Answer:
The scientist who first proposed a testable atomic theory and described atoms as hard and unbreakable was Democritus. He named the smallest indivisible particles "atomos," which means "uncuttable" in Greek. Democritus believed that atoms were the fundamental building blocks of matter and that they could not be further divided.
Aristotle, on the other hand, had a different view of matter. He believed that matter was made up of four elements: earth, air, fire, and water. According to Aristotle's theory, these elements combined in different proportions to form all substances.
John Dalton is credited with developing the modern atomic theory based on experimental evidence. He proposed that atoms were indivisible and indestructible particles that combined in specific ratios to form compounds. Dalton's atomic theory also stated that atoms of different elements had different masses and properties.
J.J. Thomson discovered the electron through his experiments with cathode rays. He observed that these rays could be deflected by electric and magnetic fields, suggesting the presence of negatively charged particles within atoms. Thomson proposed the "plum pudding" model of the atom, where electrons were embedded in a positively charged sphere.
Ernest Rutherford conducted the famous gold foil experiment, which led to the discovery of the atomic nucleus. He bombarded a thin sheet of gold foil with alpha particles and observed that some particles were deflected at large angles or even bounced back. This unexpected result led Rutherford to propose a model where most of the atom's mass was concentrated in a tiny, dense nucleus at its center.
Niels Bohr built upon Rutherford's work and developed the Bohr model of the atom. He proposed that electrons orbited the nucleus in specific energy levels or shells. Bohr's model explained why electrons did not spiral into the nucleus due to electromagnetic attraction.
In summary:
- Democritus was the first to propose a testable atomic theory and described atoms as hard and unbreakable.
- Aristotle believed that matter was made up of four elements: earth, air, fire, and water.
- John Dalton developed the modern atomic theory based on experimental evidence.
- J.J. Thomson discovered the electron and proposed the "plum pudding" model of the atom.
- Ernest Rutherford discovered the atomic nucleus through the gold foil experiment.
- Niels Bohr developed the Bohr model of the atom, which explained electron energy levels.
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