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Chapter 2: The Chemical Context of Life
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Word roots to know for this
chapter:
an- = not
co- = together
-valent = strength
electro- = electricity
iso- = equal
neutr- = neither
pro- = before
Covalentstrength togethera bond
formed by sharing the electrons between
the two atoms.
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Matteranything that has
mass and takes up space.
Elementthe simplest form of
matter. A substance that cannot
be broken down into anothersubstance by chemical means.
Compounda substance
consisting of two or more different
elements combined in a fixed ratio.
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About 25 of the 92 naturally
occurring elements are
essential to life.
Of these, carbon (C),
oxygen (O), hydrogen (H),
and nitrogen (N) make up
about 96% of living matter.
Phosphorous (P), sulfur
(S), calcium (Ca),
potassium (K), and a fewother trace elements make
up the remaining 4%.
Trace elementsrequired only in minute quantities: Iron (Fe), Iodine (I)
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Atomthe smallest unit of
matter that still retains the
properties of the element
electron
neutron
proton
Subatomic particles
building block of an
atom.
Protonspositively charged
Neutrons - neutral
Electronsnegatively charged
Protons and neutrons make up the nucleus of the atom. The nucleus is
therefore positively charged. Electrons remain outside the nucleus in a
cloud. The cloud remains near the nucleus because of the attractions
of opposite charge.
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C
62
4
Carbon
12.011
Atomic numberthe
number of protonsunique
to each element
Electron configuration
placement of electrons
in energy levels
Atomic symbol
used to represent
both the element andone atom of that
element
Atomic massthe total mass of
the atomsum of the mass of the
protons and neutrons
12
Mass number
number of protons
plus the number of
neutrons
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Each proton and each neutron have a mass of about 1 dalton.
A dalton is equivalent to 1 amu or 1.7X10-24 grams.
Because electrons are so small, their mass is negligible and notused in the atomic mass.
You can determine the number
of neutrons an atom has bysubtracting the atomic number
from the mass number.
Carbons atomic number is 6 and
mass number is 12.
Therefore, it has 6 neutrons.
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Isotopeforms of an element that contain the same number
of protons but different numbers of neutrons
Carbon has 3 isotopes.
14
6 C13
6 C12
6 C
Carbon-12 is the most
commonly found isotope
of carbon.
Both carbon-12 and carbon-13
are stable isotopes
Carbon-14 is a radioactive
isotopethe nucleusdecays spontaneously ,
giving off particles and
energy.
Atomic # is the same.
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Uses for radioactive isotopes
Radioactive isotopes can be
used as tracers to follow atoms
through metabolism.
Here radioactive hydrogen-3
is used to label and trace the
production of DNA molecules.
Radioactive isotopes function the same chemically in these molecules.
Tracers
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Positron-Emission Tomography
(PET Scan)
When radioactive tracers
are used in conjunction
with detection and
imaging systems, theycan be used to detect hot
spots of chemical
activity, here in cancer
cells.
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Energy Levels and Electrons
Energy(E) - the capacity tocause changeperforming
work.
Potential Energy (PE)
the energy matterposses because of
location or structure.
Kinetic Energy
(KE)the energy of
motion or action.
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The different states of PE
that electrons have are
called energy levels.
The electrons energy level
correlates with the average
distance from the nucleusand is represented by
electron shells.
An electron can change the shell it occupies by
either gaining or losing a specific amount of PE.
Lost energy is usually released to the environment
as light and heat.
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These atoms are arranged in periods according to the
number of electron shells they possess.
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Electrons tend to exist in the lowest available state of potential
energy.
Things to Remember
The chemical behavior of an atom depends mostly on the
electrons found in the outermost shell.
The outermost electrons are called valence electrons and theoutermost shell is called the valence shell.
Atoms with the same number of valence electrons exhibit
similar chemical properties.
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Atoms with their outer most shell filled (total of 8
valence electrons) are inert or non-reactive.
These elements, plus
Xenon (Xe), are
called the noble or
inert gases.
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Electron Orbitals
The 3-dimensional space where an electron can be found 90% of the
time is called an orbital.
There are 4 types of orbitals, with different theoretical shapess, p,
d, f.
Each orbital can hold only 2 electrons.
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Dont Forget
The first electron shell has only one orbitalan s-orbitalthat can
hold only 2 electrons.
The next electron shell has 4 orbitals, 1 s and 3 p orbitalsthat
can hold a total of 8 electrons.
Orbitals are filled with electrons singly; only when all of theorbitals in a shell have one electron will they begin to pair.
The reactivity of atoms arises from the presence of unpaired
electrons in the orbitals of the valence shells.
When atoms react and form bonds, they are completing their
valence shells, which means they contain 8 electrons.
The unpaired electrons are the ones involved in forming bonds.
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The name is BondCovalent Bond
Covalent bonds result from the
sharing or electrons between two
atoms.
Two or more elements held
together by covalent bonds are
called molecules.
One shared pair between two
atoms is a single bond, two
shared pairs are a double bond,
and three shared pairs are atriple bond.
H-H is the structural formula
and H2 is the molecular formula.
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Not all bonds are created equal!!!
Electronegativitythe measure of the affinity of an atom for
electrons. The more electronegative an atom, the higher the electron
affinity.
A bond where the electrons
are shared equally is a non-polar covalent bondH2 or
H-H.
A bond where the electrons are
not shared equally is a polar
covalent bondH2O.
O i l i h h d i k h
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Oxygen is more electronegative than hydrogen so it keeps the
valence electrons of the entire molecule closer to it.
This gives the molecule an uneven charge distribution and makes it
a polar molecule.
The Greek
symbol for
delta ( )represents
the unequal
charges.
This water molecule has polar bonds and is a polar molecule.
Are all molecules that have polar bonds polar?
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All of the bonds in this
methane (CH4) molecule are
also polar because carbon is
more electronegative than
hydrogen.
However, because the
uneven charges are evenly
distributed around the center
of the molecule methane is a
non-polar molecule.
No!!!
Another example of a non-polar molecule with polar bonds is
carbon dioxide, structural formula: O=C=O.
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Ionic Bonds
When the electronegativity of one atom far exceeds that
of another atom, the more electronegative atom will stripaway the electron(s) and add them permanently to their
valance shell.
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This results in two ions or charged particles.
Na has lost one
electron, giving
it a +1 charge.
When an ion is
positive it is
called a cation.
Cl has gained one
electron, giving it
a1 charge.
When an ion is
negative it is
called an anion.
Oppositely charge particles attract each other and an ionic bond
is formed.
Compounds formed by ionic bonds are called ionic compounds
or salts.
NaCl is sodium chloride, or table salt.
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Ionic compounds are often
found as crystalline
structures, which areaggregates of cations and
anions arranged in a three-
dimensional lattice.
These structures areextremely strong when in
their solid state but when
mixed with a solvent, like
water, the ionic attractionbetween the atoms greatly
decreases and the salt will
dissolve readily.
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Other Molecular Attractions
Most of the chemical bonds in living things are covalent ones.
The sharing of electrons, either equally or not, gives covalent
molecules the ability to form weak attractions between themselves.
These weak attractions are critical to the formation of many of the
molecules that we will be studying.
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Hydrogen Bondsa crucial attraction in living chemistry.
Hydrogen is less electronegative
than all the other atoms with which
it bonds.
Because of this, it frequently
has a partial positive charge
when in a covalent
molecule.
The partial positive of the
hydrogen attracts and holds
partial negative charges
from other polar molecules
thus creating a hydrogen
bond.
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Van der Waals Interactions
Even a molecule that is not polar can have regions of variable
charge.
These positive and negative zones move throughout the molecule
but they exist with enough frequency and duration to create
attractions between molecules.
Van der Waals forces are very weak and only happen when atoms
and molecules are very close together.
Attractions like these do not just have to happen between separate
molecules, they can happen within molecules as well. Ex: proteins.
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Molecular Shape and Function
A very important concept
that will become evident in
many of our topics is thatform defines function.
This is especially true in
biological molecules.
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Molecular shapes determines how molecules recognize and
respond to each other.
Only molecules with complimentary shapes can get close enoughto each other to interact.
Proteins of all types work because of their specific conformations.
Enzymes
Hormones
Cell surface receptors
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The making and breaking of
chemical bonds, leading to a
change in the composition of
matter, are called chemicalreactions.
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An arrow indicates the direction that the reaction occurs.
Raw materials are called reactants.
End materials are called products.
Coefficients show the number of molecules or atoms involved.
Matter is conserved during a chemical reaction.
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Some reactions go to completionall reactants are converted to
products.
C3H8 + 5O2 3CO2 + 4H2O
Most reactions are reversiblethe products of the forward reaction
become the reactants of the reverse reaction.3H2 + N2 2NH3
When the forward and reverse reactions are occurring at the samerate the system has reached dynamic equilibrium.
The reactions does not cease to occur, rather the ratio of products to
reactants remains constant.
1) Which four elements make up approximately 96% of living matter?A) b di hl i it
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A) carbon, sodium, chlorine, nitrogenB) carbon, sulfur, phosphorus, hydrogenC) oxygen, hydrogen, calcium, sodiumD) carbon, hydrogen, nitrogen, oxygenE) carbon, oxygen, sulfur, calcium
2) Each element is unique and different from other elements because of itsA) atomic mass.
B) atomic weight.C) atomic number.D) mass weight.E) mass number.
3) Nitrogen (N) is much more electronegative than hydrogen (H). Which of the following statements is correct about theatoms in ammonia (NH3)?A) Each hydrogen atom has a partial positive charge.B) The nitrogen atom has a strong positive charge.C) Each hydrogen atom has a slight negative charge.
D) The nitrogen atom has a partial positive charge.E) There are covalent bonds between the hydrogen atoms.
4) What explains most specifically the attraction of water molecules to one another.A) nonpolar covalent bondB) polar covalent bondC) ionic bondD) hydrogen bondE) hydrophobic interaction
5) Which of the following best describes chemical equilibrium?A) Forward and reverse reactions continue with no effect on the concentrations of the reactants and products. B) Concentrations of the products are higher than the concentrations of the reactants. C) Forward and reverse reactions have stopped so that the concentration of the reactants equals the concentration of theproducts.D) Reactions stop only when all reactants have been converted to products.E) There are equal concentrations of reactants and products and the reactions have stopped.
D
C
A
D
A
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