Transcript Biol 178 Lecture 2

Bio 178 Lecture 2
The Nature of Molecules
Reading
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Chapter 2
Quiz Material
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Questions on P 34
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Chapter 2 Quiz on Text Website
(www.mhhe.com/raven7)
Outline
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Atoms
Chemical Bonds
Water
Acids, Bases, and Buffers
The Chemistry of Life - Definitions
• Matter
Any substance that has mass and occupies space.
• Mass
Amount of a substance.
• Weight
How strongly mass is pulled by gravity.
Example of Mass Vs. Weight:
Astronauts in space and on earth.
The Structure of Atoms
What are Atoms?
The smallest particle into which a substance can be divided
and still retain its chemical properties.
Atomic Number
The number of PROTONS an atom possesses.
Eg. Oxygen
Atomic number = 8
Atomic Mass
Sum of the masses of protons and neutrons.
Masses:
Protons = 1.009 Da, Neutrons = 1.007 Da, Electrons = 1/1840 Da
Eg. O2
Atomic Mass = 16 Da
Elements and Isotopes
Element
Atoms with the same atomic number have the same
chemical properties.
Isotopes
Atoms of an element that have the same number of
protons but a different number of neutrons.
12
6C
13
6C
14
6C
Radioactive Isotopes
Unstable isotopes - the nucleus of the isotope has a
tendency to break into elements with different atomic
numbers.
Eg.
14C
decays to 14N (1 neutron in 146C1 proton147N)
Half-Life
Time taken for 1/2 the atoms in a sample to decay.
Example: The half-life of 146C is 5,600 yrs. How old is a
fossil that contains 1/8 the proportion of 146C in the
atmosphere?
Electrons
Determine the chemical behavior of atoms.
Neutral Atoms
Have no net charge.
Ions
Atoms in which:
# electrons  # protons  Electrical charge
Cation
Ion with > # protons than electrons.
Anion
Ion with > # electrons than protons.
Electron Orbitals
Orbital
The area around an atom’s nucleus where an electron is
most likely to be found.
Potential Energy of Electrons (Energy of Position)
Negative charge of electrons  Attraction to Nucleus
 Keeping electrons in orbit  work
Which requires more energy of position?
Orbitals close to or far away from the
nucleus?
Eg. Photosynthesis
Electron Energy Levels
Electron Orbitals
Electron Energy Levels
Redox
Oxidation-Reduction occurs when an electron is
transferred from one atom to another (OILRIG).
The Periodic Table
Chemical Bonds
Molecule
Two or more atoms held together by chemical bonds.
Compound
A molecule that contains atoms of more than one element.
Types of Chemical Bond
• Ionic
• Covalent
• Hydrogen
Ionic Bonds
*Not important in most biological molecules.
When do they occur?
When 2 atoms are so unequal in their attraction for
electrons that an electron is transferred from one atom to
the other and ions are formed.
Ionic Bonds
CRYSTALS are formed - each ion is attracted to ions of
opposite charge (NOT between particular ions, so
DISCRETE NaCl molecules do not form).
Properties
• Strong (not as
strong as covalent)
• Non-directional
Covalent Bonds
*Important in biological molecules.
When do they occur?
When electrons are SHARED between 2 atoms so that
their outer energy level is completed.
Properties
• Strong (important for
bonding within
molecules)
• Directional (bonds
between 2 specific
atoms, resulting in
specific SHAPE)
Strength of Covalent Bonds
Dependent on the number of shared electrons.
Double Bonds
2 pairs of shared electrons.
Eg. O=O
Triple Bonds
Strongest covalent bonds.
Polar Covalent Bonds
• Occur when the nucleus of one atom (involved in a
covalent bond) is more electronegative than the other
atom(s).
• This results in partial charges () on the atoms.
Example
Water
Hydrogen bonds
*Important in biological molecules
When do they occur?
When a hydrogen atom is covalently bonded to an
electronegative atom it will have a partial positive charge
and will  also be attracted to another electronegative
atom.