Science

Chapter 4: Introduction to Atoms   Study Guide   Section 1: Development of the Atomic Theory   440 B.C.   Democritus.       Greek philosopher.     -discovered atom.   THOUGHTS:     you would end up with an “uncuttable” particle.       atoms were small, hard particles.       singlematerial formed into different shapes and sizes.       He was right.   -atom : the smallest particle into which an element can be divided and still be the same substance.       John Dalton.   -British chemist and schoolteacher.       experimented with different substances.     -results suggested that elements combine in certain proportions because they are made of single atoms.   HIS ATOMIC THEORY:   1) All substances are made of atoms.       small particles that cannot be created, divided, or destroyed.     2) atoms of same element = alike .   atoms of different elements = different.   3)Atoms join with other atoms to make new substances.   -formed theory from observations and many experiments.   -theory was changed step by step into the modern atomic theory.       J.J. Thomson.   -British scientist.   -discovered that there were small particles inside the atom.       atoms could be divided into even smaller parts.     -experimented with cathode- ray tube.     discovered that a positively charged plate attracted beam.       concluded that the beam was made of particles that have negative electric charges.       -concluded that these were in every type of atom.       called electrons.     -proposed new model '   plum pudding model.   -thought electrons were mixed throughout an atom.       Ernest Rutherford.   -Thomson’s former student.   -tested Thomson’s theory with the gold-foil experiment.   -proposed that in the center of the atom is a tiny, extremely dense positively charged part.     called the nucleus.         Niels Bohr.   -Danish scientist.     studied the way atoms react to light.       results led him to propose that electrons move around the nucleus in certain paths, or energy levels.       electrons can jump...