In 1830, John Dalton proved that matter is made of tiny building blocks called atoms.
Dalton envisioned atoms as solid spheres, the smallest particles in the universe.
About a century later, scientists discovered atoms contained even smaller particles.
JJ Thompson and the Electron (1897)
JJ Thompson designed a cathode ray tube filled with low pressure gas and two metal plates.
When charged, the positively charged plate (anode) attracted a beam of particles from the negatively charged plate (cathode).
Named the beam "cathode rays."
The particles had a negative charge as they bent toward the positive plate.
They were very light, thousands of times lighter than hydrogen atoms.
Thompson discovered the electron and won a Nobel Prize; he also mentored future Nobel laureates.
Ernest Rutherford and the Nucleus
Rutherford experimented with alpha particles emitted from uranium atoms.
He fired alpha particles at a thin sheet of gold foil and monitored their behavior.
Most alpha particles passed through gold foil, indicating atoms are mostly empty space.
Some alpha particles were deflected, while 1 in 20,000 bounced back, leading to the discovery of the atomic nucleus.
The nucleus contains concentrated positive charge; over 99% of an atom's mass is in the nucleus.
If an atom were a stadium, the nucleus would be the size of a pea.
Rutherford also discovered the proton, determining atomic number is equal to the number of protons.
Discovery of Neutrons
There was evidence of extra mass in the nucleus not accounted for by protons alone.
In 1932, James Chadwick discovered neutrons, particles with about the same mass as protons but no charge.
He conducted experiments using alpha particles on beryllium atoms, knocking neutrons out of the nucleus.
This work was significant during his life, including time as a prisoner during WWI and later contributing to the atomic bomb.
Summary of Atomic Structure
Atoms consist of:
A nucleus with positively charged protons and neutral neutrons.
Light, negatively charged electrons orbiting around the nucleus at high speeds (3/4 of the speed of light).
Electric forces hold the electrons in orbit around the nucleus, much stronger than gravitational forces (10^33 times stronger).
Conclusion
Next video will cover how electrons are arranged in shells around the nucleus and how this arrangement affects the properties of elements (gas, liquid, solid, color, brittleness, metal/non-metal classification).