chapter 2 class lecture_1
TRANSCRIPT
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Biology
Biology the study of life
Is founded on the principles of
chemistry and physics
All living organisms are acollection of atoms and molecules
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Atoms
Smallest functional units of matter
Form all chemical substances and
Cannot be further broken down into othersubstances
Each specific type
of atom is a
chemical element
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Electrons occupy orbitals
Scientists initially visualized an atom asa mini solar system
Electrons travel within regionssurrounding the nucleus (orbitals) in
which the probability is high of findingthat electron
Can be depicted as a cloud
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Atoms with progressively moreelectrons have orbitals within
electron shells that are at greater andgreater distances from the center ofthe nucleus
1st shell - 1 spherical orbital (1s) - holds 2 electrons 2nd shell - 1 spherical orbital (2s) and 3 dumbbell-
shaped orbitals (2p) can hold 4 pairs of electrons
Atomic shells
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Nitrogen example
An atom of nitrogen has seven protonsand seven electrons
2 electrons fill 1st shell
5 electrons in 2nd shell2 fill 2s orbital
1 each in the 3 p orbitals
Outer 2nd shell is not full
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Atomic Number
Number of protons in an atom is itsatomic number
The atomic number is also equal to thenumber of electrons in the atom so that
the net charge is zero
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Periodic table
Organized by atomic number
Rows correspond to number of electron shells
Columns, from left to right, indicate the numbers of
electrons in the outer shell
Similarities of elements within a column occur
because they have the same number of electrons in
their outer shells, and therefore they have similarchemical bonding properties
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Mass Number
The second number associated with each atom is the
mass number. The mass number expresses the sum
of the masses of the particles in the atom.
proton has a mass number of 1, electron 0, therefore the
mass number indicates the number of proton and neutrons in
the nucleus.
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Mass Number
Atomic mass scale indicates an atoms mass relativeto the mass of other atoms
Most common form of carbon has six protons and sixneutrons, is assigned an atomic mass of exactly 12
A hydrogen atom has an atomic mass of 1, indicating that
it has 1/12 the mass of a carbon atom A magnesium atom, with an atomic mass of 24, has twice
the mass of a carbon atom
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Mass or weight?
Weight is derived from the gravitational pullon a given mass
If a man who weighs 154 pounds on earthwere standing on the moon, for example, hewould only weigh about 25 pounds, but hewould weigh 21 trillion pounds if he couldstand on a neutron star. However, his mass isthe same in all locations.
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Units
Dalton
Unit of measurement for atomic mass
One dalton (d) equals 1/12 the mass of acarbon atom
Carbon has an atomic mass of 12 Daltons
Mole1 mole of any element contains the same
number of atoms6.022 x 1023
Avogadros number
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Isotopes
Multiple forms of an element that differ
in the number of neutrons
12C contains 6 protons and 6 neutrons
14C contains 6 protons and 8 neutrons
Atomic masses are averages of the
weights of different isotopes of an
element
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Hydrogen, oxygen, carbon, and
nitrogen Typically make up about 95% of the atoms in living
organisms
Hydrogen and oxygen occur primarily in water
Nitrogen is found in proteins
Carbon is the building block of all living matter
Mineral elements - less than 1%
Trace elements - less than 0.01%
Essential for normal growth and function
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Chemical bonds and molecules
Molecule
2 or more atoms bonded together
Molecular formula
Contains chemical symbols of elementsfound in a molecule
Subscript indicates how many of eachatom are present
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3 types of bonds
1. Covalent
Polar covalent
2. Hydrogen
3. Ionic
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Covalent bonds
Atoms share a pair of electrons
Occurs between atoms whose outer electron shellsare not full
Covalent bonds are often the strongest of allchemical bonds, because the shared electronsbehave as if they belong to each atom
Can share
1 pair of electrons single bondH-F 2 pairs of electrons double bond O=O
3 pairs of electrons triple bond
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Octet rule
Atoms are stable when their outer
shell is full
For many atoms, the outer shell
fills with 8 electrons
One exception is hydrogen, whichfills its outer shell with 2 electrons
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Polar covalent bonds
When two atoms with different electronegativitiesform a covalent bond, the shared electrons aremore likely to be in the outer shell of the atom of
higher electronegativity rather than the atom oflower electronegativity
Polar covalent bonds occur because the
distribution of electrons around the atoms createsa polarity, or difference in electric charge, acrossthe molecule
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Water
Classic example of polar covalent
bonds
Electrons tend to be in the moreelectronegative oxygen atom rather
than either of the less electronegative
hydrogen atoms
Molecule has partial negative charge
region and a partial positive charge
region
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Hydrogen bonds
Weak polar covalent bonds
Represented as dashed or dotted lines
Collectively, can form strong bond overall Holds DNA strands together
Individually, weak bonds can form and break
easily Substrate and enzyme bonding
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Ionic bonds
An ion is an atom or molecule that has gainedor lost one or more electrons
It now has a net electric charge
Cations- net positive charge
Anions- net negative charge
Ionic bond occurs when a cation binds to an
anion
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Chemical Reactions
Occurs when one or more substances arechanged into other substances Reactants products
Share many properties All require a source of energy
Reactions in living organisms often require acatalyst (enzymes)
Tend to proceed in a particular direction but willeventually reach equilibrium
Occur in liquid environment - water
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Properties of water
A solution is made up of the
Solvent- liquid
Solutes- substances dissolved in solvent
Aqueous solution- water is the solvent
Ions and molecules that contain polar covalent
bonds will dissolve in water
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Hydrophillic- water-loving Readily dissolve in water Ions and molecules that contain polar covalent bonds
Hydrophobic- water-fearing
Do not readily dissolve in water Nonpolar molecules like hydrocarbon
Amphipathic molecule Have both polar or ionized regions at one or more sites
and nonpolar regions at other sites
May form micelles in water Polar (hydrophilic) regions at the surface of the micelle and
nonpolar (hydrophobic) ends are oriented toward the interior ofthe micelle
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Solutions
Concentration
Amount of a solute dissolved in a unit volume of solution
1 gram of NaCl was dissolved in 1 liter of water = 1 g/L
Molarity
Number of moles of a solute dissolved in 1 L of water
1 mole of a substance is the amount of the substance in
grams equal to its atomic or molecular mass
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H2O in 3 states of matter
Solid (ice), liquid (water), and gas (water
vapor)
Changes in state, such as changes betweenthe solid, liquid, and gas states of H2O, involve
an input or release of energy
Water is extremely stable as a liquid
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Colligative properties of water
Depend strictly on the concentration of dissolvedsolute particles and not on the specific type ofparticle
Addition of solutes to water lowers its freezing pointbelow 0C and raises its boiling point above 100C
Some animals produce antifreeze molecules thatdissolve in their body fluids, thereby lowering the
freezing point of the fluids and preventing theirblood and cells from freezing in the extreme cold
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Not just a solvent
Water has many important functions in livingorganisms
Participates in chemical reactions
Hydrolysis or dehydration
Provides force or support
Remove toxic waste components
Evaporative cooling
Cohesion and adhesion
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Acids and Bases
Pure water has the ability to ionize to a very
small extent into hydrogen ions (H+) and
hydroxide ions (OH-)
In pure water
[H+][OH-] = [10-7 M][10-7 M] 10-14 M
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Acids are molecules that release hydrogen
ions in solution
A strong acid releases more H+ than a weak acid
Bases lower the H+ concentration
Some release OH-
Others bind H+
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pH
pH = log10 [H+]
Acidic solutions are pH 6 or below
pH 7 is neutral
Alkaline solutions are pH 8 or above
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The pH of a solution can affect
The shapes and functions of molecules
The rates of many chemical reactions
The ability of two molecules to bind to each other
The ability of ions or molecules to dissolve in
water
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Buffers
Organisms usually tolerate only small changes
in pH
Buffers help to keep a constant pH
An acid-base buffer system can shift to
generate or release H+ to adjust for changes in
pH