Atoms, Molecules and Ions

The Discovery of Atomic Structure

• 1803 Dalton - the atom is a indivisible, indestructible, tiny ball
• 1850 Evidence is accumulating that the atom is itself composed of smaller particles
• The current model...

The behavior of electrically charged particles

• A charged particle moving though a magnetic field will feel a force perpendicular to the plane described by the velocity vector and magnetic field vector
• This deflects the moving charged particle according to the "right hand rule" (based on a positive charge)

• A negative charge will be deflected in the opposite direction

Electrical discharge through partially evacuated tubes produced radiation. This radiation originated from the negative electrode, known as the cathode (thus, these rays were termed cathode rays).

• The "rays" traveled towards, or were attracted to the positive electrode (anode)
• Not directly visible but could be detected by their ability to cause other materials to glow, or fluoresce
• Traveled in a straight line
• Their path could be "bent" by the influence of magnetic or electrical fields
• A metal plate in the path of the "cathode rays" aquired a negative charge
• The "cathode rays" produced by cathodes of different materials appeared to have the same properties

These observations indicated that the cathode ray radiation was composed of negatively charged particles (now known as electrons).

J.J. Thompson (1897) measured the charge to mass ratio for a stream of electrons (using a cathode ray tube apparatus) at 1.76 x 10⁸ coulombs/gram.

• Charged particle stream can be deflected by both an electric charge and by a magnetic field
• An electric field can be used to compensate for the magnetic deflection - the resulting beam thus behaves as if it were neutral
• The required current needed to "neutralize" the magnetic field indicates the charge of the beam
• The loss of mass of the cathode indicated the "mass" of the stream of electrons

Thompson determined the charge to mass ratio for the electron, but was not able to determine the mass of the electron.

However, from his data, if the charge of a single electron could be determined, then the mass of a single electron could determined.

Robert Millikan (1909) was able to successfully measure the charge on a single electron (the "Milliken oil drop experiment"). This value was determined to be 1.60 x 10^-19 coulombs.

Thus, the mass of a single electron was determined to be:

(1 gram/1.76 x 10⁸ coulombs)*(1.60 x 10^-19 coulombs) = 9.10 x 10^-28 grams

Note: the currently accepted value for the mass of the electron is 9.10939 x 10^-28 grams.

Wilhelm Roentgen (1895) discovered that when cathode rays struck certain materials (copper for example) a different type of ray was emitted. This new type of ray, called the "x" ray had the following properties:

• The could pass unimpeded through many objects
• They were unaffected by magnetic or electric fields
• They produced an image on photographic plates (i.e. they interacted with silver emulsions like visible light)

Henri Becquerel (1896) was studying materials which would emit light after being exposed to sunlight (i.e. phosphorescent materials). The discovery by Roentgen made Becquerel wonder if the phosphorescent materials might also emit x- rays. He discovered that uranium containing minerals produced x-ray radiation (i.e. high energy photons).

Marie and Pierre Curie set about to isolate the radioactive components in the uranium mineral.

Ernest Rutherford studied alpha rays, beta rays and gamma rays, emitted by certain radioactive substances. He noticed that each behaved differently in response to an electric field:

• The b-rays were attracted to the anode
• The a-rays were attracted to the cathode
• The g-rays were not affected by the electric field

The a and b "rays" were composed of (charged) particles and the g-"ray" was high energy radiation (photons) similar to x-rays

• b-particles are high speed electrons (charge = -1)
• a-particles are the positively charged core of the helium atom (charge = +2)

J.J. Thompson model of the atom (1900)

• The atom consists of a sphere of positive charge within which was buried negatively charged electrons
• Also known as the "plum pudding" model of the atom

Rutherford model of the atom (1910)

• Most of the mass of the atom, and all its positive charge, reside in a very small dense centrally located region called the "nucleus"
• Most of the total volume of the atom is empty space within which the negatively charged electrons move around the nucleus

Rutherford (1919) discovers protons - positively charged particles in the nucleus

Chadwick (1932) discovers neutron - neutral charge particles in the nucleus