1. Chemical Bonding and Properties of Water

2. Chemical Bonds
A bond is formed when electrons from two atoms interact with each other and their atoms become joined.

3. Molecules and Compounds
Molecule – when two or more atoms bond. CO2 , O2 , H2 and H2O are all molecules.
Compound – when different elements combine. CO2 and H2O are molecules, but they are also compounds because they are molecules containing more than one element.

4. Molecules and Compounds

5. The formation and function of molecules depend on chemical bonding between atoms
Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms
These interactions usually result in atoms staying close together, held by attractions called chemical bonds
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

6. Ionic Bonds
Because oppositely charged ions attract, sodium and chlorine atoms that have formed ions may react to form an ionic bond
Sodium ions (Na+) and chloride ions (Cl-) form the compound sodium chloride or table salt

7. 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

8. Fig. 2-15
Na+
Cl–
Figure 2.15 A sodium chloride crystal

9. Covalent Bonds
Atoms can bond by sharing electrons instead of exchanging them
Atoms like to have their outer shell filled. If it is not, they can share electrons with another atom who needs an electron

10. Covalent Bonds Cont.
A molecule consists of two or more atoms held together by covalent bonds
A single covalent bond, or single bond, is the sharing of one pair of valence electrons
A double covalent bond, or double bond, is the sharing of two pairs of valence electrons
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

11. Name and Molecular Formula Electron- distribution Diagram Lewis Dot
Fig. 2-12a
Name and
Molecular
Formula
Electron-
distribution
Diagram
Lewis Dot
Structure and
Structural
Formula
Space-
filling
Model
(a) Hydrogen (H2)
Figure 2.12 Covalent bonding in four molecules

12. Name and Molecular Formula Electron- distribution Diagram Lewis Dot
Fig. 2-12b
Name and
Molecular
Formula
Electron-
distribution
Diagram
Lewis Dot
Structure and
Structural
Formula
Space-
filling
Model
(b) Oxygen (O2)
Figure 2.12 Covalent bonding in four molecules

13. Polar v. Nonpolar Molecules
In a nonpolar covalent bond, the atoms share the electron equally
In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally
Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

14. Name and Molecular Formula Electron- distribution Diagram Lewis Dot
Fig. 2-12d
Name and
Molecular
Formula
Electron-
distribution
Diagram
Lewis Dot
Structure and
Structural
Formula
Space-
filling
Model
(d) Methane (CH4)
Figure 2.12 Covalent bonding in four molecules

15. Name and Molecular Formula Electron- distribution Diagram Lewis Dot
Fig. 2-12c
Name and
Molecular
Formula
Electron-
distribution
Diagram
Lewis Dot
Structure and
Structural
Formula
Space-
filling
Model
(c) Water (H2O)
Figure 2.12 Covalent bonding in four molecules

16. Hydrogen Bond
Weak electrical attraction between a hydrogen atom and another atom

17. 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

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

19. Bond Strength
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 stick to each other
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

20. Water is a Polar Molecule
Fig. 2-13
Water is a Polar Molecule
 – + H O
H2O
Figure 2.13 Polar covalent bonds in a water molecule

21. How does polarity influence the way molecules interact?
Opposites Attract!!!
Positively charged atoms
Are attracted to negatively
Charged atoms

22. More About Water Water is cohesive – it sticks together
Because of hydrogen bonding
Creates surface tension – forms droplets
Allows insects and leaves to rest on the surface of water
Water is adhesive – it sticks to other things
Capillary action
Water travels up the stem of a plant
Meniscus forms at surface in graduated cylinder