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Chapter 2:
The Chemical Context of Life
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AP Biology
Why are we studying chemistry?
Chemistry is the foundation of Biology
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Chemical Context of Life
Matter (space & mass)
Elements & Compounds
The Atom
Atomic Number (# of protons);
Mass Number (protons + neutrons)
Isotopes (different # of neutrons);
radioactive isotopes (nuclear decay)
Energy (ability to do work); energy
levels (electron states of potential
energy)
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AP BiologyProton Neutron Electron
Hydrogen1 proton
1 electron
Oxygen8 protons
8 neutrons
8 electrons
+ 0 –
Everything is made of matter
Matter is made of atoms
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AP Biology
The World of Elements
C
Different kinds of atoms = elements
H
ON
P SNa
K
Mg
Ca
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AP Biology
Life requires ~25 chemical elements
About 25 elements are essential for life
Four elements make up 96% of living
matter:
• carbon (C) • hydrogen (H)
• oxygen (O) • nitrogen (N)
Four elements make up most of
remaining 4%:
• phosphorus (P) • calcium (Ca)
• sulfur (S) • potassium (K)
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AP Biology
Bonding properties
Effect of electrons
electrons determine
chemical behavior of
atom
depends on number
of electrons in atom’s
outermost shell
valence shell
How does this
atom behave?
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AP Biology
Bonding properties
Effect of electrons
chemical behavior of an atom depends on
number of electrons in its valence shell
What’s themagic number?
How does this atom behave? How does this atom behave?
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AP Biology
Elements & their valence shells
Elements in the same row have
the same number of shells
Moving from left to right, each element has a
sequential addition of electrons (& protons)
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AP Biology
Elements & their valence shells
Elements in the same column
have the same valence &
similar chemical properties
Remembersome food chains
are built on reducing O to H2O
& some on reducing S to H2S
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AP Biology
–
–
–
Chemical reactivity
Atoms tend to
complete a partially filled valence shell
or
empty a partially filled valence shell
This tendency drives chemical reactions…
and creates bonds
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Covalent Bonding
Sharing pair of valence
electrons
Number of electrons
required to complete an
atom’s valence shell
determines how many
bonds will form
Ex: Hydrogen & oxygen
bonding in water;
methane
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Nonpolar covalent bond
Pair of electrons shared equally by 2 atoms
example: hydrocarbons = CxHx
methane (CH4 )
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are needed to see this picture.
balanced, stable,
good building block
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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AP Biology
H
H
Oxygen
Polar covalent bonds
Pair of electrons shared
unequally by 2 atoms
example: water = H2O
oxygen has stronger
“attraction” for the
electrons than hydrogen
oxygen has higher
electronegativity
water is a polar molecule
+ vs – poles
leads to many interesting
properties of water…
+
+
––
––
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AP Biology
Electronegativity
F O Cl N Br I C H
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Ionic bonding
High electronegativity
difference strips
valence electrons away
from another atom
Electron transfer
creates ions (charged
atoms)
Cation (positive ion);
anion (negative ion)
Ex: Salts (sodium
chloride)
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Hydrogen bonds
Hydrogen atom covalently
bonded to one
electronegative atom is
also attracted to another
electronegative atom
(oxygen or nitrogen)
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Hydrogen bonding
Polar water creates molecular attractions
attraction between positive H in one H2O
molecule to negative O in another H2O
also can occur wherever
an -OH exists in a larger
molecule
Weak bond
H
O
H
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van der Waals interactions
Weak interactions brought about by
localized change fluctuations
Because electrons are constantly in
motion and at any given instant, ever-
changing “hot spots” of negative or
positive charge may develop
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Strength of Bonds
Covalent
Ionic
Hydrogen
van der Waals
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Weak Bonds
1. Numbers
2. Shape
3. Duration
4. Fluidity