Chapter+Two+-+Lesson+2

Chapter Two: Atoms and Elements

Lesson Two

• The Discovery of the Electron (2.4)

• The Structure of the Atom (2.5)

2.4 The Discovery of the Electron

• John Dalton initially proposed that matter is composed of tiny indivisible particles called atoms, likened to solid billiard balls.

• Later discoveries showed that atoms are not indivisible but made of smaller particles (electrons).

  • Key developments leading to this understanding included:

    • Invention of the cathode ray tube (CRT) influenced by Johann Hittorf, William Crookes, and Karl Ferdinand Braun.

    • J. J. Thomson's cathode ray experiment.

    • Robert Millikan's oil-drop experiment.

The Cathode Ray Experiments

J.J. Thomson's Experiments

• In 1897, Thomson used a cathode ray tube (CRT) to study cathode rays emitted from a cathode (negatively charged electrode) towards an anode (positively charged electrode).

• When a magnetic field was applied, the cathode rays bent, indicating they were negatively charged, since they were attracted to the positive electrode.

• Thomson's experiments showed:

  • Cathode rays consist of negatively charged particles (electrons).

  • All metals emit cathode rays, indicating that all atoms contain electrons.

  • Atoms are electrically neutral, necessitating an equal positive charge.

  • Proposed the "plum pudding" atomic model as a visual representation of atom structure.

Determining Charge-to-Mass Ratio of Electron

Experimentation and Calculations

• By balancing the deflections of cathode rays using electric and magnetic fields, Thomson could measure the extent of the particles' deflection.

• Balancing forces led to insights about the charge-to-mass ratio of the electron:

  • Formulas used in calculations include kinetic energy (KE) and electric/magnetic field forces.

  • Charge-to-mass ratio determined to be approximately -1.76 x 10^8 C/g or -1.76 x 10^11 C/kg.

Thomson's Contributions

• Identified electrons as negatively charged low mass particles.

• Developed the charge-to-mass ratio of the electron.

• Proposed the plum pudding model for atomic structure.

Millikan's Oil Drop Experiment: The Charge of the Electron

Overview of the Experiment

• Robert Millikan conducted the oil drop experiment to determine the charge of a single electron.

  • Utilized an electric field to balance the downward gravitational force and upward electric force on charged oil droplets.

• When forces were balanced, oil droplets remained suspended in equilibrium.

Experimental Findings

• Millikan adjusted voltage to determine droplet charge and measured outcomes like:

  • Charges found to be multiples of 160 zeptocoulombs (zC).

  • Fundamental value determined as the charge of a single electron: -1.60 x 10^-19 C.

Key Discoveries

• Established the exact charge of an electron.

• Calculated the mass of the electron at approximately 9.11 x 10^-31 kg.

2.5 The Structure of the Atom

Thomson's Atomic Model

• Described the atom as comprising a positively charged sphere with electrons embedded within, likening it to a plum pudding.

Rutherford's Gold Foil Experiment

Overview and Findings

• Conducted by Ernest Rutherford in 1909, aimed to validate the plum pudding model.

• Most alpha particles passed through gold foil; some deflected and a few bounced back, indicating a surprising atomic structure, suggesting atoms possess significant empty space.

Nuclear Theory Proposal

• Rutherford proposed three key components:

  1. Nucleus: Contains most of the atom's mass and all positive charge.

  2. Empty Space: Most of the atom's volume consists of empty space with electrons dispersed throughout.

  3. Neutrality: Equal numbers of protons in the nucleus balance electrons outside, maintaining atomic neutrality.

The Neutrons

Discovery and Characteristics

• Neutrons, discovered by James Chadwick, explain the discrepancy in mass between helium and hydrogen.

• The presence of neutrons accounts for the additional mass of helium (four times that of hydrogen).

• Neutrals in the nucleus have the same mass as protons but no electrical charge.