OpenStax Chemistry: Atoms First 2e
2.2: Evolution of Atomic Theory
Chemistry: Atoms First 2e
2: Atoms, Molecules, and Ions
2.2: Evolution of Atomic Theory
5. The existence of isotopes violates one of the original ideas of Dalton’s atomic theory. Which
one?
Solution
Dalton originally thought that all atoms of a particular element had identical properties, including
mass. Thus, the concept of isotopes, in which an element has different masses, was a violation of
the original idea. To account for the existence of isotopes, the second postulate of his atomic
theory was modified to state that atoms of the same element must have identical chemical
properties.
6. How are electrons and protons similar? How are they different?
Solution
Both are charged particles that are components of an atom. Although charges are the same size,
the signs of the charges are opposite. Protons are much more massive than electrons. Protons are
located within an atom’s nucleus, whereas electrons are located outside the nucleus.
7. How are protons and neutrons similar? How are they different?
Solution
Both are subatomic particles that reside in an atom’s nucleus. Both have approximately the same
mass. Protons are positively charged, whereas neutrons are uncharged.
8. Predict and test the behavior of α particles fired at a “plum pudding” model atom.
(a) Predict the paths taken by particles that are fired at atoms with a Thomson’s plum pudding
model structure. Explain why you expect the particles to take these paths.
(b) If particles of higher energy than those in (a) are fired at plum pudding atoms, predict how
their paths will differ from the lower-energy particle paths. Explain your reasoning.
(c) Now test your predictions from (a) and (b). Open the URL
http://phet.colorado.edu/en/simulation/rutherford-scattering and select the “Plum Pudding Atom”
tab. Set “Alpha Particles Energy” to “min,” and select “show traces.” Click on the gun to start
firing particles. Does this match your prediction from (a)? If not, explain why the actual path
would be that shown in the simulation. Hit the pause button, or “Reset All.” Set “Alpha Particles
Energy” to “max,” and start firing particles. Does this match your prediction from (b)? If not,
explain the effect of increased energy on the actual paths as shown in the simulation.
Solution
(a) The plum pudding model indicates that the positive charge is spread uniformly throughout the
atom, so we expect the particles to (perhaps) be slowed somewhat by the positive-positive
repulsion, but to follow straight-line paths (i.e., not to be deflected) as they pass through the
atoms. (b) Higher-energy particles will be traveling faster (and perhaps slowed less) and will
also follow straight-line paths through the atoms. (c) The particles followed straight-line paths
through the plum pudding atom. There was no apparent slowing of the particles as they passed
through the atoms.
9. Predict and test the behavior of α particles fired at a Rutherford atom model.
(a) Predict the paths taken by particles that are fired at atoms with a Rutherford atom model
structure. Explain why you expect the particles to take these paths.
(b) If particles of higher energy than those in (a) are fired at Rutherford atoms, predict how
their paths will differ from the lower-energy particle paths. Explain your reasoning.
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2.2: Evolution of Atomic Theory
Chemistry: Atoms First 2e
2: Atoms, Molecules, and Ions
2.2: Evolution of Atomic Theory
5. The existence of isotopes violates one of the original ideas of Dalton’s atomic theory. Which
one?
Solution
Dalton originally thought that all atoms of a particular element had identical properties, including
mass. Thus, the concept of isotopes, in which an element has different masses, was a violation of
the original idea. To account for the existence of isotopes, the second postulate of his atomic
theory was modified to state that atoms of the same element must have identical chemical
properties.
6. How are electrons and protons similar? How are they different?
Solution
Both are charged particles that are components of an atom. Although charges are the same size,
the signs of the charges are opposite. Protons are much more massive than electrons. Protons are
located within an atom’s nucleus, whereas electrons are located outside the nucleus.
7. How are protons and neutrons similar? How are they different?
Solution
Both are subatomic particles that reside in an atom’s nucleus. Both have approximately the same
mass. Protons are positively charged, whereas neutrons are uncharged.
8. Predict and test the behavior of α particles fired at a “plum pudding” model atom.
(a) Predict the paths taken by particles that are fired at atoms with a Thomson’s plum pudding
model structure. Explain why you expect the particles to take these paths.
(b) If particles of higher energy than those in (a) are fired at plum pudding atoms, predict how
their paths will differ from the lower-energy particle paths. Explain your reasoning.
(c) Now test your predictions from (a) and (b). Open the URL
http://phet.colorado.edu/en/simulation/rutherford-scattering and select the “Plum Pudding Atom”
tab. Set “Alpha Particles Energy” to “min,” and select “show traces.” Click on the gun to start
firing particles. Does this match your prediction from (a)? If not, explain why the actual path
would be that shown in the simulation. Hit the pause button, or “Reset All.” Set “Alpha Particles
Energy” to “max,” and start firing particles. Does this match your prediction from (b)? If not,
explain the effect of increased energy on the actual paths as shown in the simulation.
Solution
(a) The plum pudding model indicates that the positive charge is spread uniformly throughout the
atom, so we expect the particles to (perhaps) be slowed somewhat by the positive-positive
repulsion, but to follow straight-line paths (i.e., not to be deflected) as they pass through the
atoms. (b) Higher-energy particles will be traveling faster (and perhaps slowed less) and will
also follow straight-line paths through the atoms. (c) The particles followed straight-line paths
through the plum pudding atom. There was no apparent slowing of the particles as they passed
through the atoms.
9. Predict and test the behavior of α particles fired at a Rutherford atom model.
(a) Predict the paths taken by particles that are fired at atoms with a Rutherford atom model
structure. Explain why you expect the particles to take these paths.
(b) If particles of higher energy than those in (a) are fired at Rutherford atoms, predict how
their paths will differ from the lower-energy particle paths. Explain your reasoning.
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