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Scientist Democritus—440 B.C John Dalton—1800 J.J. Thomson—1898 Ernest Rutherford—1911 Niels Bohr—1913 History of Atomic Theory Greek Philosopher who was the first person to suggest the idea that atoms caused the changes in matter that could be observed. He also proposed the name “atom”. “Atom” comes from a Greek word that means “cannot be destroyed.” Thought that atoms: 1. Were always moving 2. Were infinite in number 3. Could be joined together Conducted experiments on gases that led to the development of the first atomic theory. 1. All elements are composed of small, solid balls called atoms, 2. Said that atoms cannot be created or destroyed. 3. Atoms of the same element are exactly alike, but atoms of different elements are different from each other. 4. The atoms of two or more elements can join together to form types of matter called compounds. First scientist to suggest that atoms were made up of smaller particles. Discovered negatively charged rays when passing electric currents through gases. He found that all atoms have positive centers and negatively charged particles evenly scattered throughout. These negative particles are called electrons. Discovered atoms were mostly empty space surrounding a positive center in his gold foil experiment. Concluded that the positively charged particles were contained in a small central core called nucleus surrounded by electrons. Suggested the modern atomic theory Electrons in an atom move in specific energy levels and orbit the nucleus. Electrons only exist in certain orbits at certain energy levels. Energy levels are a certain distance from the nucleus and contain a specific number of electrons (Level 1—2, Level 2—8, Level 3—18)