Introduction to Atomic Theory and Key Discoveries

Atomic theory has evolved significantly over the last 2500 years.
Understanding atomic theory allows for a deeper comprehension of atomic structure compared to the knowledge scientists possessed in ancient Greece.

Early Proposals:
- Greek philosophers Leucippus and Democritus (around 450 BCE) first posited that matter consists of indivisible particles called "atoms" (derived from the Greek word meaning "uncuttable").
- They believed that different substances were made of different atom types (e.g., iron atoms, clay particles).
Properties Attribution:
- They assigned physical properties to atoms based on their material type:
- Iron atoms thought to be hard, sticky.
- Clay atoms viewed as softer and flexible.
- Cheese atoms imagined as squishy.
Philosophical and Scientific Limitations:
- The understanding of atomic nature was rudimentary and lacked the empirical evidence provided by modern science.

Law of Conservation of Mass:
- French chemist Antoine Lavoisier proposed that mass remains constant in a closed system despite changes in form.
Dalton's Atomic Theory:
- English chemist John Dalton established that elements consist of discrete packets of matter.

1870s Developments:
- Use of discharge tubes to probe matter.
- Cathode rays produced from negative electrodes were discovered.
- Eugen Goldstein (1886) found positive rays in discharge tubes, indicating a positive charge in matter.
JJ Thomson’s Contributions:
- Conducted measurements of cathode rays, concluding they consisted of small, negatively charged particles named electrons.
- Proposed the Plum Pudding Model of the atom, where negative electrons were embedded in a positively charged "soup".
Rutherford's Gold Foil Experiment (1909):
- Ernest Rutherford discerned the nucleus concept through experiments involving alpha particles and thin gold foil.
- Most particles passed through, but some deflected at large angles, leading to the conclusion that atoms contain a dense nucleus that holds positive charge.
- This established the atom mostly consists of empty space.

Rutherford identified protons as fundamental particles after bombarding nitrogen with alpha particles, leading to hydrogen ion production.

Bohr integrated mathematical principles of electromagnetic radiation into atomic theory, predicting electron arrangements in orbits around the nucleus.
- Suggested electrons exist in specific orbits with defined energy levels.
- Addressed electron energy transitions with discrete energy absorption and emission during these orbit changes.

Robert Millikan pioneered the measurement of electron charge using an apparatus to suspend oil droplets charged by electrons.
- Key setup: A gas chamber with charged plates and an atomizer for fine oil sprays.
Established that oil droplets would carry negative charge due to ionization, allowing control of their movement via electrostatics.
Determined the charge of an electron: q = -1.6 imes 10^{-19} coulombs.
Calculated the mass of an electron from the charge-to-mass ratio as derived by Thomson:
- Mass of electron: 9.1 imes 10^{-28} grams.

James Chadwick identified the neutron by conducting experiments with alpha particles and low atomic mass materials (e.g., beryllium).
The neutron was confirmed as a neutral particle within the nucleus, reconciling the discrepancy between atomic mass and the number of protons.

Ongoing exploration in atomic theory resulted in the development of quantum mechanics and provides a central context for modern chemistry and physics.
Upcoming topics will explore nuclear chemistry and organic chemistry, grounding them in a solid understanding of atomic structure and behavior.