This confirmed that the cathode rays were composed of negatively charged particles. Thomson replaced the magnetic field with an electric field in a cathode ray tube with an improved vacuum, and he showed that the cathode rays were attracted to the positive plate and repelled by the negative plate. Crookes also showed that the results of his experiments were independent of the nature of the gas in the tube. In 1879, William Crookes found that the cathode rays were deflected by a magnetic field, so he postulated that the rays were composed of negatively charged particles. Plücker observed a green glow on the wall of the tube and suggested that it was due to a ray coming from the cathode. There are two electrodes in the tube, and a strong voltage sends an electric current between the electrodes. A cathode ray tube consists of a sealed tube that has most of the air pumped out of it. As we will see, Thomson based his theory of the atom in part on experiments done with a cathode ray tube, which was developed by the glassblower Heinrich Geissler and German physicist Julius Plücker in the 1850s. I’ll just describe a few of the steps that led to Thomson’s work. Thomson is generally given credit for the discovery of the electron, but like with most scientific discoveries, the work of others made his discovery possible. A series of experiments done in the late 19th century and the early 20th century showed that atoms are composed of protons, neutrons, and electrons. The clue to the explanation of the differences among atoms of different elements lay in the discovery of the internal structure of the atom. For example, his theory could not explain why lithium and sodium are so similar when sodium is almost three times heavier than lithium or why elements that are closer size, such as lithium and helium are so different. It was improbable that size or mass alone could account for the widely varying chemical and physical properties of the elements. Because compounds are always composed of the same elements in specific ratios with respect to the number of atoms of each element, and because atoms of the same element have the same mass and atoms of different elements have different masses, specific compounds must be composed of the same elements in the same definite proportion by mass.ĭalton was not able to provide an adequate explanation for why atoms of one element are different from another. Because the atoms carry the mass, and because the atoms are conserved, the mass is conserved.ĭalton’s Atomic Theory can also be used to explain the Law of Definite Proportions. In Dalton's view, chemical reactions involve a rearrangement of the atoms, but the atoms themselves are conserved. Compounds are composed of combinations of atoms of two or more elements with the atoms combining in a specific ratio with respect to the number of atoms of each element.ĭalton’s Atomic Theory can be used to explain the Law of Conservation of Mass. Chemical reactions involve the separation and combination of atoms, but the atoms themselves are never created, destroyed, or even changed.Ĥ. (It was later found that isotopes of the same element can have atoms with different masses.)ģ. Atoms of the same element have the same mass, and atoms of different elements have different masses. Elements are composed of tiny, indivisible particles of matter called atoms.Ģ. With help from other scientists, in the early 19th century, John Dalton developed a theory that can be used to explain the Law of Conservation of Mass and the Law of Definite Proportions. The importance of these laws for us is that they provided 19th and 20th century scientists with the challenge of how to explain them. (The modern version qualifies this a bit.) The second scientific law that concerns us is the Law of Definite Proportions (or Constant Composition) that states that a specific compound always contains the same elements in the same definite proportions by mass. The first scientific law that affected the development of atomic theory is the Law of Conservation of Mass (or Matter) that states that matter and thus mass are neither created nor destroyed in the process of normal chemical reactions. One way to distinguish between scientific laws and scientific theories is that scientific laws describe what happens, and scientific theories explain why things happen. Scientific Laws are general statements about nature that are based on repeated experiments or observations. Our story starts with two of the scientific laws suggested in the late 18th and early 19th centuries. The purpose of this webpage is supplement what I did in the text by describing this history. When I was writing my text, I decided that I wanted to devote more time and space to describing the modern model of the atom instead of including the traditional description of the historical development of atomic theory.
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