached when the forward and reverse reaction rates are equal
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

17. Electrons (– charge) form negative cloud and determine
Fig. 2-UN3
Nucleus
Protons (+ charge)
determine element
Electrons (– charge) form negative cloud
and determine
chemical behavior
Neutrons (no charge)
determine isotope
Atom

18. Fig. 2-UN4

19. Single covalent bond Double covalent bond
Fig. 2-UN5
Single
covalent bond
Double
covalent bond

20. Ionic bond Electron transfer forms ions Na Sodium atom Cl
Fig. 2-UN6
Ionic bond
Electron
transfer
forms ions Na
Sodium atom Cl
Chlorine atom
Na+
Sodium ion
(a cation)
Cl–
Chloride ion
(an anion)

21. Fig. 2-UN7

22. Fig. 2-UN9

23. Fig. 2-UN10

24. Fig. 2-UN11

25. You should now be able to:
Identify the four major elements
Distinguish between the following pairs of terms: neutron and proton, atomic number and mass number, atomic weight and mass number
Distinguish between and discuss the biological importance of the following: nonpolar covalent bonds, polar covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interactions
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings