Chapter 2 Part A 2.1 Dalton?s Atomic Theory - if we look at a piece of copper with a powerful microscope, we cannot see atoms. The copper appears to be continuous. Lots of evidence was needed to show that matter is composed of the atoms we cannot see. Evidence for Atoms -Law of conservation of mass mass is not gained or lost in a chemical reaction Law of definite proportions a compound always has the same relative amounts of the elements that compose it for example, when water is broken down by electolysis into oxygen and hydrogen, the mass ratio is always 8 to 1. Dalton?s Atomic Theory In 1808 John Dalton published a paper on atomic theory, summarized by the following postulates: -All matter is composed of small, indivisble particles, called atoms. all atoms of a given element are identical both in mass and in chemical properties. Atoms of different elements have different masses and different chemical properties. atoms are not created or destroyed in chemical reactions atoms combine in simple, fixed, whole-number ratios to form compounds. STM Image of Gold -the scanning tunneling microscope, invented in 1981, allows us to create images of matter at the atomic level. Subatomic Particles -atoms are not as indivisble as Dalton imagined. -atoms are composed of subatomic particles. The three most important are: Proton, Neutron, Electron Evidence for the Electron in 1897 J.J Thomson?s experiments with cathode-ray tubes showed that matter is composed of negatively charged particles. The beam of particles is attracted to the positively charged plate. The Proton -scientists reasoned that if atoms have negatively charged particles, they must also have positively charged particles (protons)
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