Cathode Ray Experiment and Electron Properties

Cathode Ray Experiment

• Invisible rays emitted from the cathode
• The rays cause a green fluorescent glow on hitting the tube wall
• The rays act like negatively charged particles in an electric field
• The rays travel in straight lines and are independent of the cathode material

Properties of Cathode Ray Particles

• Negatively charged

Cathode Ray: Charge/Mass Ratio (e/m)

• J.J. Thomson calculated the charge-to-mass ratio (e/m) using electric and magnetic fields
• The calculated ratio is -1.76 x 10^8 coulombs per gram (C/g)

Millikan Oil Drop Experiment

• Used to determine the charge (e) of an electron
• Measured the charge on oil droplets falling through an electric field
• Found that the charge on the droplets was always a multiple of a basic charge

Charge of Electron

• Millikan discovered that all charges were multiples of a basic charge, 1.6 x 10^-19 Coulombs
• The charge of an electron is -1.6 x 10^-19 Coulombs

Mass of Electron

• Using the e/m ratio (-1.76 x 10^8 C/g) and the calculated charge of the electron, the mass of the electron was determined
• The mass of the electron is 9.11 x 10^-28 g

Anode Ray (Positive Ray) (Canal Ray)

• The atom is electrically neutral, meaning there are positively charged particles to balance the negative charge of the electrons
• Eugen Goldstein experimentally proved the existence of protons
• The positive ray consists of positive ions of the gas used in the tube

Canal (anode) Rays

• Eugen Goldstein experimentally confirmed the existence of the canal rays

Explanation of production of positive rays

• High voltage ionizes the gas in the tube
• Positive ions of the gas constitute the canal ray
• They are the nuclei of the gas atoms and possess properties different from the cathode rays

Properties of anode rays

• Travel in straight lines
• Positively charged
• The Charge/Mass (q/m) ratio varies depending on the gas used
• The velocity of anode rays is much smaller than the velocity of cathode rays

Thomson’s Atomic Model

• The atom is a solid sphere of uniform positive electric charge (pudding)
• Negatively charged electrons (plums) are embedded in the sphere
• This model is sometimes referred to as the plum pudding model

Rutherford’s Scattering Experiment (Gold Foil Experiment)

• Rutherford fired alpha particles at a thin gold foil
• He observed that most of the alpha particles passed through the foil
• Some particles were deflected at small angles
• A very small number of particles were deflected at large angles

Rutherford Conclusions

• Most of the atom is empty space
• The nucleus is small, dense, and positively charged
• Electrons revolve around the nucleus

Rutherford Conclusions

• The atom contains a dense positively charged center called the nucleus
• The nucleus occupies only a small fraction of the atom's volume
• Electrons move around the nucleus in closed circular paths
• The number of electrons is equal to the number of protons, maintaining electrical neutrality

Planetary Model of Atom

• The atomic model resembles the solar system, with a positively charged nucleus at the center (like the sun) and negatively charged electrons orbiting it (like the planets)

Drawbacks of Rutherford’s Model

• The model could not explain the stability of the atom -- An orbiting electron should continually lose energy and spiral into the nucleus.
• The model could not explain the radiation emitted from some atoms
• Did not explain how the positive charges within the nucleus were held together.

Extra Mass

• The mass of a nucleus is greater than the sum of the masses of its protons and neutrons
• This extra mass is due to the binding energy of the nucleons

Discovery of Neutrons (Chadwick Experiment)

• James Chadwick proved the existence of neutrons (neutral particles)
• Chadwick bombarded Beryllium with alpha particles.
• The resulting radiation was neutral and was able to knock protons out of other atoms, revealing the presence of the neutron.

Chadwick's observations and conclusions

• When Beryllium was bombarded with alpha particles, neutral radiation was produced
• The radiation was able to knock protons out of other atoms, indicating the presence of material particles with no charge.

Chadwick’s observations and conclusions

• The radiation was able to move a paddle wheel, further suggesting the وجود جسيمات مادية (a material particle).
• The radiation did not deflect in an electric field, confirming the neutrality of the particles.

Atomic Number and Mass Number

• The atomic number (Z) is the number of protons in the nucleus of an atom.
• The mass number (A) is the total number of protons and neutrons in the nucleus of an atom.

Isotopes

• Isotopes are atoms of the same element with the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons)

Hydrogen Isotopes

• Protium (¹H): One proton
• Deuterium (²H): One proton, one neutron
• Tritium (³H): One proton, two neutrons

Carbon Isotopes

• Carbon-12 (¹²C): Six protons, six neutrons
• Carbon-13 (¹³C): Six protons, seven neutrons
• Carbon-14 (¹⁴C): Six protons, eight neutrons

Oxygen Isotopes

• Oxygen-16 (¹⁶O): Eight protons, eight neutrons
• Oxygen-17 (¹⁷O): Eight protons, nine neutrons
• Oxygen-18 (¹⁸O): Eight protons, ten neutrons

Atomic Mass (Weight)

• The relative mass of an atom of an element, expressed in atomic mass units.

Atomic Mass (Weight) Scale

• Determined by the abundance of each isotope of an element in nature
• The average atomic mass is typically measured in atomic mass units (amu)

Atomic Mass Unit (amu)

• One atomic mass unit (amu) is equal to 1/12 the mass of a carbon-12 atom (¹²C)

Atomic Mass (Weight) of Chlorine (Cl)

• Two isotopes: ³⁵Cl and ³⁷Cl
• Average atomic mass of chlorine is calculated to be 35.45 amu based on the abundance of each isotope

Atomic Mass of Carbon (C)

• Average atomic mass of carbon is 12.011 amu, calculated considering the abundance of ¹²C, ¹³C, and ¹⁴C