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Chapter 4 Chapter 4- Section 1 Covers: Atomic Theory Elecron Nucleus Different Models “Atomos” – Greek for indivisible or unable to be cut or divided They determine the properties of matter Matter is made up of particles called atoms Democritus – thought atoms were indivisible & indestructible Lacked experimental support th 4 century B.C. John Dalton – (1766-1844) Dalton’s Atomic Theory 1. All elements are composed of tiny, indivisible atoms 2. Atoms of the same element are identical. Atoms of 1 element are different from another element Dalton’s Atomic Theory (cont) 3. Atoms of different elements can either physically mix or chemically combine in simple, whole number ratios to form compounds Dalton’s Atomic Theory (cont) 4. Chemical Reactions occur when atoms are separated, joined or rearranged. Atoms of 1 element cannot change into another element by a chemical reaction. Dalton’s Theory is mostly accepted today Except that we now know atoms CAN be divided - into subatomic particles (i.e. electrons, protons, neutrons) JJ Thompson (1856-1940) discovered electrons using a device called a cathode ray tube Cathode Ray Tube •He sealed gases in a tube fitted at both ends with electrodes (metal disks) •Connected to a source of high voltage electricity A glowing beam formed between the 2 electrodes He called it the cathode ray CATHODE has a negative charge Cathode Ray Tube The cathode ray is attracted to metal plates that have a positive charge It is repelled by negatively charged plates From Catode(-) to Anode(+) The ray must be composed of negatively charged particles moving at a high speed He called these particles electrons ANODE has a positive charge JJ Thomson’s model of the atom is called the plum-pudding model He thought the electrons were randomly placed throughout the atom, like the currants in a plum pudding Ernest Rutherford discovered the nucleus He used a thin sheet of gold foil and bombarded it with alpha particles (helium nuclei) Most particles passed through the foil as he expected However, some were deflected slightly and came through at an angle Some particles bounced back at him He hypothesized that there must be a dense positive part of the atom, but most of the atom is empty space Rutherford’s Gold Foil Experiment Bohr – proposed that electrons have a fixed energy and move in energy levels around the nucleus – which is why they don’t fall into the nucleus The energy levels are like the rungs of a ladder – electrons cannot be in between levels, and need a specific amount of energy to move from one to another Dalton – thought atoms were solid and indivisible JJ Thomson – discovered the electron, & made the plum-pudding model Rutherford – discovered the nucleus Bohr – proposed electrons in orbitals around nucleus What do you already know? What are the subatomic particles? How can you distinguish between the subatomic particles?
