1.1 What is Chemistry?

http://www.ck12.org Chapter 4. Atomic Structure

target and studying the effects. From his analysis, he concluded that the nucleus also contains a particle which has
equal mass to the proton, but unlike the proton, is electrically neutral - hence the nameneutron. Here is a short
video clip describing Chadwick’s experiment: http://www.youtube.com/watch?v=HnmEI94URK8 (2:14).

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Chadwick’s work resulted in a new understanding of the nucleus of the atom; it is comprised of both protons and
neutrons. Because the masses of subatomic particles are so small, a new unit, called an atomic mass unit (amu),
was defined. Protons and neutrons each have a mass of approximately one amu. TheTable4.2 describes the
characteristics of the three subatomic particles we have discussed.

TABLE4.2:Characteristics of Subatomic Particles

Particle Charge (C) Mass (kg) Mass (amu) Location

proton +1.6022× 10 −^19 1.67262× 10 −^27 1.0073 (~1) nucleus

electron (e−) -1.6022× 10 −^19 9.10938× 10 −^31 5.4858× 10 −^4 (~0) outside nucleus

neutron 0 1.67493× 10 −^27 1.0087 (~1) nucleus

Lesson Summary

• Experiments conducted during the early twentieth century revealed that the atom is comprised of subatomic
  particles called electrons, neutrons, and protons.


• In 1877, William Crookes discovered cathode rays, which later became known as electrons. Crookes discov-
  ered these rays using an apparatus he developed called the Crookes tube or cathode-ray tube.


• Electrons, which were initially called cathode rays, are negatively charged and have a very small mass
  compared to the masses of other subatomic particles.


• In 1895, Wilhelm Conrad Roentgen discovered X-rays.


• In 1897, J. J. Thomson showed that cathode rays are deflected in a magnetic field and proposed that cathode
  rays are streams of negatively charged particles. Thomson proposed the plum-pudding model of the atom. This
  model described the atom as a disperse field of positive charge containing small negatively charged particles.


• In 1909, the magnitude of the charge carried by an electron was determined by Robert Millikan in an experi-
  ment known as the oil drop experiment. Information from this experiment was later used to also calculate the
  mass of an electron.


• In 1919, Rutherford discovered the presence of a positively charged nucleus with his famous gold foil experi-
  ment.


• Rutherford proposed a new atomic model that described the atom as comprised of a positively charged nucleus
  surrounded by negatively charged electrons. In this model, most of the atom was thought to be empty
  space.*Protons are positively charged and have a relatively large mass compared to electrons. Protons can
  be found in the nucleus of the atom.


• In 1932, Chadwick discovered the neutron, a particle with a mass similar to that of the proton but without any
  electrical charge.


• Neutrons are particles with a mass similar to that of the proton, but they have no electrical charge. Neutrons
  also reside in the nucleus.