Ch. 4 Chemical Basis of Life
4.1 Life requires about 25
chemical elements.
I. Elements
A. matter: anything that occupies space and has
mass
B. element: pure substance that cannot be
broken down into other substances by chemical
or physical means Ex: gold, helium, mercury,
etc.
Elements
C. oxygen (O), carbon (C), hydrogen (H), and
nitrogen (N)—make up about 96 percent of the
living matter in your body. Calcium (Ca),
phosphorus (P), potassium (K), sulfur (S), and
a few other elements account for most of the
remaining 4 percent
D. trace element: element critical to health that
makes up less than 0.01 percent of body mass,
Ex: iodine (I), iron (Fe), copper (Cu), Flourine
(F), manganese (Mn), selenium (Se)
Elements in
the body
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II. Compounds
A. compound: substance containing two or more
elements chemically combined in a fixed ratio,
Ex: water (H2O) is a compound that always
contains the same ratio of hydrogen combined
with oxygen
B. Compound's properties may differ greatly from
those of its component elements. The white
crystals of table salt, or sodium chloride (NaCl),
on a pretzel look very different from the silvery
gray sodium metal and yellowish-green chlorine
gas.
4.2 Chemical properties are
based on the structure of
atoms.
I. Atoms
A. atom: smallest particle of an element
B. subatomic particles
1. proton: + charge, atomic weight of 1
2. electron: - charge, atomic weight considered
zero
3. neutron: no charge, atomic weight of 1
4. nucleus: protons and neutrons packed
together at core, + charge keeps the – charged
electrons orbiting, Ex: Helium atom
Helium atom
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Atoms
C. atomic number: number of protons in an
atom's nucleus; is unique for each element
D. atoms often loses or gain electrons, but do
not usually change the number of protons or
neutrons
II. Isotopes
A. isotope: one of several forms of an element,
each containing the same number of protons in
their atoms but a different number of neutrons,
Ex: 12C, 13C, 14C
B. radioactive isotope: isotope in which the
nucleus decays (breaks down) over time, giving
off radiation in the form of matter and energy,
used in medicine and radiometric dating, can
also damage cells
C. Atomic mass or atomic weight is found by
averaging the percentages of each isotope for an
element
Isotopes
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III. Electrons and Reactivity
A. Electrons determine how atoms react with
each other, usually electrons in the highest
energy levels (outer layers) are responsible
for reactions
B. 1st energy level – holds 2 electrons, 2nd – 8,
3rd – 18, 4th – 32, 5th – 32, 6th – 32, 7th – 32
C. atoms want to fill their energy levels by
losing or gaining electrons to make them
more stable
Electrons
4.3 Chemical bonds join
atoms to one another.
I. Ionic Bonds
A. ionic bond: chemical bond that occurs when
an atom transfers an electron to another atom
B. atoms give or take electrons to giving each
an individual charge, the positive ion is called
a cation, the negative an anion, see Ex: NaCL
tablesalt
C. ion: atom that has become electrically
charged as a result of gaining or losing an
electron, Ex: Na+1, Cl-1
Ionic Bonding
II. Covalent Bonds
A. covalent bond – two or more atoms share
electrons
B. valence electrons (outer layer) are
represented in dot diagrams, where to shared
dots are equivalent to a covalent bond, Ex:
H2
C. The number of bonds an atom can form
usually equals the number of additional
electrons needed to fill is highest energy
level. Ex: Hydrogen need 1 electron, so one
covalent bond
Covalent Bonds
D. molecule: two or more atoms held
together by covalent bonds
E. chemical formula tells the specific
amount of each atom in the molecule,
structural formula tells how the atom are
arranged
Covalent Bonding
III. Chemical Reactions
A. atoms in molecules are constantly rearranged
when bonds broken (releasing energy) and
bonds created (absorbing energy)
B. chemical reaction: breaking of old and
formation of new chemical bonds that result in
new substances, Ex: see water reaction
C. Chemical equation shows the staring materials
(reactants) and ending materials (products) in
the appropriate quantities
Modeling Molecules
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Chemical Reaction
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4.4 Life depends on the
unique properties of water.
I. The Structure of Water
A. cells are 70-95% water, and are suspended in
solutions mostly containing water
B. Water’s special properties are because the
electrons in the covalent bonds of the molecule
are not shared equally, oxygen pulls them more
than hydrogen, also the shape of the molecule is
bend, not linear making it polar (slight charge)
C. polar molecule: molecule in which opposite
ends have opposite electric charges
The Structure of Water
D. this polarity makes water molecules
attracted to each other forming a weak bond
between molecules, hydrogen bond
E. hydrogen bond: bond created by the weak
attraction of a slightly positive hydrogen atom
to a slightly negative portion of another
molecule
Water Molecule
II. Water’s Life Supporting
Properties
A. cohesion: tendency of molecules of the same kind
to stick to one another Ex: surface tension, and
adhesion: attraction between unlike molecules, Ex:
capillary action, both in plants
B. Temperature moderation – caused by hydrogen
bonds in water either absorbing or releasing energy
1. thermal energy: total amount of energy
associated with the random movement of atoms
and molecules in a sample of matter
2. temperature: measure of the average energy of
random motion of particles in a substance
Cohesion
& Adhesion
Q u ic k T im e ™ a n d a
T I F F ( Un c o m p r e s s e d ) d e c o m p r e s s o r
a r e n e e d e d t o s e e t h is p ic t u r e .
Q u ic k T im e ™ a n d a
T I F F ( Un c o m p r e s s e d ) d e c o m p r e s s o r
a r e n e e d e d t o s e e t h is p ic t u r e .
Q u ic k T im e ™ a n d a
T I F F ( Un c o m p r e s s e d ) d e c o m p r e s s o r
a r e n e e d e d t o s e e t h is p ic t u r e .
Water’s Life Supporting
Properties
C. low density of ice a result of water’s hydrogen bonds
forming a group of four waters in a square
D. Waters ability to dissolve other substances
1. solution: uniform mixture of two or more substances
2. solvent: substance in a solution that dissolves the
other substance and is present in the greater amount
3. solute: substance in a solution that is dissolved and is
present in a lesser amount
4. aqueous solution: solution in which water is the
solvent
5. water dissolves ionic solids, salts, also nonionic
compounds like sugar
Ice &
Water
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Water as a Solvent
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III. Acids, Bases, and pH
A. in aqueous solutions a small amount of the
water molecules break apart into positively
charged hydrogen ions (H+) and negatively
charged hydroxide ions (OH-)
B. acid: compound that donates H+ ions to an
aqueous solution and measures less than 7 on
the pH scale, Ex: HCl
C. base: compound that removes H+ ions from an
aqueous solution and that measures more than 7
on the pH scale Ex: NaOH
Acids, Bases, and pH
D. pH scale: a range of numbers used to
describe how acidic or basic a solution is;
ranges from 0 (most acidic) to 14 (most
basic)
E. buffer: substance that maintains a fairly
constant pH in a solution by accepting H+
ions when their levels rise and donating H+
ions when their levels fall, resist change in
pH
pH
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IV. An Environment Fit for Life
• A. Earth is unique in its many properties
for supporting life