Transcript Chapter 2 Presentation-The Chemical Context of Life

Chapter 2
The Chemical Context of
Life
Matter
• Matter is the amount of “stuff” in an
object.
• Matter is anything that contains
mass and occupies space.
• Mass is the amount of matter in an
object, weight is the force of gravity
acting upon the object.
Elements
• Elements are the fundamental units
of matter. When you get to the
atomic level, you cannot break it
down any further and still maintain
properties of the element. Going
further will give you protons, neutron,
and electrons.
Compounds
• Compounds are mixtures of two or
more different atoms combined in
fixed ratios.
Atoms
• Are the fundamental units of matter.
• They are comprised of protons,
neutrons, and electrons.
Living Matter
• 96% of living matter is made up of C,
H, N, and O.
• The remaining 4% comes from P, S,
Ca, K and a few trace elements.
Trace Elements
• Trace elements are elements that
are required in extremely small
quantities for normal bodily
functioning.
• They are usually cofactors that assist
in enzyme functioning.
Atomic Number
• This is the number of protons within
an atom.
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Mass Number
• The mass number is the number of
protons plus the number of neutrons.
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Isotopes
• Isotopes are atoms of the same
element with different numbers of
neutrons.
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Atomic Mass
• The atomic mass of an atom is the
average mass of a mixture of its
isotopes.
• Even though the mass of the
element is different, the atom
behaves exactly the same in
chemical reactions.
Radioactive Isotopes
• These are isotopes in which the nucleus
decays giving off particles and energy.
• Radioactive carbon is very commonly
used by biologists as is radioactive H.
• These are called tracers and often are
used to date old objects and assist in the
mapping of metabolic processes.
Radioactive Isotopes-Decay
• There are three main types of
radioactive decay of radioactive
isotopes:
a decay
b decay
 Decay
a and b decay are of most
importance.
a Decay
• Alpha decay occurs
when an atom emits a
He atom with no
electrons, He2+.
• These particles are
hazardous if ingested, or
if any substance which
produces them is
ingested (radon).
• Otherwise they are not
very harmful because
your dead skin cells
absorb them and they
cause no harm.
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b-decay
• In b-decay, the
decaying element
ejects a positron (same
mass as an electron,
but with a positive
charge) from the
nucleus--it is called a bparticle.
• These are what are
often used in
patients/organisms as
tracers.
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b-decay and Tracers
• The metabolically active tissue of
interest takes up the tracer.
• When it undergoes decay, a
positron is emitted and shortly after
combines with an electron
producing a photon that can be
detected and formed into an image
by the scanner.
Interactions of Atoms
• Most atoms with the exception of
the Noble gases are reactive
because they contain an
incomplete outer energy shell.
• The interaction of the elements is
what gives rise to a chemical bond.
• There are two main categories of
chemical bonds:
• 1. Ionic
• 2. Covalent
Covalent Bonds
• In a covalent bond, electrons are
shared and the result is a molecule.
• Single bonds, double bonds, and
triple bonds can occur resulting in
molecules.
• H2 is a single bond, H-H
• O2 is a double bond, O=O
• C2H2 is a triple bond, H-C≡C-H
Covalent Bonds
• Covalent bonds
Electronegativity
• Remember that electronegativity is
the number that represents an
atoms “desire” to have electrons.
• The more electronegative an
element, the more likely it is to steal
an electron and form an ionic
compound with an element.
Covalent Bonds
• There are two types:
– Polar covalent--electrons are not
shared equally. The more
electronegative atom hogs the
electrons giving it a slightly negative
charge. H2O for example. H is slightly
positive, O is slightly negative.
– Non-polar covalent--atoms share
electrons equally. CO2, and CH4.
Ionic Bonds
• In ionic bonds, atoms steal electrons from
one another forming compounds that
have very high melting points.
• When an electronegative atom steals an
electron, it has an overall negative
charge and is called an anion.
• The atom that gave up the electron is
now positive and is called a cation.
• Since opposites attract, an ionic
compound is formed and is called a salt.
Ionic Bonds
• Ionic Bonds
Chemical Reactions
• When chemical reactions occur,
reactants combine to form
products.
• 6CO2 + 6H2O --> C6H12O6 + 6O2
• Reactants
Products
Van der Waals Interactions
• Van der Waals interactions are
weak intermolecular forces that
occur between atoms of
compounds and molecules.
• There are three different types:
• 1. Dispersion forces.
• 2. Dipole interactions.
• 3. Hydrogen bonds.
Van der Waals Interactions
• Van der Waals forces are very weak
individually, but in large number
they are very strong.
• The gecko example from the book
and H-bonding that creates surface
tension that allows water striders to
walk on water.
1. Dispersion Forces
• Dispersion forces are the weakest:
these forces generally increase as
the number of electrons increases.
• The halogens are an example.
• Chlorine is a gas, bromine is a liquid,
and iodine is a solid.
2. Dipole Interactions
• Dipole interactions are a little
stronger and these result from the
interactions of polar molecules.
• SiO2 is a very polar molecule, they
are attracted to other SiO2
molecules.
3. Hydrogen Bonds
• Hydrogen bonds are the strongest
dipole interactions. Hydrogen
bonds are seen in H-containing
compounds that are bonded to
very electronegative atoms.
• H2O
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