1. The Chemical Context of Life and Properties of Water
Chapter 2
The Chemical Context of Life
and Properties of Water

2. Why are we studying chemistry?
Chemistry is the foundation of Biology

3. Basics You Should Know Matter Elements Compounds Atoms
Subatomic Particles
Atomic Number
Atomic Mass
Isotopes
Electron Shells/Energy Levels
Electron Orbitals

4. Everything is made of matter Matter is made of atoms
Hydrogen
1 proton
1 electron
Oxygen
8 protons
8 neutrons
8 electrons
Proton +
Neutron
Electron –

5. Different kinds of atoms = elements
The World of Elements H C N O Na Mg P S K Ca
Different kinds of atoms = elements

6. Life requires ~25 chemical elements
About 25 elements are essential for life
Four elements make up 96% of living matter:
• carbon (C) • hydrogen (H)
• oxygen (O) • nitrogen (N)
Four elements make up most of remaining 4%:
• phosphorus (P) • calcium (Ca)
• sulfur (S) • potassium (K)

8. Deficiencies
If there is a deficiency of an essential element, disease results
(a) Nitrogen deficiency
(b) Iodine deficiency (Goiter)

9. Trace Elements
Trace elements Are required by an organism in only minute quantities
Minerals such as Fe and Zn are trace elements

10. How does this atom behave?
Bonding properties
Effect of electrons
electrons determine chemical behavior of atom
depends on number of electrons in atom’s outermost shell
valence shell
How does this atom behave?

11. Bonding properties How does this atom behave?
What’s the magic number?
Bonding properties
Sulfur on the LEFT
Magnesium on the RIGHT
How does this atom behave?
How does this atom behave?

12. Elements & their valence shells
Elements in the same row have the same number of shells
Moving from left to right, each element has a sequential addition of electrons (& protons)

13. Elements & their valence shells
Elements in the same column have the same valence & similar chemical properties
Oxygen has medium electronegativity so doesn’t pull electrons all the way off hydrogen whereas chlorine would.
So oxygen forms a polar covalent bond.
Carbon has only a weak electronegativity so forms a nonpolar covalent bond

14. This tendency drives chemical reactions…
Chemical reactivity
Atoms tend to
complete a partially filled valence shell or
empty a partially filled valence shell
This tendency drives chemical reactions…
and creates bonds – –

15. Ionic bonding
High electronegativity difference strips valence electrons away from another atom
Electron transfer creates ions (charged atoms)
Cation (positive ion); anion (negative ion)
Ex: Salts (sodium chloride)

16. Covalent Bonding Sharing pair of valence electrons
Number of electrons required to complete an atom’s valence shell determines how many bonds will form
Ex: Hydrogen & oxygen bonding in water; methane

17. Nonpolar covalent bond
Pair of electrons shared equally by 2 atoms
example: hydrocarbons = CxHx
methane (CH4 )
balanced, stable, good building block

18. Polar covalent bonds Pair of electrons shared unequally by 2 atoms
example: water = H2O
oxygen has higher electronegativity
oxygen has stronger “attraction” for the electrons than hydrogen
water is a polar molecule
+ vs – poles
leads to many interesting properties of water… H
Oxygen + – – – – +

19. Hydrogen bonds
Hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom (oxygen or nitrogen)

20. On Earth, it exists in three phyiscal states
Properties of Water
Capillary Action
High Specific Heat
Surface Tension
Supports Life
Adhesion
Good Solvent
On Earth, it exists in three phyiscal states
Cohesion
Solid
Gas
liquid

21. Properties of Water Earth is ¾ Water Supports All of Life
Most animals and plants contain more than 60% water by volume.
All living organisms require water more than any other substance

22. Properties of Water Cohesion/Adhesion Surface tension
Temperature Moderation
High specific heat
Evaporative cooling
Universal solvent

23. Effects of Changes in pH
Water can dissociate Into hydronium ions (H+ or H3O+) and hydroxide (OH-) ions
Changes in the concentration of these ions Can have a great affect on pH in living organisms H
Hydronium
ion (H3O+)
Hydroxide
ion (OH–) + –
Figure on p. 53 of water dissociating

24. Acids and Bases
An acid
Is any substance that increases the hydrogen ion concentration of a solution
A base
Is any substance that reduces the hydrogen ion concentration of a solution (more OH- ions)

25. Buffers The internal pH of most living cells Buffers
Must remain close to pH 7
Buffers
Are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution