Bonding and Water Properties
General Biology
A chemical compound consists of atoms oftwo or more elements
Compounds are held together by chemicalbonds
Chemical Compounds
Chemical bonds
Forces of attraction that hold atoms of a compoundtogether
The three types of bonds are:
Covalent bonds
Ionic bonds
Hydrogen bonds
Bonding
Covalent bonds
Share electrons between atoms
Each atom has a filled outer energy level
2 types of covalent bonds
Nonpolar covalent – electrons are shared equally
Example: Oxygen gas
Polar covalent – electrons are shared unequally and pulledtowards one atom making a neg. charge, or pole, at oneend of the atom
Example: Water
Bonding
Covalent bonds
Covalent bonds can be nonpolar or polar
Form between a positive ion and a negativeion Example: Na+ and Cl- bond to create NaCl
Ionic Bonds
Ionic bonding
Hydrogen bonds
Weak bonds between molecules
Positively charged hydrogen atom in one covalentlybonded molecule is attracted to a negatively chargedarea of another molecule.
Example: Hydrogen bonding between watermolecules
Hydrogen Bonding
Hydrogen
bonding
Because of hydrogen bonding, water hassome very unique properties.
Water
Fig. 3-2
Hydrogen
bond
–
H
+
H
O
——
——
+
+
+
–
–
–
Water exists in 3 forms: gas, liquid, or solid
Hydrogen bonds are formed or broken aswater changes state
Water Properties
Ionic and polar substances dissolve well in water b/cof water’s hydrogen bonds.
Water facilitates chemical reactions
Hydrophilic substances—interact readily withwater, such as table salt
Hydrophobic substances—not disrupted ordissolved by water, such as fats
Water is an excellent solvent.
Fig. 3-7
Cl–
Na
Cl–
+
+
+
+
+
+
+
+
–
–
–
–
–
–
–
–
Na
+
–
–
–
+
Changes temperature very slowly.
Must add a lot of energy to heat or remove a lot ofenergy to cool/freeze water.
Water has a high heat capacity
Less dense as a solid than a liquid… due to hydrogenbonding
Ice floats!
Fig. 3-6a
Hydrogen
bond
Liquid water
Hydrogen bonds break and re-form
Ice
Hydrogen bonds are stable
Cohesion – attraction between like substances
Occurs b/c of hydrogen bonding between watermolecules
Leads to a high surface tension
Creating a water surface that is very firm
Example: insects ‘walking’ on water
Water has strong cohesion and highsurface tension.
Fig. 3-4
Attraction of unlike substances
Capillary action
Water adhering to the narrow tubing or absorbentsolids
Water has strong adhesion.
Fig. 3-3
Water-conducting
cells
Adhesion
Cohesion
150 µm
Direction
of water
movement