Chapter 2
The Chemical Context of Life
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Chapter 2 The Chemical Context of Life
1. What is an atom?
–
–
Smallest unit of matter that retains the physical & chemical
properties of its element
Element – a substance that cannot be broken down into other
substances by chemical rxn
2. What is an atom made of?
–
–
–
Proton – (+1) charge, found in nucleus, 1 amu
Neutron – no charge, found in nucleus, 1 amu
Electron – (-1) charge, orbit nucles, mass neglible
Figure 2.4 Simplified models of a helium (He) atom
Cloud of negative
charge (2 electrons)
Electrons
Nucleus
(a)
(b)
Chapter 2 The Chemical Context of Life
1. What is an atom?
2. What is an atom made of?
3. What do these numbers mean?
23Na
12C
11Na
6C
4. What is an isotope? Radioisotope?
–
–
Atoms of an element that have the same atomic # but different
atomic mass
Unstable isotope where the nucleus spontaneously decays
emitting subatomic particles &/or energy as radioactivity.
5. What are radioisotopes used for?
Chapter 2 The Chemical Context of Life
1.
2.
3.
4.
5.
6.
7.
What is an atom?
What is an atom made of?
What do these numbers mean?
What is an isotope? Radioisotope?
What are radioisotopes used for?
How are atoms held together?
What are the different types of bonds?
– Covalent – sharing of electrons
•
•
Polar covalent – UNequal sharing
Nonpolar covalent – equal sharing
– Ionic – complete transfer of electrons
Chapter 2 The Chemical Context of Life
1.
2.
3.
4.
5.
6.
7.
8.
What is an atom?
What is an atom made of?
What do these numbers mean?
What is an isotope? Radioisotope?
What are radioisotopes used for?
How are atoms held together?
What are the different types of bonds?
What is electronegativity?
– Atom’s ability to attract & hold electrons
• Formation of a covalent bond
Hydrogen atoms (2 H)
1
2
3
Figure 2.10
In each hydrogen
atom, the single electron
is held in its orbital by
its attraction to the
proton in the nucleus.
When two hydrogen
atoms approach each
other, the electron of
each atom is also
attracted to the proton
in the other nucleus.
The two electrons
become shared in a
covalent bond,
forming an H2
molecule.
+
+
+
+
+
+
Hydrogen
molecule (H2)
• Single and double covalent bonds
Name
(molecular
formula)
(a) Hydrogen (H2).
Two hydrogen
atoms can form a
single bond.
(b) Oxygen (O2).
Two oxygen atoms
share two pairs of
electrons to form
a double bond.
Figure 2.11 A, B
Electronshell
diagram
Structural
formula
H
H
O
O
Spacefilling
model
• Covalent bonding in compounds
Name
(molecular
formula)
(c) Water (H2O).
Two hydrogen
atoms and one
oxygen atom are
joined by covalent
bonds to produce a
molecule of water.
(d) Methane (CH4).
Four hydrogen
atoms can satisfy
the valence of
one carbon
atom, forming
methane.
Electronshell
diagram
Structural
formula
O
H
H
H
H
C
H
Figure 2.11 C, D
H
Spacefilling
model
• In a polar covalent bond
– The atoms have differing electronegativities
– Share the electrons unequally
Because oxygen (O) is more electronegative than hydrogen (H),
shared electrons are pulled more toward oxygen.
d–
d–
This results in a
partial negative
charge on the
oxygen and a
partial positive
charge on
the hydrogens.
O
Figure 2.12
d+
H
H
H2O
d+
• An ionic bond
– Is an attraction between anions and cations
The lone valence electron of a sodium
atom is transferred to join the 7 valence
electrons of a chlorine atom.
1
2 Each resulting ion has a completed
valence shell. An ionic bond can form
between the oppositely charged ions.
+
Na
Na
Figure 2.13
Sodium atom
(an uncharged
atom)
Cl
Cl
Chlorine atom
(an uncharged
atom)
Na
Na+
Sodium on
(a cation)
–
Cl
Cl–
Chloride ion
(an anion)
Sodium chloride (NaCl)
Hydrogen Bonds
• A hydrogen bond
– Forms when a hydrogen atom covalently
bonded to one electronegative atom is also
attracted to another electronegative atom
d–
d+
H
Water
(H2O)
O
H
d+
d–
Ammonia
(NH3)
N
H
d+
Figure 2.15
H
H
d+
d+
A hydrogen
bond results
from the
attraction
between the
partial positive
charge on the
hydrogen atom
of water and
the partial
negative charge
on the nitrogen
atom of
ammonia.
• Molecular shape
– Determines how biological molecules recognize
and respond to one another with specificity
Carbon
Nitrogen
Hydrogen
Sulfur
Oxygen
Natural
endorphin
Morphine
(a) Structures of endorphin and morphine. The boxed portion of the endorphin molecule (left) binds to
receptor molecules on target cells in the brain. The boxed portion of the morphine molecule is a close match.
Natural
endorphin
Brain cell
Figure 2.17
Morphine
Endorphin
receptors
(b) Binding to endorphin receptors. Endorphin receptors on the surface of a brain cell
recognize and can bind to both endorphin and morphine.
Chapter 2 The Chemical Context of Life
1.
2.
3.
4.
5.
6.
7.
8.
9.
What is an atom?
What is an atom made of?
What do these numbers mean?
What is an isotope? Radioisotope?
What are radioisotopes used for?
How are atoms held together?
What are the different types of bonds?
What is electronegativity?
How are bonds created between atoms?
• Chemical reactions
– Convert reactants to products
+
2 H2
Reactants
+
O2
Reaction
2 H2O
Product
• Chemical equilibrium
– Is reached when the forward and reverse
reaction rates are equal