Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Development of the Atomic Theory The atomic model The Greek Model Democritus was the first to propose an atomic theory around 440 B.C. He theorized that if you cut, for example, a coin in half and cut that half in half and that half in half and so on, eventually you would end up with an “uncuttable” particle. He named this particle the atom from the Greek word atomos meaning indivisible. He claimed Atoms are small, hard particles made up of a single material. Atoms are always moving Atoms join together to form different materials Dalton’s Model In the late 1700’s, John Dalton performed many experiments and from them developed a theory. All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms in specific proportions to make new substances. Thomson’s Model In 1887, J. J. Thomson performed an experiment that identified an error in Dalton’s theory and proposed the Plum-Pudding model. Atoms can be divided into smaller pieces. Atoms contain negatively charged particles he called corpuscles that are identical. Today they are called electrons. Atoms also contain a positively charged substance because atoms are neutral. Atoms are made up of a positively charged pudding like material with electrons located throughout like plum-pudding. Thomson’s Experiment Rutherford’s Model In 1909, Rutherford performs an experiment changing Thomson’s model. The atom has a small, dense, positively charged center he called the nucleus. The atom is mostly empty space Electrons orbit the nucleus in definite energy levels like planets orbit the sun. Rutherford’s Experiment The Bohr Model In 1913, Neils Bohr proposed a new theory. While electrons had to be in “special orbits””, electrons could jump from one level to another. Today’s Atomic Model Many twentieth-century scientists have contributed to today’s atomic model. The atom contains a small, positively charged center called the nucleus. The nucleus contains both positively charged protons and neutrally charged neutrons. Negatively charged electrons orbit the nucleus in an electron cloud. The location is impossible to determine as they gain and lose energy constantly. As electrons gain energy they move away from the nucleus. As they lose energy they move closer to the nucleus. The atom is made up of mostly empty space. Let’s Review!