99.9% of an atom's mass is concentrated in an incredibly dense nucleus. Have you ever heard of the 'plum pudding' model of the atom? It sounds quaint today, but it was generally accepted at the start of the 20th century that atoms were made of a mix of negatively charged electrons (behaving like plums) embedded in a positively-charged medium (which behaved like pudding) that filled all of space. Electrons could be stripped off or stolen, explaining the phenomenon of static electricity. For years, J.J. Thomson's model of a composite atom, with small electrons in a positively charged substrate, was generally accepted. Until, that is, it was put to the test by Ernest Rutherford.
By firing high-energy, charged particles (from radioactive decays) at a very thin sheet of gold foil, Rutherford fully expected that all the particles would pass through. And most of them did, but a few spectacularly bounced back! As Rutherford recounted:
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.
What Rutherford discovered was the atomic nucleus, containing virtually all the mass of an atom, confined to a volume one-quadrillionth (10-15) the size of the entire thing. It was the birth of modern physics, and it paved the way for the quantum revolution of the 20th century.
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