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The Atom What is an atom? • An atom is the smallest part of an element • The atom remained mostly a mystery because it is unable to be seen with even a microscope • Over time there have been many models of what an atom looks like Ancient Greek Model • Greek philosophers asked “What would happen if you kept cutting a piece of paper in half until there is nothing left?” • They believed that at some point a single, smallest particle, that could not be divided would remain Ancient Greek Model • The Greek model was not highly accepted at the time • Aristotle refuted these claims and most people agreed with his argument Dalton’s Atomic Theory • In 1766, Dalton concluded that gas is made up of billions of particles • He gathered his evidence by measuring the masses of elements in compound form Dalton’s Atomic Theory • He concluded that no matter how large or small the sample of a compound is, the ratio of the masses of the elements is always the same • Compounds have fixed compositions Dalton’s Atomic Theory • Dalton developed a theory – “All matter is made up of individual particles called atoms, which cannot be divided.” Dalton’s theory cont. • All elements are composed of atoms • All atoms of the same element have the same mass, and atoms of different elements have masses • Compounds contain atoms of more than one element • In particular compounds, atoms of different elements always combine in the same way Dalton’s Atomic Model Thomson’s Atomic Model • In 1856, J.J Thomson used an electrical current to learn more about the atom • He used an electrical current, metal discs, and a beam of light to study his theory Thomson’s Atomic Model • He found that particles in a beam of light have a negative charge • He found that these particles of light were even smaller than an atom Thomson’s Atomic Model • Thomson’s experiments provided the first evidence that atoms are made up of even smaller particles • We call these “Subatomic Particles” Thomson’s Atomic Model • Thomson concluded that an atom has a neutral charge (Neither positive, nor negative) • He designed an atom that has negative charges scattered throughout a positively charged mass Thomson’s Atomic Model Rutherford’s Atomic Theory • In 1899 Rutherford discovered that uranium emits fast moving particles that have a positive charge • He named theses particles “Alpha particles” • He studied what would happen to alpha particles as they pass through a thin sheet of gold. Rutherford’s Atomic Theory • 1/20,000 alpha particles were deflected 90 degrees off of the gold • The rest bounced straight off of the gold foil Rutherford’s Atomic Theory • Rutherford concluded that the deflected alpha particles must have come close to a charged object • After studying this theory he concluded that atoms contain a nucleus The Nucleus • A dense, positively charged mass located at the center of an atom • According to Rutherford, all of an atom’s positive charge is concentrated in its nucleus