Content Benchmark P.8.A.7
Students know the characteristics of electrons, protons, and neutrons. E/S
An effective tactic for teaching the fundamental particles of the atom is to illustrate the work of
the men and women who uncovered them. Often the fundamental characteristics are simply
listed in a table and students see them as discrete facts disconnected from their history. A
timeline of historical events is seen in Figure 1.
Figure 1. Subatomic particles discovered between 1898 – 1964
(From http://particleadventure.org/other/history/quantumt.html)
The Electron: Discovery and Basic Properties
Electrons were first discovered by a British scientist, J. J. Thomson in 1897 while he was
studying cathode rays. Thomson found that the unusual rays he was studying were attracted to
the positive end of a cathode ray tube. He theorized that the rays were composed of particles
which he called corpuscles and thought they would be the fundamental building blocks of the
atom. Thomson was correct that his cathode ray particles were one of the particles that compose
the atom. Later, these small particles were named electrons.
Figure 2. A simple cathode ray tube.
(From http://www.aip.org/history/electron/jjhome.htm)
Thomson continued experimenting with the cathode ray tube in order to determine some of the
characteristics of the electron. Because the ray of electrons bent towards the positive electrode of
the cathode ray tube, Thomson knew that the electron was negatively charged. He was unable to
determine the precise mass of the electron, but did determine its charge to mass ratio. Later, it
was determined that the mass of the electron was 1/1836th the mass of a proton or
9.11 × 10−31 kg. The charge on a single electron is −1.602 × 10−19 Coulomb.
The electron is in the class of subatomic particles called leptons, which are believed to be
fundamental particles of matter. Electrons are bound to the atom through electromagnetic
attractions to positively charged protons. Electrons moving freely in a vacuum can be focused
into a beam such as Thomson’s cathode ray tube.
For more information about the properties and behavior of electrons one helpful site with concise
information is http://www.visionlearning.com/library/module_viewer.php?mid=50.
Thomson proposed a theory that suggested that the atom was composed of a positive sphere of
electricity with the electrons evenly spaced around it. This theory was called the “plum pudding”
model of the atom.
The Proton: Discovery and Basic Properties
Ernest Rutherford was a student of Thomson and continued Thomson’s research of cathode ray
tubes. In his famous gold foil experiment of 1911, Rutherford discovered positively charge
particles inside the atom. Based on his experimental results, he formulated a new atomic theory
that suggested that the atom consisted of a positive core with electrons surrounding this core.
Figure 3. Interpreting Rutherford’s Gold Foil Experiment
(From http://www.visionlearning.com/library/module_viewer.php?mid=50)
Further information on this topic and a diagram of the chamber used in his experiment can be
found at http://www.rsc.org/chemsoc/timeline//pages/1911.html.
Characteristics of the proton include that it is positively charged, has a mass of 1.67×10−27 kg,
and in terms of an atom, is located in the nucleus. The charge on a single proton is
+1.602 × 10−19 Coulomb (exactly the same magnitude, but opposite in sign to a single electron).
A hydrogen atom minus its electron (i.e., hydrogen ion) is also known as a proton.
The Neutron: Discovery and Basic Properies
The neutron was an elusive particle, and before it was discovered, its presence was predicted by
Ernest Rutherford in 1920. He suggested that a neutral particle could be generated by the capture
of an electron by a proton. He theorized that the particle was in the nucleus of the atom and that
it had approximately the same mass as the proton, but did not carry a charge.
The scientist team of Irene and Frederick Joliot-Curie made an interesting discovery during one
of their experiments, but misinterpreted the results. Although their experiment intrigued the
science community, it was met with some skepticism. James Chadwick who worked in the
Cavendish laboratory repeated Joliot-Curie’s experiment and re-interpreted the results suggesting
the observations made by Joliot-Curie were actually due to a neutral particle which was a
component of the radiation being emitted by a beryllium metal. He called the particle a neutron
and received the Nobel Prize in 1935 for his work. The neutron has approximately the same mass
as a proton, carries no charge, and is located in the nucleus of an atom.
A full explanation of this experiment can be found at
http://www.chemcases.com/nuclear/nc-01.htm.
A nice animation is available through
http://wwwoutreach.phy.cam.ac.uk/camphy/neutron/neutron2_1.htm
and, http://web.visionlearning.com/custom/chemistry/animations/CHE1.2-an-atoms.shtml.
Figure 4. Model of the atom based on Chadwick’s discovery.
This model is often called the Rutherford-Bohr Model of the Atom.
(From http://library.thinkquest.org/27954/neutron.html)
Summary-Characteristics of Sub-Atomic Particles
Electron
Protons
Neutrons
Location
Outside nucleus
Inside nucleus
Inside nucleus
Relative Charge
-1
+1
0
Relative Mass (amu)
1/1836th
1
1
Protons and neutrons do not have exactly the same mass as each other. On the carbon-12 scale, a
proton has a mass of 1.0073 atomic mass units (amu) and a neutron has a mass of 1.0087 amu.
Relative Size of Sub-atomic Particles
The radius of a hydrogen atom consisting of one electron and one proton is approximately
5 × 10−9 meters. The nucleus is approximately 1 × 10−15 meters in diameter. The "classical"
radius of a "free" electron is taken to be about 3 × 10−15 meters, and the "classical" radius of a
"free" proton is taken to be about 1 × 10−15 meters.
This information was obtained from the following site, which also contains more details about
atomic and subatomic sizes see
http://www.newton.dep.anl.gov/askasci/phy00/phy00494.htm.
Modern Models of the Atom
The modern model of the atom is a probability model in which electrons are located in charge
clouds outside the nucleus of the atom, which contains the protons and neutrons (nucleons). The
Standard Theory of the Atom includes discussions of smaller building blocks of matter: quarks,
force carrier particles, and leptons. Electrons are leptons and both protons and neutrons are
composed of combinations of quarks.
For more information on the Standard Model of the Atom, go to
http://particleadventure.org/frameless/standard_model.html.
A final site that summarizes the characteristics of protons, neutrons and electrons can be found at
http://web.jjay.cuny.edu/~acarpi/NSC/3-atoms.htm.
For another helpful website which contains the history of the atom in paragraph form and which
is also available in Spanish, please visit
http://www.visionlearning.com/library/module_viewer.php?mid=50.
Nuclear Symbols
Nuclear symbols were developed as a model to represent the numbers of protons, neutrons and
electrons in an atom.
Figure 5. Nuclear symbol for Helium
(From http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/Nuclear-Symbol.html)
Figure 6. Nuclear Symbol for Uranium
(From http://dbhs.wvusd.k12.ca.us/webdocs/Radioactivity/Nuclear-Symbol-Example.GIF).
The mass number of an element represents the total number of protons and neutrons in an atom.
The mass number is sometimes referred to as the A number. The atomic number represents the
total number of protons in an atom. It is also known as the Z number. In neutral atoms, the
atomic number is the same as the total number of electrons in the atom. In the example of
uranium-238, there are 92 protons, 92 electrons and 146 neutrons.
To assist students in calculating the numbers of protons, neutrons and electrons in different
atoms, http://science.widener.edu/svb/tutorial/protons.html provides an interactive table for them
to complete.
Content Benchmark P.8.A.7
Students know the characteristics of electrons, protons, and neutrons. E/S
Common misconceptions associated with this benchmark
1. Students incorrectly think that there is an edge, or boundary, to an atom.
Atoms and orbitals have no hard edges. Instead, the contour of the orbital, as we show it in
pictures and models, corresponds to the space of highest probability for electron location. Care
must be taken during classroom instruction to discuss the strengths and limitations of models.
Ask students to draw and interpret a model of the atom. From their interpretations, the teacher
can address the misconceptions. Using vocabulary such as “electron shell” reinforces this
misconception. Explain to students that some vocabulary does not reflect the current
understandings of the atom and has its basis in the history of the atom.
This information was obtained from
http://intro.chem.okstate.edu/ChemSource/Atomic/concpt4.htm.
Figure 7. The Fuzzy Atom
(From http://www.uq.edu.au/_School_Science_Lessons/TWFig4.GIF)
To learn more about this misconception and other science misconceptions, go to
http://educ.queensu.ca/~science/main/concept/chem/c07/C07CDTL1.htm
and, http://www.uq.edu.au/_School_Science_Lessons/TWImagesatoms.html.
2. Students incorrectly think that air exists between particles (protons, neutrons, and
electrons) in atoms.
There is no matter, air or otherwise, between atomic particles, because these particles are the
matter from which air and all other gases is comprised. This common misconception can be
addressed by building models that are more representative of the charge cloud model of the atom
rather than “ball and stick” images.
More information about this misconception can be found at
http://intro.chem.okstate.edu/ChemSource/Atomic/concpt4.htm.
3. Students incorrectly think that all atoms are charged.
Atoms are neutral because there are equal numbers of electrons and protons. Ions of atoms form
when the atom gains or loses electrons and ions have a net positive or negative charge. Protons
cannot be lost by an atom in normal chemical reactions. It is helpful to use the nuclear symbols
of atoms to assist students in understanding that an equal number of electrons and protons exist
in an atom. A simplified discussion of cations and anions can help students understand what
charges are.
To learn more about this misconception and other science misconceptions, go to
http://www.seed.slb.com/qa2/FAQView.cfm?ID=572.
4. Students incorrectly think that all models accurately describe the atom.
Models are ways for us to understand the physical world. They are simplified representations.
Models may have limitations, which can often cause misconceptions. As scientific research
finds new evidence, models are modified. But, regardless of the model, all do not fully represent
reality.
To learn more about this misconception and other science misconceptions, go to
http://education.jlab.org/qa/atom_model.html
and, http://www.regentsprep.org/Regents/physics/phys05/catomodel/default.htm.
Content Benchmark P.8.A.7
Students know the characteristics of electrons, protons, and neutrons. E/S
Sample Test Questions
Questions and Answers to follow on separate document
Content Benchmark P.8.A.7
Students know the characteristics of electrons, protons, and neutrons. E/S
Answers to Sample Test Questions
Questions and Answers to follow on separate document
Content Benchmark P.8.A.7
Students know the characteristics of electrons, protons, and neutrons. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. The Discovery of the Electron Exhibit
This site provides a museum presentation of the experiments by J.J. Thomson in 1897 that led to
the discovery of a fundamental building block of matter. Historical images give the students and
the teacher the feel for the laboratory equipment prevalent during the 1890’s.
To access these activities, go to http://www.aip.org/history/electron/.
2. Rutherford’s Experiment
This site provides a narration that explains the history of the atom and Thomson’s theory.
Rutherford and his colleagues had tried to test Thomson’s theory by bombarding gold foil. The
animation assists the student in viewing Rutherford’s experiment and his analysis of the
experimental results.
To access these activities, go to
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf.
3. Exploring Life at Bio dot Edu
This site contains a detailed essay and diagrams about the historical development of Rutherford’s
atomic theory. The essay begins with Becquerel’s discovery of radioactivity and leads the reader
through his experiment complete with information not usually included in textbook descriptions
of the famous Gold Foil Experiment. This is a great nature of science reading.
To access these activities, go to
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C3/C3_Protons.html.
4. Famous Experiments: The Discovery of the Neutron
This site provides a brief explanation of the work of scientists that led to James Chadwick’s
discovery of the neutron. The diagrams nicely assist the student in understanding how Chadwick
designed his experiment. The essay helps students understand how science works to nullify
hypotheses which often lead to new discoveries.
To access these activities, go to
http://dev.physicslab.org/Document.aspx?doctype=3&filename=AtomicNuclear_ChadwickNeutr
on.xml
5. The Particle Adventure: The Fundamentals of Matter and Force
Students are engaged with an interactive site that explores modern understandings of the atom as
compared to the more commonly known history of the discovery of the subatomic particles
provided in most classrooms. This site developed by the Particle Data Group of Lawrence
Berkeley National Laboratory tours quarks, neutrinos, antimatter, dark matter, etc. It is a great
site for more in-depth discussions with students about the nature of the atom.
To access these activities, go to
http://www.particleadventure.org/ and http://pdg.lbl.gov/quarkdance/.
6. Fermilabyrinth
This site provides a game that allows students to propel a particle through an accelerator.
Developed by scientists at the Lederman Science Center at the Fermi Laboratory, this animation
loads easily and students can control various aspects of the accelerator while learning how it
operates. In addition to the games, there are video interviews with scientists and resources for
both educators and students.
To access these activities, go to
http://ed.fnal.gov/projects/labyrinth/games/