Chemical Bonding and Properties of Water

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

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.

Molecules and Compounds

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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