PowerPoint Lectures
Campbell Biology: Concepts & Connections, Eighth Edition REECE • TAYLOR • SIMON • DICKEY • HOGAN
Chapter 2
Lecture by Edward J. Zalisko
The Chemical Basis of Life
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 2.0-2
Chapter 2: Big Ideas
Elements, Atoms,
and Compounds
Chemical Bonds Water’s Life-
Supporting Properties
O
H H
Introduction
• Chemicals are the raw materials that make up
• our bodies,
• the bodies of other organisms, and
• the physical environment.
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Introduction
• Life’s chemistry is tied to water.
• Life first evolved in water.
• All living organisms require water.
• Cells consist of about 75% water.
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2.1 Organisms are composed of elements, in combinations called compounds
• Living organisms are composed of matter, which is
anything that occupies space and has mass
(weight).
• Matter is composed of chemical elements.
• An element is a substance that cannot be broken
down to other substances by ordinary chemical
means.
• There are 92 elements in nature—only a few exist
in a pure state.
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2.1 Organisms are composed of elements, in combinations called compounds
• A compound is a substance consisting of two or
more different elements in a fixed ratio.
• Compounds are more common than pure
elements.
• Sodium chloride, table salt, is a common
compound of equal parts of sodium (Na) and
chlorine (Cl).
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2.1 Organisms are composed of elements, in combinations called compounds
• About 25 elements are essential for human life.
• Four elements make up about 96% of the weight of
most living organisms. These are
• oxygen,
• carbon,
• hydrogen, and
• nitrogen.
• Trace elements are essential but are only needed
in minute quantities.
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2.2 CONNECTION: Trace elements are common additives to food and water
• Some trace elements are required to prevent
disease.
• Without iron, your body cannot transport oxygen.
• An iodine deficiency prevents production of thyroid
hormones, resulting in goiter.
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2.2 CONNECTION: Trace elements are common additives to food and water
• Fluoride is usually added to municipal water and
dental products to help reduce tooth decay.
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2.2 CONNECTION: Trace elements are common additives to food and water
• Several chemicals are added to food to
• help preserve it,
• make it more nutritious, and/or
• make it look better.
• Check out the nutrition facts label on foods and
drinks you purchase.
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2.3 Atoms consist of protons, neutrons, and electrons
• Each element consists of one kind of atom.
• An atom is the smallest unit of matter that still
retains the properties of an element.
• Three subatomic particles in atoms are relevant to
our discussion of the properties of elements.
• Protons are positively charged.
• Electrons are negatively charged.
• Neutrons are electrically neutral.
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2.3 Atoms consist of protons, neutrons, and electrons
• Neutrons and protons are packed into an atom’s
nucleus.
• Electrons orbit the nucleus.
• The negative charge of electrons and the positive
charge of protons keep electrons near the nucleus.
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Figure 2.3
Nucleus
Electron
cloud
Nucleus Protons
Neutrons
Electrons
2
2e−
2
2
−
+
−
+
+
+
+
−
2.3 Atoms consist of protons, neutrons, and electrons
• The number of protons is the atom’s atomic
number.
• An atom’s mass number is the sum of the number
of protons and neutrons in the nucleus.
• The atomic mass is approximately equal to its
mass number.
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Figure 2.3
Nucleus
Electron
cloud
Nucleus Protons
Neutrons
Electrons
2
2e−
2
2
−
+
−
+
+
+
+
−
2.3 Atoms consist of protons, neutrons, and electrons
• Although all atoms of an element have the same
atomic number, some differ in mass number.
• Different isotopes of an element have
• the same number of protons
• but different numbers of neutrons.
• Different isotopes of an element behave identically
in chemical reactions.
• In radioactive isotopes, the nucleus decays
spontaneously, giving off particles and energy.
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2.4 CONNECTION: Radioactive isotopes can help or harm us
• Living cells cannot distinguish between isotopes of
the same element.
• Therefore, radioactive compounds in metabolic
processes can act as tracers.
• This radioactivity can be detected by instruments.
• By using these instruments, the fate of radioactive
tracers can be monitored in living organisms.
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2.4 CONNECTION: Radioactive isotopes can help or harm us
• Radioactive tracers are frequently used in medical
diagnosis.
• Sophisticated imaging instruments are used to
detect them.
• An imaging instrument that uses positron-emission
tomography (PET) detects the location of injected
radioactive materials.
• PET is useful for diagnosing heart disorders and
cancer and in brain research.
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2.4 CONNECTION: Radioactive isotopes can help or harm us
• In addition to benefits, there are also dangers
associated with using radioactive substances.
• Uncontrolled exposure can cause damage to some
molecules in a living cell, especially DNA.
• Chemical bonds are broken by the emitted energy,
which causes abnormal bonds to form.
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2.5 The distribution of electrons determines an atom’s chemical properties
• Of the three subatomic particles—protons,
neutrons, and electrons—only electrons are
directly involved in the chemical activity of an
atom.
• Electrons can be located in different electron
shells, each with a characteristic distance from the
nucleus.
• An atom may have one, two, or more electron
shells.
• Information about the distribution of electrons is
found in the periodic table of the elements. © 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 2.5b-0
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
First
shell
Second
shell
Third
shell
Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon
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Figure 2.5b-1
Hydrogen
Lithium Beryllium Boron
Sodium Magnesium Aluminum
Carbon
Silicon
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Figure 2.5b-2
Helium
Nitrogen Oxygen Fluorine Neon
Phosphorus Sulfur Chlorine Argon
2.5 The distribution of electrons determines an atom’s chemical properties
• The number of electrons in the outermost shell,
called the valence shell, determines the chemical
properties of the atom.
• Atoms whose outer shells are not full tend to
interact with other atoms in ways that enable them
to complete or fill their valence shells.
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2.5 The distribution of electrons determines an atom’s chemical properties
• When two atoms with incomplete outer shells
react, each atom will share, donate, or receive
electrons, so that both partners end up with
completed outer shells.
• These interactions usually result in atoms staying
close together, held by attractions called chemical
bonds.
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2.6 Covalent bonds join atoms into molecules through electron sharing
• In a covalent bond, two atoms, each with an
unpaired electron in its outer shell, actually share a
pair of electrons.
• Two or more atoms held together by covalent
bonds form a molecule.
• A covalent bond connects two hydrogen atoms in a
molecule of the gas H2.
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2.6 Covalent bonds join atoms into molecules through electron sharing
• There are four alternative ways to represent
common molecules.
• The hydrogen atoms in H2 are held together by a
pair of shared electrons.
• In an oxygen molecule (O2), the two oxygen atoms
share two pairs of electrons, forming a double
bond, indicated in a structural formula by a pair of
lines.
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Figure 2.6-0
Molecular
Formula
Electron Distribution
Diagram
Structural
Formula
Space-Filling
Model
O2
Oxygen
CH4
Methane
H2O
Water
Polar covalent bonds
in a water molecule
Single bond
Double bond
Nonpolar covalent
bonds
Polar covalent
bonds
(slightly −)
(slightly +) (slightly +)
H H
H
H
H
H
H H
O O
O
C
O
H H
H2
Hydrogen
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Figure 2.6-1
Molecular
Formula
Electron Distribution
Diagram
Structural
Formula Space-Filling
Model
O2
Oxygen
H H
O O
H2
Hydrogen Single bond
Double bond
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Figure 2.6-2
Molecular
Formula
Electron Distribution
Diagram
Structural
Formula
Space-Filling
Model
CH4
Methane
H2O
Water
Nonpolar covalent
bonds
Polar covalent
bonds
H
H
H
H H
O
C H
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Figure 2.6-3
Polar covalent bonds
in a water molecule
(slightly +) (slightly +)
O
H H
(slightly −)
2.6 Covalent bonds join atoms into molecules through electron sharing
• H2 and O2 are molecules composed of only one
element.
• Methane (CH4) and water (H2O) are compounds,
substances composed of two or more different
elements.
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2.6 Covalent bonds join atoms into molecules through electron sharing
• Atoms in a covalently bonded molecule continually
compete for shared electrons.
• The attraction (pull) for shared electrons is called
electronegativity.
• More electronegative atoms pull harder.
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2.6 Covalent bonds join atoms into molecules through electron sharing
• In molecules of only one element, the pull toward
each atom is equal, because each atom has the
same electronegativity.
• The bonds formed are called nonpolar covalent
bonds.
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2.6 Covalent bonds join atoms into molecules through electron sharing
• Water has atoms with different electronegativities.
• Oxygen attracts the shared electrons more strongly
than hydrogen.
• So the shared electrons spend more time near
oxygen.
• The oxygen atom has a slightly negative charge
and the hydrogen atoms have a slightly positive
charge.
• The result is a polar covalent bond.
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2.7 Ionic bonds are attractions between ions of opposite charge
• An ion is an atom or molecule with an electrical
charge resulting from gain or loss of one or more
electrons.
• When an electron is lost, a positive charge results.
• When an electron is gained, a negative charge
results.
• Two ions with opposite charges attract each other.
• When the attraction holds the ions together, it is
called an ionic bond.
• Salt is a synonym for an ionic compound.
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Figure 2.7a-2
Na
Sodium atom
Cl
Chlorine atom
Na+
Sodium ion
Cl−
Chloride ion
Na+ Na Cl− Cl
− +
Sodium chloride (NaCl)
2.8 Hydrogen bonds are weak bonds important in the chemistry of life
• In living organisms, most of the strong chemical
bonds are covalent, linking atoms to form a cell’s
molecules.
• Crucial to the functioning of a cell are weaker
bonds within and between molecules.
• One of the most important types of weak bonds is
the hydrogen bond, which is best illustrated with
water molecules.
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2.8 Hydrogen bonds are weak bonds important in the chemistry of life
• The hydrogen atoms of a water molecule are
attached to oxygen by polar covalent bonds.
• Because of these polar bonds and the wide V
shape of the molecule, water is a polar
molecule—that is, it has an unequal distribution of
charges.
• This partial positive charge allows each hydrogen
to be attracted to a nearby atom that has a partial
negative charge.
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2.8 Hydrogen bonds are weak bonds important in the chemistry of life
• Weak hydrogen bonds form between water
molecules.
• Each hydrogen atom of a water molecule can form
a hydrogen bond with a nearby partially negative
oxygen atom of another water molecule.
• The negative (oxygen) pole of a water molecule can
form hydrogen bonds to two hydrogen atoms.
• Thus, each H2O molecule can hydrogen-bond to as
many as four partners.
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Figure 2.8
(−)
(+)
(−) (+)
(−)
(+) (−)
(+)
Hydrogen
bond
Polar covalent
bonds
2.9 Chemical reactions make and break chemical bonds
• Remember that the structure of atoms and
molecules determines the way they behave.
• Atoms combine to form molecules.
• Hydrogen and oxygen can react to form water:
2 H2 + O2 2 H2O
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2.9 Chemical reactions make and break chemical bonds
• The formation of water from hydrogen and oxygen
is an example of a chemical reaction.
• The reactants (H2 and O2) are converted to H2O,
the product.
• Chemical reactions do not create or destroy
matter.
• Chemical reactions only rearrange matter.
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2.9 Chemical reactions make and break chemical bonds
• Photosynthesis is a chemical reaction that is
essential to life on Earth.
• Carbon dioxide (from the air) reacts with water.
• Sunlight powers the conversion of these reactants
to produce the products glucose and oxygen.
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2.10 Hydrogen bonds make liquid water cohesive
• The tendency of molecules of the same kind to
stick together is cohesion.
• Cohesion is much stronger for water than for other
liquids.
• Most plants depend upon cohesion to help
transport water and nutrients from their roots to
their leaves.
• The tendency of two kinds of molecules to stick
together is adhesion.
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2.10 Hydrogen bonds make liquid water cohesive
• Cohesion is related to surface tension—a
measure of how difficult it is to break the surface of
a liquid.
• Hydrogen bonds give water high surface tension,
making it behave as if it were coated with an
invisible film.
• Water striders stand on water without breaking the
water surface.
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2.11 Water’s hydrogen bonds moderate temperature
• Thermal energy is the energy associated with the
random movement of atoms and molecules.
• Thermal energy in transfer from a warmer to a
cooler body of matter is defined as heat.
• Temperature measures the intensity of heat—that
is, the average speed of molecules in a body of
matter.
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2.11 Water’s hydrogen bonds moderate temperature
• Heat must be absorbed to break hydrogen bonds.
• Heat is released when hydrogen bonds form.
• To raise the temperature of water, hydrogen bonds
between water molecules must be broken before
the molecules can move faster. Thus,
• when warming up, water absorbs a large amount of
heat and
• when water cools, water molecules slow down,
more hydrogen bonds form, and a considerable
amount of heat is released.
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2.11 Water’s hydrogen bonds moderate temperature
• Earth’s giant water supply moderates
temperatures, helping to keep temperatures within
limits that permit life.
• Water’s resistance to temperature change also
stabilizes ocean temperatures, creating a favorable
environment for marine life.
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2.11 Water’s hydrogen bonds moderate temperature
• When a substance evaporates, the surface of the
liquid that remains behind cools down; this is the
process of evaporative cooling.
• This cooling occurs because the molecules with
the greatest energy leave the surface.
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2.12 Ice floats because it is less dense than liquid water
• Water can exist as a gas, liquid, or solid.
• Water is less dense as a solid than a liquid
because of hydrogen bonding.
• When water freezes, each molecule forms a stable
hydrogen bond with its neighbors.
• As ice crystals form, the molecules are less densely
packed than in liquid water.
• Because ice is less dense than water, it floats.
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Figure 2.12-0
Hydrogen
bond
Ice
Hydrogen bonds are stable.
Liquid water
Hydrogen bonds constantly break and re-form.
2.13 Water is the solvent of life
• A solution is a liquid consisting of a uniform
mixture of two or more substances.
• The dissolving agent is the solvent.
• The substance that is dissolved is the solute.
• An aqueous solution is one in which water is the
solvent.
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2.13 Water is the solvent of life
• Water’s versatility as a solvent results from the
polarity of its molecules.
• Polar or charged solutes dissolve when water
molecules surround them, forming aqueous
solutions.
• Table salt is an example of a solute that will go into
solution in water.
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Figure 2.13
Salt crystal
Na+ Cl−
− +
Na+ Cl−
−
+
+ +
+
+
+
+
−
−
−
−
−
− −
Positive hydrogen ends of
water molecules attracted
to negative chloride ion
Negative oxygen ends of
water molecules attracted
to positive sodium ion
2.14 The chemistry of life is sensitive to acidic and basic conditions
• In liquid water, a small percentage of water
molecules break apart into ions.
• Some are hydrogen ions (H+).
• Some are hydroxide ions (OH–).
• Both types are very reactive.
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2.14 The chemistry of life is sensitive to acidic and basic conditions
• A substance that donates hydrogen ions to
solutions is called an acid.
• A base is a substance that reduces the hydrogen
ion concentration of a solution.
• The pH scale describes how acidic or basic a
solution is.
• The pH scale ranges from 0 to 14, with 0 the most
acidic and 14 the most basic.
• Each pH unit represents a 10-fold change in the
concentration of H+ in a solution.
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2.14 The chemistry of life is sensitive to acidic and basic conditions
• A buffer is a substance that minimizes changes in
pH. Buffers
• accept H+ when it is in excess and
• donate H+ when it is depleted.
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© 2015 Pearson Education, Inc.
Figure 2.14-0
Acidic
solution
H+
OH−
H+ H+
H+
H+
OH− OH−
OH−
OH−
OH−
OH− H+
H+ H+
H+
H+ H+
H+
H+
OH−
H+
H+
OH− OH− OH−
OH−
OH− OH−
Neutral
solution
Basic
solution
NEUTRAL
[H+] = [OH−]
Inc
rea
sin
gly
AC
IDIC
(Hig
he
r H
+ c
on
ce
ntr
ati
on
)
Inc
rea
sin
gly
BA
SIC
(Hig
he
r O
H− c
on
ce
ntr
ati
on
)
pH scale
Battery acid
Lemon juice,
gastric juice
Vinegar, cola
Tomato juice
Rainwater
Human urine
Saliva
Pure water
Human blood, tears Seawater
Milk of magnesia
Household ammonia
Household bleach
Oven cleaner
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
© 2015 Pearson Education, Inc.
Figure 2.14-1
NEUTRAL
[H+] = [OH−]
Incre
asin
gly
AC
IDIC
(Hig
her
H+ c
on
cen
trati
on
)
pH scale
Battery acid
Lemon juice,
gastric juice
Vinegar, cola
Tomato juice
Rainwater
Human urine
Saliva
Pure water
0
1
2
3
4
5
6
7
© 2015 Pearson Education, Inc.
Figure 2.14-2
NEUTRAL
[H+] = [OH−]
Inc
reasin
gly
BA
SIC
(Hig
her
OH
− c
on
cen
trati
on
)
Pure water
Human blood, tears
Seawater
Milk of magnesia
Household ammonia
Household bleach
Oven cleaner
7
8
9
10
11
12
13
14
pH scale
© 2015 Pearson Education, Inc.
Figure 2.14-3
Acidic
solution
H+
OH−
H+ H+ H+
H+
OH− OH−
OH−
OH−
OH−
OH− H+
H+ H+
H+
H+
H+
H+
H+ OH−
H+
H+
OH− OH− OH−
OH−
OH− OH−
Neutral
solution
Basic
solution
2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO2 on coral reef ecosystems
• Carbon dioxide is
• the main product of fossil fuel combustion,
• increasing in the atmosphere, and
• linked to global climate change.
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2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO2 on coral reef ecosystems
• About 25% of this human-generated CO2 is
absorbed by the vast oceans.
• CO2 dissolved in seawater lowers the pH of the
ocean in a process known as ocean acidification.
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2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO2 on coral reef ecosystems
• As seawater acidifies, the extra hydrogen ions (H+)
combine with carbonate ions (CO32–) to form
bicarbonate ions (HCO3–).
• This reaction reduces the carbonate ion
concentration available to corals and other shell-
building animals.
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2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO2 on coral reef ecosystems
• In a controlled experiment, scientists looked at the
effect of decreasing carbonate ion concentration
on the rate of calcium deposition by reef
organisms.
• The lower the concentration of carbonate ions, the
lower the rate of calcification, and thus the slower
the growth of coral animals.
• The results from experimental and observational
field studies of sites where pH naturally varies have
dire implications for the health of coral reefs and the
diversity of organisms they support.
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© 2015 Pearson Education, Inc.
Figure 2.15a
220 240 260 280
20
10
0
[CO32−] (μmol/kg of seawater)
Ca
lcif
ica
tio
n r
ate
(mm
ol C
aC
O3/m
2 ×
da
y)
Source: Adaption of figure 5 from “Effect of Calcium Carbonate Saturation State
on the Calcification Rate of an Experimental Coral Reef” by C. Langdon, et al., from
Global Biogeochemical Cycles, June 2000, Volume 14(2). Copyright © 2000 by
American Geophysical Union. Reprinted with permission of Wiley Inc.
2.16 EVOLUTION CONNECTION: The search for extraterrestrial life centers on the search for water
• The emergent properties of water support life on
Earth.
• When astrobiologists search for signs of
extraterrestrial life on distant planets, they look for
evidence of water.
• The National Aeronautics and Space
Administration (NASA) has found evidence that
water was once abundant on Mars.
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You should now be able to
1. Describe the importance of chemical elements to living
organisms.
2. Explain the formation of compounds.
3. Describe the structure of an atom.
4. Distinguish between ionic, hydrogen, and covalent
bonds.
5. Define a chemical reaction and explain how it changes
the composition of matter.
6. List and define the life-supporting properties of water.
7. Explain the pH scale and the formation of acid and base
solutions. © 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 2.UN01
Protons (+ charge)
determine element −
+ −
+ Electrons (− charge)
form negative cloud
and determine
chemical behavior
Neutrons (no charge)
determine isotope Atom
Nucleus
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Figure 2.UN02
Liquid water:
Hydrogen bonds
constantly break
and re-form
Ice: Stable
hydrogen bonds
hold molecules
apart
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Figure 2.UN03-0
Atoms
atomic number
of each element
(a)
Chemical
Bonds
ions
nonpolar
covalent bonds
water
(b) (c)
(d)
(e)
(f) (g)
(h)
Na
have positively
charged have neutral have negatively
charged
number present
equals
number may
differ in
number in outer
shell determines
formation of
H
H H
−
−
Cl
H H
H
O
O
(−)
(−)
(+) (+)
(+)
electron transfer
between atoms
creates
electron sharing
between atoms
creates
attraction between
ions creates
unequal
sharing creates
equal
sharing creates
example is can lead to
has important
qualities due
to polarity and
© 2015 Pearson Education, Inc.
Figure 2.UN03-1
Atoms
atomic number
of each element
(a) (b) (c)
(d)
have positively
charged have neutral have negatively
charged
number present
equals
number may
differ in
number in outer
shell determines
formation of
H
−
Chemical
Bonds
© 2015 Pearson Education, Inc.
Figure 2.UN03-2
Chemical
Bonds
ions
nonpolar
covalent bonds
water
(e)
(f) (g)
(h)
Na H H
−
Cl
H H
H
O
O
(−)
(−)
(+) (+)
(+)
electron transfer
between atoms
creates
electron sharing
between atoms
creates
attraction between
ions creates unequal
sharing creates
equal
sharing creates
example is can lead to
has important
qualities due
to polarity and