Atoms: The building
blocks of Matter
The Structure of the Atom
Dalton’s Atomic Theory
Democritus – atomos
Lavoisier
Law of Conservation of Matter
Proust
Law of Definite Proportions
Dalton
Atomic Theory
Dalton’s Atomic Theory
Questions:
1. What accounts for
differences in
properties between
atoms of different
elements?
2. What forces hold
atoms together in
compounds?
Dalton’s Model: Hard,
round object
Discovery of the Electron
Experiments by J.J. Thomson with cathode ray
tubes showed that atoms contain electrons. (1897)
J.J. Thomson:
The Plum Pudding Model
Atoms are
mostly
empty
space.
Millikan’s Oil Drop Experiment-1909
• Confirmed that
electrons have
a negative
charge
• The mass of an
electron was
determined (a
small fraction of
the mass of an
atom).
Two important inferences:
1. Atoms are neutral, so they must
contain positive charge to balance
the negatively charged electrons.
2. The small mass of electrons
compared to atoms indicated that
there must be other particles which
account for an atom’s mass.
Discovery of the Atomic
Nucleus
Rutherford’s Experiment (1911)
Alpha Particle Scattering
Experiment
Expected results
Actual Results
Rutherford’s Conclusion
• Atom’s contained a
densely packed bundle
of matter with a
positive charge.
• This bundle of matter
was called the nucleus
• Suggested that
electrons move around
the nucleus like planets
around the sun
Other Important Discoveries
• (Mosely) Atoms of each element have a unique
positive charge
• Protons are positively charged particles in the nucleus
• Neutrons are neutral particles in the nucleus
• The charge of a proton is equal but opposite of the
charge of an electron
• Atoms contain an equal number of protons and
electrons.
• The # of protons in the nucleus gives each element its
identity.
• Strong nuclear forces hold protons and neutrons
together in the nucleus