Atomic Structure History - Study Notes

Democritus and the Idea of Atoms

Democritus (460–370 BC) was a Greek philosopher who first thought matter was made of tiny, indivisible parts called atoms.
He believed atoms couldn't be broken down or destroyed, but he had no experiments to prove this.

John Dalton and Experimental Atomic Theory

Around 1803, John Dalton used experiments to show that atoms existed.
His work showed gases combine in specific ways, leading to Dalton’s atomic theory.

Dalton’s Atomic Theory (Main Ideas)

Matter is made of very small particles called atoms.
For an element, all atoms are identical. (Note: This was later found to be incorrect because of isotopes).
In chemical reactions, atoms are rearranged but never made or destroyed.

From Atoms to Smaller Parts (Electron, Proton, Neutron)

JJ Thomson and the Electron (1897)

Using a cathode ray tube (CRT), JJ Thomson found atoms could be divided.
He discovered negatively charged particles called electrons.
He proposed the "plum pudding model," where electrons were scattered in a positive substance, like plums in pudding.

Millikan’s Oil Drop Experiment (early 1900s)

Goldstein and the Proton (1886)

Eugene Goldstein found positively charged particles in CRTs, called protons.
A proton is about 1840 times heavier than an electron ( $m<em>p \approx 1.84 \times 10^3 m</em>e$ ).

Chadwick and the Neutron (1932)

Rutherford and the Nuclear Model (1911)

Ernest Rutherford performed the famous gold foil experiment.
He shot alpha particles at thin gold foil.
What he expected (Plum Pudding Model): Most particles would go straight through or deflect slightly.
What he found: Most passed through, but some bounced off at large angles or even came back.
Conclusion: Atoms are mostly empty space but have a tiny, dense, positively charged center called the nucleus.

Rutherford’s Nuclear Atom: Key Ideas

The nucleus is tiny, dense, and positively charged, holding most of the atom’s mass (protons and neutrons).
Electrons orbit far away from the nucleus in mostly empty space.
If the nucleus were the size of a person, the atom's edge would be miles away.

Bohr’s Planetary Model and Energy Levels (1913)

Niels Bohr suggested electrons move around the nucleus in specific paths, like planets orbiting the sun.
Electrons live in distinct energy levels (orbits).
Electrons in Period 1 elements have 2 electrons in the first shell, Period 2 elements have up to 8 in the second, and Period 3 elements have up to 8 in the third.

From Fixed Orbits to Orbitals: Quantum Mechanical Model

The modern model (electron cloud model) says electrons don't travel in fixed orbits.
Instead, electrons occupy probability regions called orbitals, meaning we can only know where they are likely to be found.
Schrödinger (1926) and Heisenberg (1927) helped develop this model.

Atomic Theories Timeline

Key Takeaways

Atomic theory evolved from ideas (Democritus) to experiments (Dalton) and then to understanding smaller parts.
We moved from atoms being indivisible to finding electrons, protons, and neutrons inside them.
The shift from fixed orbits (Bohr) to probable orbitals (quantum model) changed how we see electron behavior.
These models help explain chemistry, light, and how matter works at tiny scales.

Important Numbers

Electron mass: $m_e = 9.11 \times 10^{-28} \text{ g}$
Electron charge: $e = -1.602 \times 10^{-19} \text{ C}$
Proton mass vs. electron: $m<em>p \approx 1840 \, m</em>e$
Bohr energy levels: $E_n = -\frac{13.6\ \text{eV}}{n^2}$
Bohr electron capacities: Period 1 (2 electrons); Period 2 (8 electrons); Period 3 (8 electrons).

Real-World & Ethical Importance

Knowing about atoms helps in technology (like semiconductors and medical imaging) and nuclear safety.
The history shows how science builds knowledge through experiments and open-mindedness.