Transcript Electronic Absorption Spectroscopy of Organic Compounds

Chapter 2
The Atomic Nature of
Matter
Department of Chemistry
and Biochemistry
Seton Hall University
Atomic Theory
• Bulk of atomic theory was
described by Dalton
• Basic key is the definition of a
compound (anybody
remember?)
• We can “see” individual atoms
via STM, and it confirms our
expectations (consider: what if
it didn’t?)
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Dalton’s Theory
• All matter is composed of tiny
particles called atoms
• All atoms of a given element have
identical chemical properties that are
characteristic of that element
• Atoms form chemical compounds by
combining in whole-number ratios
• Atoms can change how they are
combined, but they are neither
created nor destroyed in chemical
reactions (the Law of the
Conservation of Mass)
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Several Related Points
• Matter and energy are
interconverted, the amount of
mass involved is unmeasureable
• Atoms and molecules are
continually in motion
• Diffusion - the passage of a
particle
• Dynamic equilibrium - balanced
motion of particles (consider
evaporation, diffusion)
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Atomic Architecture
• Matter is held together by forces
– gravitational
– electrical
– magnetic
• Atoms consist of:
– nucleus - contains protons and
neutrons
– electrons
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Atomic Architecture
• Nucleus contains essentially all
of the mass of the atom, but
essentially none of the volume
• Electrons have virtually none of
the mass, but occupy basically
all of the volume
• An atom the size of a baseball
stadium will have a nucleus the
size of a pea
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Atomic Diversity
• An element is identified by the charge of
its nucleus (the number of protons)
• Isotopes have the same number of protons,
but different number of neutrons
• X - element symbol
• A - atomic mass (number of protons and
neutrons)
• Z - atomic number (number of protons)
A
Z
X
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Isotopes
• Most elements occur naturally
as two or more isotopes, hence
the fractional atomic mass
• Isotopes of hydrogen
– protium - one proton, one electron
– deuterium - one proton, one
neutron, one electron
– tritium - one proton, two
neutrons, one electron
• Mass spectrometry - see fig 53;
measures mass very precisely
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Charged species
• Charged chemical species are
called ions
– cations - positively charged
– anions - negatively charged
• Ions can be atoms or molecules
• Compounds formed from two or
more ions are called ionic
compounds or salts
• Salts often dissolve in liquids to
give separated ions
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Forms of energy
•
•
•
•
•
•
Kinetic - E=0.5mv2
Potential
Thermal
Chemical
Radiant
Electrical
q1q2
Ek
r
2
kg m
1J  1 2
s
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Conservation of Energy
• Energy is neither created nor
destroyed in a chemical process,
but may be transferred from one
body to another or changed
from one form to another
• End of Chapter 2
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