Video 2/3 #2 - Subatomic Particles

By the end of the nineteenth century, scientists had established that matter is composed of atoms.
Further experimentation revealed that atoms themselves consist of smaller fundamental particles.

Key Figure: J.J. Thompson
- J.J. Thompson conducted experiments that led to the identification of the electron as the first subatomic particle.
- His model of the atom is referred to as the Plum Pudding Model.

The model's name is derived from a traditional English dessert.
Description:
- The atom is envisioned as a spherical cloud of positive charge.
- Embedded within this positive cloud are electrons, which are negatively charged.
- The positive charge is represented as a sphere filled with positive charges (symbolized in red in the original model).
- Electrons are depicted as circles within the sphere representing the negative charge.

Atoms, previously thought to be indivisible, are actually composed of subatomic particles, specifically electrons.
The ability to produce cathode rays with different metals implies that all atoms must contain these negatively charged, low-mass particles.
Charge to Mass Ratio of Electron:
- Determined to be $-1.76 imes 10^8$ coulombs per gram.

Following the understanding of electrons' charge-to-mass ratio, the next objective was to ascertain the charge of a single electron.
Conducted by: Robert Millikan
- Millikan used an apparatus known as the oil drop experiment.
- The apparatus included a positively charged plate and an oil atomizer that converted oil into a fine spray, crucial for understanding how atoms form compounds.
Charge of One Electron:
- Found to be $-1.6 imes 10^{-19}$ coulombs.
Utilized the previously established charge-to-mass ratio to derive the mass of one electron.

Alongside discoveries of subatomic particles, the phenomenon of radioactivity was also identified by various researchers, including the Curies.
Definition of Radioactivity:
- The emission of small, energetic particles from the cores of unstable atoms.

Key Types of Radioactivity:
- Alpha Particles: Positively charged particles.
- Beta Particles: Negatively charged particles.
- Gamma Rays: Uncharged, high-energy electromagnetic radiation.

In 1909, further work using alpha particles aimed to verify the Plum Pudding Model.
Results contradicted the Plum Pudding Model, leading to the introduction of the Nuclear Theory of the Atom.
- Proposed by Ernest Rutherford.
Key Points of Nuclear Theory:
- Most of the mass and all positively charged particles of an atom are located in a small dense region called the nucleus.
- The majority of the volume of the atom is empty space.
- Electrons, which are negatively charged, are dispersed throughout this empty space, forming an electron cloud around the nucleus.
Electrically Neutral Atoms:
- Atoms maintain electrical neutrality by having an equal number of negatively charged electrons and positively charged protons within the nucleus.
Visual Model:
- The visual representation includes a dense nucleus containing protons and neutrons, surrounded by an electron cloud with an equal number of electrons to balance the positive charge of the nucleus.

The understanding of atomic structure progressed from the Plum Pudding Model to the Nuclear Theory due to experimental evidence.
While significant insights have been gained, the Nuclear Theory was considered more comprehensive but still incomplete in fully explaining atomic structure.