From Ancient Greece to the Atomic Nucleus

Around 440 BCE, Greek philosopher Democritus[^1] proposed a radical idea:

“Everything is composed of tiny, indivisible particles called atomos (ἄτομος) — meaning ‘uncuttable’ or ‘indivisible’.”

The concept:

• Matter cannot be divided infinitely
• Eventually you reach fundamental, indivisible units
• These atoms are eternal, unchangeable, and invisible
• Different arrangements create different substances

The problem:

• Pure philosophy—no experiments, no evidence
• Rejected by Aristotle (who favored continuous matter)
• Forgotten for over 2,000 years

J.J. Thomson[^2] discovers electrons using cathode ray tubes—the first subatomic particle ever identified.

What it proved:

• Atoms weren’t indivisible after all (Democritus was wrong about that)
• But atoms DO exist as fundamental building blocks (Democritus was right about that)

The new question: If electrons exist inside atoms, what does the atom actually look like?

Thomson proposed: an atom is like a plum pudding[^3]

• A blob of positive charge (the “pudding”)
• Tiny negative electrons embedded throughout (the “plums”)
• The positive charge is evenly spread across the entire atom

Result: This seemed reasonable. Everyone accepted it.

77 years earlier…

Botanist Robert Brown[^4] looked at pollen grains floating in water under a microscope.

What he saw: The grains were jiggling around randomly, constantly, for no apparent reason.

The mystery: No one could explain why. The puzzle sat unsolved for 78 years.

Albert Einstein[^5] solved it.

His explanation:

• Water molecules are real
• They’re constantly moving
• They’re constantly bombarding the pollen grain from all sides
• The jiggling you see IS the proof

The impact:

• He gave precise mathematical predictions
• Experiments confirmed them
• This finally convinced the last skeptics: atoms were real

Now that atoms were proven real, what was inside them?

Ernest Rutherford[^6] and his team fired alpha particles[^7] (tiny, positively charged bullets) at a super-thin sheet of gold foil—only a few atoms thick.

This experiment would change everything.

The Setup:

• Alpha particles: Positively charged particles from radioactive decay (helium nuclei)
• Shot from a radioactive source through a narrow slit
• Aimed at ultra-thin gold foil (only a few atoms thick)
• Detected by a fluorescent screen that flashes when hit

What they expected (based on plum pudding model):

• Particles would pass straight through
• Like bullets through tissue paper
• The positive charge was spread out, so nothing should stop them

What actually happened:

• Most particles passed through
• But some bounced straight back

Rutherford later said:

“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

The atom doesn’t have evenly spread positive charge.

Instead:

• There’s a tiny, incredibly dense, positively charged nucleus at the center
• Almost all the atom’s mass is packed into that tiny core
• The electrons orbit around it
• Most of the atom is empty space

Result:

• The plum pudding model was dead
• The nuclear model was born

Nucleus(tiny, dense, positive)

e⁻e⁻

Most of the atom is empty space