![]() The idea that matter is made up of discrete units is a very old idea, appearing in many ancient cultures, including Greece and India. Particles which are truly indivisible are now referred to as "elementary particles". John Dalton applied the term to the basic units of mass of the chemical elements under the mistaken belief that chemical atoms are the fundamental particles in nature it was another century before scientists realized that Dalton's so-called atoms have an underlying structure of their own. ![]() The term "atom" comes from the Greek word atomos, which means "uncuttable". ![]() By the end of the 19th century, atomic theory had gained widespread acceptance in the scientific community. The concept that matter is composed of discrete particles is an ancient idea, but gained scientific credence in the 18th and 19th centuries when scientists found it could explain the behaviors of gases and how chemical elements reacted with each other. The current theoretical model of the atom involves a dense nucleus surrounded by a probabilistic "cloud" of electronsĪtomic theory is the scientific theory that matter is composed of particles called atoms. For the modern view of the atom which developed from atomic theory, see atomic physics. For a history of the study of how atoms combine to form molecules, see history of molecular theory. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom's interior, which he called the nucleus.This article is about the historical development of understanding the existence and behavior of atoms. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space. Rutherford needed to come up with an entirely new model of the atom in order to explain his results. In a famous quote, Rutherford exclaimed that it was "as if you had fired a 15-inch shell at a piece of tissue and it came back and hit you." No prior knowledge had prepared them for this discovery. Some were even redirected back toward the source. Surprisingly, while most of the alpha particles were indeed not deflected, a very small percentage (about 1 in 8000 particles) bounced off the gold foil at very large angles. Rutherford found that a small percentage of alpha particles were deflected at large angles, which could be explained by an atom with a very small, dense, positively-charged nucleus at its center (bottom).Īccording to the accepted atomic model, in which an atom's mass and charge are uniformly distributed throughout the atom, the scientists expected that all of the alpha particles would pass through the gold foil with only a slight deflection or none at all. (B) According to the plum pudding model (top), all of the alpha particles should have passed through the gold foil with little or no deflection. \): (A) The experimental setup for Rutherford's gold foil experiment: A radioactive element that emitted alpha particles was directed toward a thin sheet of gold foil that was surrounded by a screen which would allow detection of the deflected particles.
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