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Chapter 4 Pg. 100-118 ISN page 39 Studying Atoms Ancient Greek-1897 Book page 100-103 ISN pg. 41 Ancient Greek Models of Atoms • Democritus– believed that all matter consisted of extremely small particles that could not be divided – called particles atoms from the Greek word atomos, which means “uncut” or “indivisible” – thought there were different types of atoms with specific sets of properties Ancient Greek Models of Atoms • Aristotle– thought that all substances were built up from only 4 elements: earth, air, fire, and water – matter could be divided an unlimited number of times Dalton’s Atomic Theory • John Dalton – born in England in 1766 – Teacher – Interest in predicting the weather, led to his study of the behavior of gases in air -- Based on the way gases exert pressure, he correctly concluded that a gas consists of individual particles Dalton’s Atomic Theory • Evidence for Atoms – Measured the masses of elements that combine when compounds form. – Noticed that no matter how large or small the sample, the ratio of the masses of the elements in the compound is always the same (aka fixed composition) Dalton’s Atomic Theory • Dalton’s Theory- Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided. • Dalton’s Model- solid spheres with different masses Dalton’s Atomic Theory • Main Points of Theory 1. All elements are composed of atoms. 2. All atoms of the same element have the same mass, and atoms of different elements have different masses. 3. Compounds contain atoms of more than one element. 4. In a particular compound, atoms of different elements always combine in the same way Dalton’s Atomic Theory • Not all of Dalton’s ideas about atoms were completely correct, but scientists did not discard his theory, they revised it to take into account new discoveries Dalton’s Atomic Theory Thomson’s Model of the Atom • When rubbed, some materials gain the ability to attract or repel other materials. • Based on their behavior, they are said to have either positive or negative electric charges. • Objects with like charges repel or push apart. Thomson’s Model of the Atom • Objects with opposite charges attract, or pull together. • Some charged particles can flow from one location to another. • Thomson’s Experiment: used electric current to learn more about atoms. A flow of charged particles is called an electric current Thomson’s Model of the Atom Thomson’s Model of the Atom • Thomson’s Experiment (pg. 102) – Hypothesized that the beam was a stream of charged particles that interacted with the air in the tube and cause the air to glow. – Placed charged metal plates at either end-> caused beam to deflect or bend – Observed beam: repelled by negatively charged plate and attracted by positively charged plate Thomson’s Model of the Atom • Evidence for Subatomic Particles – Concluded that the particles in the beam had a negative charge because they were attracted to the positive plate. – Thomson’s experiments provided the first evidence that atoms are made of even smaller particles. – Revised Dalton’s model to account for the subatomic particles Thomson’s Model of the Atom • Thomson’s Model – Atom is neutral: meaning it has neither a negative nor a positive charge – Negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter – “Plum pudding” model (may want to think of chocolate chip ice cream) Thomson’s Plum Pudding Model