Chapter 2:Atoms, Molecules & Life
Chapter 2: Atoms, Molecules & Life
What Are Atoms?
Answer: The smallest unit of matter
Composed of:1) Nucleus (in the middle)
• Protons (positively charged particles)• Neutrons (neutrally charged particles)
2) Electrons (negatively charged particles)• Orbit the nucleus
An atom is electrically neutral (protons = electrons)
Chapter 2: Atoms, Molecules & Life
An example ofan atom:
Chapter 2: Atoms, Molecules & Life
Periodic Table of Elements:
Element = Substance that can’t be broken down or converted to another substance
Atomic Number = Number of protons in the nucleus
Chapter 2: Atoms, Molecules & Life
Atomic Mass = Number of protons & neutrons in nucleus
Isotope = The same element with a differentnumber of neutrons
• C14 • P32 • U235
Chapter 2: Atoms, Molecules & Life
Chapter 2: Atoms, Molecules & Life
Electrons:
A) are repelled from one another (- charge)B) are attracted to the nucleus (+ charge)
Electrons orbit the nucleus in 3-dimensional spaceforming electron shells:
• First shell (2 electrons)• Second shell (8 electrons)• Third shell (8 electrons)
Chapter 2: Atoms, Molecules & Life
Inert Atoms = Atoms with their outermost shell either completely full or empty
Reactive Atoms = Atoms with their outermost shell only partially filled
HydrogenCarbonOxygen
HeliumNeonArgon
Chapter 2: Atoms, Molecules & Life
How would you make these atoms stable?
(Figure 2.3)
Chapter 2: Atoms, Molecules & Life
Chemical Bonds:Interaction of atoms to stabilize their outermostelectron shells (attractive force)
Molecule = 2 or more atoms of any type held together byinteractive forces
H HO
Water (H2O)
HH
Hydrogen (H2)
Compound = 2 or more atoms of different types held together by interactive forces
Chemical Reaction = making or breaking chemical bonds • Essential for life (and modern society)
Chapter 2: Atoms, Molecules & Life
Chemical Bond Types:
1) Ionic Bond:Attractive force between atoms that have lost or gained electrons (Ions - charged molecules)
Transfer ofelectrons
Chapter 2: Atoms, Molecules & Life
Sodium Chloride (NaCl - Figure 2.3)
Ionic Bonds = Weak Bonds
Chapter 2: Atoms, Molecules & Life
Chemical Bond Types:
2) Covalent Bond:Attractive force between atoms that share electrons
Sharing ofelectrons
Most biological molecules utilize covalent bonding:
ProteinsCarbohydratesLipidsNucleic Acids
Covalent Bonds = Strong Bonds
Chapter 2: Atoms, Molecules & Life
Examples of Covalent Bonds:
H - H
Share one pair of electrons (H2)• Single covalent bond
Share two pairs of electrons (O2)• Double covalent bond
O = O
Share three pairs of electrons (N2)• Triple covalent bond N N
Chapter 2: Atoms, Molecules & Life
Bonding Patterns (Table 2.3):
Carbon (C)
Oxygen (O)
Chapter 2: Atoms, Molecules & Life
Non-polar Covalent Bonds:Equal sharing of electrons
Polar Covalent Bonds:Unequal sharing of electrons
Molecule is electrically neutralbut
poles are charged due to differences in nuclear attraction for electrons
Chapter 2: Atoms, Molecules & Life
Chemical Bond Types:
3) Hydrogen Bond:Attractive force between water molecules
(Figure 2.5)
Positive end of water molecule (H’s) attract negative end (O)
of other water molecule
Due to polar covalent bonds:
Weak Bond
Chapter 2: Atoms, Molecules & Life
Water = Good Stuff!
Life most likely arose in waterLiving organisms 60 - 90% water
Why is Water so Important to Life?
Chapter 2: Atoms, Molecules & Life
Importance of Water:
1) Water is an excellent solvent:• liquid capable of dissolving other substances in itself
Dissolving Ionic Bonds:(Salt)
(Figure 2.6)
Polar nature of water
Solution = Fluid containing dissolved substances
Chapter 2: Atoms, Molecules & Life
• Molecules electricallyattracted to water
• Ions, polar molecules
Hydrophilic Molecules:Water-loving
Hydrophobic Molecules:Water-fearing
• Molecules electricallyneutral (fats / oils)
Dissolving Polar Covalent Bonds:(e.g. glucose)
+ -
• Molecules tend to clumptogether in water(Figure 2.7)
Chapter 2: Atoms, Molecules & Life
Importance of Water:
2) Water molecules tend to stick together (cohesion):• Surface Tension: Tendency for a water surface to
resist breaking
Walk onwater
Flow againstgravity
• Adhesion: Tendency for water to stick to walls of surfaces
Chapter 2: Atoms, Molecules & Life
Importance of Water:
3) Water Can Form H+ and OH- Ions (ionization):
H2O H+ OH-+Pure water contains equal amounts of H+ and OH-
HCl Acidic
H+ = OH-
Water
H+ = OH-
Water
NaOHBasic
OH-H+ <H+ > OH-
Chapter 2: Atoms, Molecules & Life
The pH of a solution describes its degree of acidity:
(Figure 2.8)
Chapter 2: Atoms, Molecules & Life
Buffers maintain a solution at relatively constant pH:• Stable pH essential for normal function
• Buffers either accept or release a H+ in response to changes in pH
Example: Bicarbonate ion (HCO3-)
Too acidic?
H+Hydrogen ion
+HCO3-
Bicarbonate ion
H2CO3Carbonic Acid
Too basic?
OH-Hydroxide ion
+H2CO3Carbonic Acid
H20HCO3- +
Bicarbonate ion Water
Chapter 2: Atoms, Molecules & Life
Importance of Water:
3) Water Moderates Temperature Changes:
• Slow molecules = Cool temperatures• Fast molecules = Warm temperatures
• Temperature = Speed of moleculesBackground:
A) Water Heats Slowly• Energy first initiates breaking of hydrogen bonds…• Specific Heat = energy needed to heat 1 gram of
a substance 1°C • Specific HeatWater = 1 cal• Specific HeatAlcohol = 0.6 cal• Specific HeatGranite = 0.02 cal
Chapter 2: Atoms, Molecules & Life
Importance of Water:
3) Water Moderates Temperature Changes:
A) Water heats slowlyB) Water is an effective coolant
• Heat of Vaporization: Heat needed to convert liquidwater to water vapor
• 529 calories/gram (very high!)By evaporating 1 g of water,
539 grams of human body cools 1° C
C) Water freezes slowly• Moderates the effects of low temperatures
D) Water forms ice (less dense than fluid water):• Acts as an insulator for life below
Transfer ofelectrons
Ionic Bonds:
Sharing ofelectronsCovalent
Bonds:
Chapter 2: Atoms, Molecules & Life
The Importance of Water:
1) Excellent Solvent2) Cohesion / Adhesion3) Forms Ions (H+ / OH-)4) Moderates Temp.