Review, Hydrogen Bonding and Water

AP Biology

Biology, Sixth Edition Chapter 2, Atoms and Molecules

Ionic Bonds

• electrons are donated by one atom to another

• An electronegative atom steals an electron from another atom to fill its valence shell

Biology, Sixth Edition Chapter 2, Atoms and Molecules

Ionic Bonds in Salt

Biology, Sixth Edition Chapter 2, Atoms and Molecules

Covalent Bonds• two electrons are shared per bond• The outer shells of both atoms become filled • More than one bond can occur between two atoms

Biology, Sixth Edition Chapter 2, Atoms and Molecules

Thinking About Covalent BondsThe Lewis structure uses dots

to indicate the number of electrons in the valence shells of atoms:

In contrast, a structural model uses a line to represent the shared pair of electrons:

methane

2 types of covalent bonds

1. Nonpolar – share electrons equally2. Polar – share electrons unequally

Biology, Sixth Edition Chapter 2, Atoms and Molecules

• Electronegative O pulls e- from H

• This causes a partial (+) charge on the H, and a partial (-) charge on the O

• Partial charges are indicated by lower case deltas ()

• The bonds between O and H are polar covalent bonds

Polar Covalent Bonds

Biology, Sixth Edition Chapter 2, Atoms and Molecules

• Partial charges interact

• Hs attracted to Os

• Causes water to self-associate

Water Forms Hydrogen Bonds

Biology, Sixth Edition Chapter 2, Atoms and Molecules

Hydrogen Bonding

• Gaseous water molecules interact very little

• Liquid water molecules are hydrogen bonded, but can slip past each other

• As water freezes, it forms a crystalline lattice because H bonding lasts longer

Biology, Sixth Edition Chapter 2, Atoms and Molecules

Water is “Sticky”

• Water is sticky because of H bonding• Cohesive – sticks to itself• Adhesive – sticks to other things

– Meniscus

Biology, Sixth Edition Chapter 2, Atoms and Molecules

Water has High Surface Tension• Water sticks to itself strongly and so has

high surface tension– Forms meniscus– Forms droplets– Small animals can ‘skate’ on surface

Other water properties

• High specific heat• High heat of vaporization• Great solvent