Chp 2: The Chemistry of Life
Key Concepts: -­‐ Atoms Make Up All Ma5er Elements, Isotopes, Atomic Number, Energy Shells Covalent, Ionic, Hydrogen, Chemical ReacGons -­‐ Chemical Bonds Link Atoms -­‐ Water’s ProperGes Cohesion, Dissolving, Expands when Solid, -­‐ Acids and Bases -­‐ Organic Molecules Carbs, Lipids, Proteins, Nucleic Acids What defense might this Ra/lebox moth have against it’s spider predator? 2!
Atoms Make Up All Ma5er Matter consists of one
or more elements. An element is a
substance that cannot
be broken down by
chemical means into
other substances.
Life requires 25
essential elements
(CHON are most
abundant)
3!
What are the 4 most abundant elements in your body?
4!
What important elements are
displayed on nutrition labels?
5!
An atom is composed of
three smaller particles:
protons, neutrons, and
electrons. What element
is this?
6!
An element’s atomic number indicates how many protons are in each atom of that element. Figure 2.1
7!
An atom’s mass number is the total number of protons and neutrons in its nucleus. 8!
Isotopes Differ in the # of Neutrons
 The nucleus of an element has a fixed number of protons, but the number of neutrons may vary.  Ex: For example, Hydrogen exists as 9!
Cobalt is interesGng  Cobalt 60 is used in radiaGon therapy.  Decay: 5 years  Cobalt: Atomic number 27. Mass 59. Protons: 27 Neutrons 32 How many electrons would this atom contain? How many protons, neutrons and electrons in Co 60? 10!
Question #1
The atomic mass of nitrogen is very near 14,
indicating that most nitrogen atoms have a
mass number of 14. How many neutrons
does the average nitrogen atom have?
A. 0
B. 7
C. 8
D. 14
E. Not enough information to determine. 11!
2.1 Mastering Concepts
If a carbon atom had a mass
number of 14, and we knew it
had 6 protons, how many
neutrons would this isotope
have? © 1996 PhotoDisc, Inc./Getty Images/RF
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
12!
How is a covalent bond represented in a structural formula? How about in the electron distribuBon diagrams? 13!
Chemical Bonds Link Atoms
Atoms are organized
into molecules O
H
H
H
O
H
Each of these water
molecules is a made
of two hydrogen
atoms and one oxygen
atom 14!
Electrons determine bonding. The number and distribuGon of electrons around an atom determines whether atoms react with one another. Figure 2.3
Section 2.2
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15!
Valence Shells are Unfilled
Electrons exist in energy shells of various distances from the atom’s nucleus. The shell farthest from the nucleus is important for bonding Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
16!
Within each energy shell, electrons are arranged in pairs. Unpaired electrons form bonds with other atoms. Atoms are most stable when their outer shells have no vacancies. Bonding with other atoms fills the vacancies. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
17!
For example, C has four vacancies in its outer shell. H has one vacancy. When atoms share electrons, as in this methane molecule, covalent bonds are formed. Figure 2.4
Section 2.2
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18!
Unequal Bonding: Polar Covalent Bonds
In methane, all atoms equally
share electrons.
In water, the O atom pulls the
electrons more strongly than the
H atoms.
Fig 2.4 Methane and Water
molecules
19!
Polar Covalent Bonds Electronegativity is a measure
of an atom’s ability to attract
electrons.
Figure 2.6
Section 2.2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
20!
The C and H atoms that make up methane have similar
electronegativities. Neither pulls electrons much more
strongly than the other.
Methane is therefore held together by
nonpolar covalent bonds.
Figure 2.6
21!
O & H have very different electronegativities!
Oxygen attracts electrons more strongly than
hydrogen.
Figure 2.6
22!
Water Has Polar Covalent Bonds
Oxygen atom has a partial negative charge. The
hydrogen atoms have a partial positive charge.
These charges lead to Hydrogen bonds
23!
H Bonds are Weak, But Important  The
charged regions of
one H2O molecule are
attracted to an oppositely
charged region on a
nearby H2O molecule
 Hydrogen bond
These (weak) attractions
are called hydrogen bonds
Hydrogen bonds between water
molecules
What properties of water
are due to H bonds?
24!
Hydrogen Bonds occur Between Molecules (or parts of molecules) - They are drawn as dotted or dashed lines
Figure 2.8
25!
Sometimes one atom
completely pulls an
electron from the other!
Creates ions!
Figure 2.7
26!
Ionic Bonds The transfer of electrons resulting in oppositely charged
ions that attract each other
Figure 2.7
27!
How many different bonds can you see in this chlorophyll molecule? How many different elements? Chlorophyll
molecule
Plant cells
28!
2.2 Mastering Concepts How does a polar covalent bond differ from a hydrogen bond? 29!
Properties of Water
Cohesion is the tendency of water
molecules to stick to one another.
This can create surface tension.
Water is an excellent solvent
Water dissolves hydrophilic (“waterloving”) solutes.
Section 2.3
30!
What is going on here? Figure 2.12
Section 2.3
31!
Water Expands when it Freezes
- Ice is less dense than
liquid water
- Aquatic life survives
the winter under the
ice cap
Figure 2.13
32!
Clicker QuesGon #3 Which property contributes to the high surface tension of water? A. hydrogen bonding B. polar covalent bonds C. cohesion D. polar covalent bonds and cohesion E. All of the choices are correct. 33!
Chemical Reactions Rearrange Atoms
 In
a chemical reaction,
– atoms
in reactant molecules are rearranged to form
new/different molecules
– Chemical
__________ are broken and new ones form
2 H2
O2
2 H2O
Reactants Products Breaking and making of bonds in a chemical reaction
34!
 Living
cells carry out thousands of chemical
reactions that rearrange atoms
What foods contain get beta-­‐carotene? Beta-carotene
Vitamin A
(2 molecules)
Chemical reacGon converGng beta-­‐carotene to vitamin A 35!
Life is Sensitive to Acids and Bases
 In
water, a few molecules break apart into ions
– Some become hydrogen ions (H+)
– Some become hydroxide ions (OH–)
 An
acidic solution has more H+ than OH–
pH scale is used to measure H+ concentration in a
solution
 The
– pH ranges from 0 (very acidic) to 14 (very __________ )
– A solution with a pH = ______ is neutral
36!
– Every
unit is a
ten-fold jump
What 2 other common
scales are logarithmic?
H+
H+
OH–
H+
OH– H+
H+ H+
H+ H+
Acidic solution
OH–
OH–
H+
H+
–
OH– OH
+
+
H
H+ H
How is your body
sensitive to pH?
OH–
low pH is high
________ !
1
2
Lemon juice, gastric
3
Grapefruit juice, soft drink
4
Tomato juice
juice
5
6 Human urine
OH–
Neutral solution
 Remember:
Increasingly ACIDIC
(Higher concentration of H+)
 The
OH–
OH– H+ OH–
OH– OH– –
OH
H+
Basic solution
NEUTRAL
[H+]=[OH–]
7
Pure water
Human blood
8
Increasingly BASIC
(Lower concentration of H+)
pH scale is
logarithmic
pH scale
0
Seawater
9
10
Milk of magnesia
11
Household ammonia
12
Household bleach
13
Oven cleaner
14
Figure 2.15 The pH scale
37!