Topic 1 – Structure of the Atom
I.
Understanding the composition of matter.
A. Antoine Lavoisier (1774) – “Law of Conservation of Matter” from before and after
measurements of chemical reactions.
B. Joseph Proust (1799) – “Law of Constant Composition” shows that no matter how a
compound is created, it has the same chemical composition.
C. John Dalton’s atomic theory (1805)
1. All matter is composed of tiny solid spheres called atoms that cannot be created or
destroyed.
2. All atoms of the same element are identical and have different properties than all
other elements.
3. Atoms can be rearranged to form compounds containing simple whole-number ratios
of atoms.
D. Dalton’s “Law of Multiple Proportions” states that 2 or more elements can combine in
different ratios to form different compounds.
II.
Important experiments related to atomic structure.
A. J.J. Thomson determined that rays in a CRT (cathode ray tube) were a fundamental part
of matter he called electrons.
1. Negatively charged.
2. Thomson determined the charge-to-mass ratio for electrons
B. Robert Millikan’s oil drop experiment.
1. determined the charge on a single electron by suspending charged oil droplets in a
carefully measured electric field.
2. Combined with Thomson’s ratio, the mass of a single electron was calculated to be
9.11 x 10-28 g.
C. Ernest Rutherford’s gold foil experiment.
1. Positively charged alpha particles were fired at high speeds at a super-thin sheet of
gold foil.
2. About 1 in 10,000 particles bounced straight back which suggested a very dense
region in the atom with a positive charge (the nucleus).
D. Neils Bohr used spectral lines of energized elements to show that the electrons had to be
in specific “rings” around the nucleus.
III.
Models of the Atom
A. Dalton’s billiard ball model – atom is nothing more than a solid sphere with no subatomic
particles.
B. Thomson’s plum pudding model – electrons immersed in the positively charged sphere
like chocolate chips in cookie dough.
C. Rutherford’s nuclear model – electrons randomly orbit a dense, positively charged
nucleus.
D. Bohr’s solar system model – electrons in specific rings around the dense, positively
charged nucleus.
E. Modern electron cloud model – electrons spend most of their time in clouds of a specific
shape.
Additional Resources
Read textbook pages 102-114
http://northspringer.tripod.com/HistoryofAtom/index.html
Practice questions
1. _____When alpha particles are used to bombard gold foil, most of the alpha particles pass
through un-deflected. This result indicates that most of the volume of a gold atom consists of
____.
A. electrons
B. protons
C. neutrons
D. empty space
E. rings
2. _____When electrons in an atom in an excited state fall to lower energy levels, energy is…
A. always absorbed
B. always released
C. neither absorbed or released
D. both absorbed and released E. sometimes absorbed and sometimes released
3. _____Dalton's Atomic Theory includes all of the following except…
A. Matter is made up of tiny, indivisible particles called atoms.
B. The atoms of one element are all identical to one another.
C. All atoms contain a dense nucleus with a positive charge.
D. Atoms of different elements are different from one another.
E. Atoms can combine in simple, whole-number ratios to form compounds.
4. _____Bohr's model of the atom was based, in part, on which of the following experiments?
A. The deflection of cathode rays by an electric field.
B. The scattering of alpha particles through a sheet of gold foil.
C. Observations on the bright-line spectrum for hydrogen.
D. The suspension of charged oil droplets in a magnetic field.
E. Sending a sample of a substance through a mass spectrometer.
5. _____Which alpha particle from Rutherford’s gold foil experiment provides the best evidence
that atoms contain a nucleus?
A. The one in 10,000 that bounces straight back
B. The ones that are deflected slightly
C. The ones that pass through un-deflected
D. All of the above
E. None of the above
6. _____Which was used to determine the exact charge of a single electron?
A. The gold foil experiment. B. Deflection of cathode rays. C. Oil drop experiment.
D. Mass spectrometer
E. Bright-line spectrum
7. _____Which of the following suggested that Thompson's model of the atom was not quite
correct?
A. The gold foil experiment.
B. Deflection of cathode rays. C. Oil drop experiment.
D. Mass spectrometer
E. Bright-line spectrum
8. _____ Which of the following led Thompson to conclude that the atom contained negative
particles.
A. The gold foil experiment.
B. Deflection of cathode rays. C. Oil drop experiment.
D. Mass spectrometer
E. Bright-line spectrum