Atomic Theory Chapter 3

Dalton (1803) Proposed that atoms are the smallest particles of an element. Proposed that atoms are the smallest particles of an element. All atoms in that element are identical but they differ from those of other elements. All atoms in that element are identical but they differ from those of other elements. There is a difference between a model of atoms and a theory of atoms. A model focuses on describing what the atoms are like, whereas the theory not only talks about what the atoms are like but how they interact with one another and so forth. There is a difference between a model of atoms and a theory of atoms. A model focuses on describing what the atoms are like, whereas the theory not only talks about what the atoms are like but how they interact with one another and so forth. Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were. Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

Thompson (1896) Found that cathode rays could be deflected by an electric field Found that cathode rays could be deflected by an electric field Showed that cathode "rays" were actually particles Showed that cathode "rays" were actually particles Electron - (originally called corpuscles by Thomson) particles given off by the cathode; fundamental unit of negative electricity Electron - (originally called corpuscles by Thomson) particles given off by the cathode; fundamental unit of negative electricity Raisin (Plum) Pudding Model - Raisin (Plum) Pudding Model - Matter is electrically neutral and electrons are much lighter than atoms. Matter is electrically neutral and electrons are much lighter than atoms. Conclusion: There must be positively charged particles which also must carry the mass of the atom. The main finding is that negatively charged electrons carried the cathode ray towards the positively charged anode. Conclusion: There must be positively charged particles which also must carry the mass of the atom. The main finding is that negatively charged electrons carried the cathode ray towards the positively charged anode.

Cathode Ray Tube Experiment Sealed tube experiments of gases under a high voltage showed a stream of particles called cathode rays moving from cathode to anode. Sealed tube experiments of gases under a high voltage showed a stream of particles called cathode rays moving from cathode to anode. These rays were deflected towards the positive plate of an electrical field showing that they are negatively charged. These rays were deflected towards the positive plate of an electrical field showing that they are negatively charged. Regardless of the type of the gas inside the tube unique cathode rays were produced. Cathode rays are a stream of electrons. Regardless of the type of the gas inside the tube unique cathode rays were produced. Cathode rays are a stream of electrons. This experiment leads to the discovery of electrons and Thomson's "Plum-pudding" (blueberry muffin) model of the atom. This experiment leads to the discovery of electrons and Thomson's "Plum-pudding" (blueberry muffin) model of the atom.

Cathode Ray Tube

Millikan (1909) Millikan measured the charge on an electron with his oil- drop apparatus. Millikan measured the charge on an electron with his oil- drop apparatus. An "atomizer" from a perfume bottle sprayed oil or water droplets into the sample chamber. Some of the droplets fell through the pinhole into an area between two plates (one positive and one negative). This middle chamber was ionized by x-rays. Particles that did not capture any electrons fell to the bottom plate due to gravity. Particles that did capture one or more electrons were attracted to the positive upper plate and either floated upward or fell more slowly. An "atomizer" from a perfume bottle sprayed oil or water droplets into the sample chamber. Some of the droplets fell through the pinhole into an area between two plates (one positive and one negative). This middle chamber was ionized by x-rays. Particles that did not capture any electrons fell to the bottom plate due to gravity. Particles that did capture one or more electrons were attracted to the positive upper plate and either floated upward or fell more slowly. Conclusion: The charge on a drop was always a multiple of 1.59 x Coulombs. He proved Thomson's hypothesis that the mass of an electron is at least 1000 times smaller than the smallest atom Conclusion: The charge on a drop was always a multiple of 1.59 x Coulombs. He proved Thomson's hypothesis that the mass of an electron is at least 1000 times smaller than the smallest atom

Millikan’s Apparatus

Rutherford (1909) Studied the deflection of alpha particles as they were targeted at thin gold foil sheets. Studied the deflection of alpha particles as they were targeted at thin gold foil sheets. Most of the alpha particles penetrated straight through. Most of the alpha particles penetrated straight through. However few were deflected at slight angles. However few were deflected at slight angles. Conclusion: The positive charge and mass of an atom were mainly in the center and only made up a small fraction of the atom. He named this concentrated center the nucleus. Conclusion: The positive charge and mass of an atom were mainly in the center and only made up a small fraction of the atom. He named this concentrated center the nucleus. Rutherford was also able to estimate the charge of an atom by studying the deflection of alpha particles. He found that the positive charge on the atom was approximately half of the atomic weight. Rutherford was also able to estimate the charge of an atom by studying the deflection of alpha particles. He found that the positive charge on the atom was approximately half of the atomic weight.

Alpha Scattering Experiment

Niels Bohr (1911) The Bohr Model is known as the "planetary model" of the atom. The Bohr Model is known as the "planetary model" of the atom. In the Bohr Model the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). In the Bohr Model the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). Led to the calculation of possible energy levels for these orbits. Led to the calculation of possible energy levels for these orbits.

Quantum Mechanical Model Max Planck (1900) suggests that radiation is quantized (it comes in discrete amounts.) Max Planck (1900) suggests that radiation is quantized (it comes in discrete amounts.) QUANTUM NUMBERS QUANTUM NUMBERS Albert Einstein (1905), one of the few scientists to take Planck's ideas seriously, proposes a quantum of light (the photon) which behaves like a particle. Einstein's other theories explained the equivalence of mass and energy, the particle- wave duality of photons, the equivalence principle, and special relativity. Albert Einstein (1905), one of the few scientists to take Planck's ideas seriously, proposes a quantum of light (the photon) which behaves like a particle. Einstein's other theories explained the equivalence of mass and energy, the particle- wave duality of photons, the equivalence principle, and special relativity.

Atomic Models Thomson: Thomson: Rutherford: Rutherford:

Atomic Models Bohr Model: Bohr Model: Quantum Mechanical Model: Quantum Mechanical Model:

Electrons: Particles or Waves? Sometimes light displays particle-like behavior, and sometimes it acts like a wave; it all depends on what sort of experiment you're doing. This is known as wave/particle duality. Sometimes light displays particle-like behavior, and sometimes it acts like a wave; it all depends on what sort of experiment you're doing. This is known as wave/particle duality. If we begin to think of electrons as waves, we'll have to change our whole concept of what an "orbit" is. Instead of having a little particle whizzing around the nucleus in a circular path, we'd have a wave sort of strung out around the whole circle. If we begin to think of electrons as waves, we'll have to change our whole concept of what an "orbit" is. Instead of having a little particle whizzing around the nucleus in a circular path, we'd have a wave sort of strung out around the whole circle.

Remember… Scientists Scientists Dalton Dalton Thomson – Cathode Ray Tube Thomson – Cathode Ray Tube Millikan – Oil dropper Apparatus Millikan – Oil dropper Apparatus Rutherford – Gold Foil Alpha Scattering Rutherford – Gold Foil Alpha Scattering Bohr Bohr Planck Planck Einstein Einstein Models Models Thomson Model Rutherford Model Bohr Model Quantum Mechanical

Sub-Atomic Particles Protons – positive charge, found in the nucleus. Protons – positive charge, found in the nucleus. Neutrons – no charge, found in the nucleus. Neutrons – no charge, found in the nucleus. Electrons – negatively charged, found around the nucleus (electron cloud). Electrons – negatively charged, found around the nucleus (electron cloud).

Atomic Structure Isotopes – same elements but different number of neutrons. Isotopes – same elements but different number of neutrons. The mass number will INDIRECTLY give you the number of neutrons. The mass number will INDIRECTLY give you the number of neutrons. Ions – charged particles. Ions – charged particles. The loss of electrons gives the atom a positive charge (+). The loss of electrons gives the atom a positive charge (+). The gaining of electrons gives the atom a negative charge. The gaining of electrons gives the atom a negative charge. The charge goes in the top right hand corner in a chemical symbol. The charge goes in the top right hand corner in a chemical symbol.