Anode Rays and Proton Discovery

Anode Rays and the Discovery of Protons

• Atoms are electrically neutral, containing negatively charged particles (electrons).
• Scientists sought positively charged particles to balance the negative charges.
• In 1886, Goldstein observed positively charged particles originating from the anode (positive electrode) of a discharge tube.
• These particles, called anode rays, travelled in a direction opposite to cathode rays (electrons).
• Rutherford's gold foil experiment further contributed to the discovery of the proton.

Crediting Rutherford with the Discovery of the Proton

• Rutherford, Geiger, and Marsden performed the gold foil experiment in 1909.
• Alpha particles were directed at a thin gold foil.
• Observations revealed some alpha particles were significantly deflected, contrary to expectations based on the prevailing model.
• Rutherford inferred a concentrated, positively charged nucleus within atoms.
• In 1919, Rutherford directed alpha particles at nitrogen gas, resulting in hydrogen nucleus emission.
• This established the positively charged particle as the proton.

Production of Positive Rays in a Discharge Tube

• A discharge tube, with low-pressure air, a perforated cathode, and an anode, is used to produce positive rays.
• A high voltage is applied between the electrodes.
• Cathode rays (electrons) leave the cathode.
• Positively charged particles (anode rays) pass through the holes in the cathode.
• These particles cause a glow on the opposite anode side.

Properties of Positive Rays

• Positive rays are positively charged.
• The charge-to-mass ratio (e/m) of positive rays is smaller than that of electrons.
• Positive rays create flashes on a ZnS plate.
• They travel in a straight line opposite the cathode.
• The charge-to-mass ratio varies based on the gas in the discharge tube.
• The particle with the lowest mass-to-charge ratio originates from hydrogen gas and is termed the proton.

Discovery of the Neutron

• Up to 1932, the atomic model included protons and electrons.
• The masses of different atoms couldn't be entirely explained by protons and electrons.
• Rutherford predicted a neutral particle with a mass similar to a proton's.
• In 1930, Becker and Bothe observed penetrating radiation from alpha particle impacts on light elements.
• In 1932, Chadwick identified these particles as neutrons.
• This neutral particle, called a neutron, resides within the nucleus of atoms. This helped scientists explain the remaining mass and stability of atomic structures.

Properties of Neutrons

• Neutrons have no charge making them neutral.
• Neutrons have a mass similar to a proton.
• Free neutrons decay into a proton, an electron, and an antineutrino.
• Neutrons are very penetrating but less so than cosmic rays.

Uses of Neutrons

• Neutrons can cause transmutation in elements.
• When slow-moving neutrons hit certain substances, they can cause the emission of high-energy gamma radiation.

Moseley and Atomic Number

• Moseley (1913) bombarded elements with cathode rays.
• This caused X-ray emission.
• Moseley found a relationship between X-ray frequencies and atomic mass.
• This implied a fundamental, increasing property (atomic number) as atomic mass increases.
• Moseley defined this increasing property as the number of protons in the atom's nucleus.

Values of Charge and Mass (Protons, Neutrons, Electrons)

• Tables with detailed values for charge and mass of protons, neutrons, and electrons

Rutherford's Gold Foil Experiment

• Rutherford conducted an experiment using a gold foil and alpha particles.
• Most alpha particles passed straight through the foil, indicating the atom is mostly empty space.
• Some alpha particles were deflected, suggesting the concentrated positive charge within the atom, called the nucleus.

Rutherford's Nuclear Model of the Atom

• The nuclear model proposes that the atom:
• Contains a dense, positively charged nucleus.
• Has electrons orbiting the nucleus, implying a largely empty space in the rest of the atom.

Limitations of Rutherford's Nuclear Model

• Rutherford's model predicted that electrons orbiting the nucleus should lose energy and spiral into the nucleus, making atoms unstable.
• Experimentally, atoms are stable. This limitation led to the development of Bohr.

Bohr's Atomic Model

• Bohr introduced certain postulates to address the shortcomings of Rutherford's model.
• Electrons orbit the nucleus in specific energy levels.
• Electrons don't continuously lose energy or collapse into the nucleus while in these specific orbits.