Atomic TheoryAtomic Theory

Chapter 3Chapter 3

Dalton (1803)Dalton (1803) Proposed that atoms are the smallest particles of an Proposed that atoms are the smallest particles of an

element.element. All atoms in that element are identical but they differ from All atoms in that element are identical but they differ from

those of other elements.those of other elements. There is a difference between a model of atoms and a There is a difference between a model of atoms and a

theory of atoms. A model focuses on describing what the theory of atoms. A model focuses on describing what the atoms are like, whereas the theory not only talks about atoms are like, whereas the theory not only talks about what the atoms are like but how they interact with one what the atoms are like but how they interact with one another and so forth. another and so forth.

Dalton's model was that the atoms were tiny, indivisible, Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined mass, size, and chemical behavior that was determined by what kind of element they were. by what kind of element they were.

Thompson (1896)Thompson (1896) Found that cathode rays could be deflected by Found that cathode rays could be deflected by

an electric field an electric field Showed that cathode "rays" were actually Showed that cathode "rays" were actually

particles particles Electron -  (originally called corpusclesElectron -  (originally called corpuscles by by

Thomson) particles given off by the cathode; Thomson) particles given off by the cathode; fundamental unit of negative electricity fundamental unit of negative electricity

Raisin (Plum) Pudding Model - Raisin (Plum) Pudding Model - Matter is electrically neutral and electrons are Matter is electrically neutral and electrons are

much lighter than atoms. much lighter than atoms. Conclusion:Conclusion:  There must be positively charged  There must be positively charged

particles which also must carry the mass of theparticles which also must carry the mass of theatom.  The main finding is that atom.  The main finding is that negatively negatively charged electrons carried the cathode ray charged electrons carried the cathode ray towards the positively charged anodetowards the positively charged anode. .

Cathode Ray Tube ExperimentCathode Ray Tube Experiment Sealed tube experiments of gases under a high Sealed tube experiments of gases under a high

voltage showed a stream of particles called voltage showed a stream of particles called cathode rays moving from cathode to anode.cathode rays moving from cathode to anode.

These rays were deflected towards the positive These rays were deflected towards the positive plate of an electrical field showing that they are plate of an electrical field showing that they are negatively charged. negatively charged.

Regardless of the type of the gas inside the tube Regardless of the type of the gas inside the tube unique cathode rays were produced. Cathode unique cathode rays were produced. Cathode rays are a stream of electrons. rays are a stream of electrons.

This experiment leads to the discovery of This experiment leads to the discovery of electrons and Thomson's "Plum-pudding" electrons and Thomson's "Plum-pudding" (blueberry muffin) model of the atom. (blueberry muffin) model of the atom.

Cathode Ray TubeCathode Ray Tube

Millikan (1909)Millikan (1909) Millikan measured the charge on an electron with his oil-Millikan measured the charge on an electron with his oil-

drop apparatus. drop apparatus. An "atomizer" from a perfume bottle sprayed oil or water An "atomizer" from a perfume bottle sprayed oil or water

droplets into the sample chamber.  Some of the droplets droplets into the sample chamber.  Some of the droplets fell through the pinhole into an area between two plates fell through the pinhole into an area between two plates (one positive and one negative).  This middle chamber (one positive and one negative).  This middle chamber was ionized by x-rays.  Particles that did not capture any was ionized by x-rays.  Particles that did not capture any electrons fell to the bottom plate due to gravity.  Particles electrons fell to the bottom plate due to gravity.  Particles that did capture one or more electrons were attracted to that did capture one or more electrons were attracted to the positive upper plate and either floated upwardthe positive upper plate and either floated upwardor fell more slowly. or fell more slowly.

Conclusion:  Conclusion:  The charge on a drop was always a The charge on a drop was always a multiple of 1.59 x 10-19 Coulombs.  He proved multiple of 1.59 x 10-19 Coulombs.  He proved Thomson's hypothesis that the mass of an electron is at Thomson's hypothesis that the mass of an electron is at least 1000 times smaller than the smallest atomleast 1000 times smaller than the smallest atom

Millikan’s ApparatusMillikan’s Apparatus

Rutherford (1909)Rutherford (1909) Studied the deflection of alpha particles as they Studied the deflection of alpha particles as they

were targeted at thin gold foil sheets. were targeted at thin gold foil sheets. Most of the alpha particles penetrated Most of the alpha particles penetrated

straight through. straight through. However few were deflected at slight However few were deflected at slight

angles. angles. Conclusion:Conclusion:  The positive charge and mass of  The positive charge and mass of

an atom were mainly in the center and only an atom were mainly in the center and only made up a small fraction of the atom.  He named made up a small fraction of the atom.  He named this concentrated center the nucleus.this concentrated center the nucleus.

Rutherford was also able to estimate the charge Rutherford was also able to estimate the charge of an atom by studying the deflection of alpha of an atom by studying the deflection of alpha particles.  He found that the positive charge on particles.  He found that the positive charge on the atom was approximately half of the atomic the atom was approximately half of the atomic weight. weight.

Alpha Scattering ExperimentAlpha Scattering Experiment

Niels Bohr (1911)Niels Bohr (1911)

The Bohr Model is known as the "planetary The Bohr Model is known as the "planetary model" of the atom. model" of the atom.

In the Bohr Model the neutrons and protons In the Bohr Model the neutrons and protons occupy a dense central region called the occupy a dense central region called the nucleus, and the electrons orbit the nucleus nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately are not confined to a plane as is approximately true in the Solar System). true in the Solar System).

Led to the calculation of possible energy levels Led to the calculation of possible energy levels for these orbits.for these orbits.

Quantum Mechanical ModelQuantum Mechanical Model

Max Planck (1900) suggests that radiation is Max Planck (1900) suggests that radiation is quantized (it comes in discrete amounts.)quantized (it comes in discrete amounts.) QUANTUM NUMBERSQUANTUM NUMBERS

Albert Einstein (1905), one of the few scientists Albert Einstein (1905), one of the few scientists to take Planck's ideas seriously, proposes a to take Planck's ideas seriously, proposes a quantum of light (the photon) which behaves like quantum of light (the photon) which behaves like a particle. Einstein's other theories explained the a particle. Einstein's other theories explained the equivalence of mass and energy, the particle-equivalence of mass and energy, the particle-wave duality of photons, the equivalence wave duality of photons, the equivalence principle, and special relativity.principle, and special relativity.

Atomic ModelsAtomic Models

Thomson:Thomson: Rutherford:Rutherford:

Atomic ModelsAtomic Models

Bohr Model:Bohr Model: Quantum Mechanical Quantum Mechanical Model:Model:

Electrons: Particles or Waves?Electrons: Particles or Waves?

Sometimes light displays particle-like behavior, Sometimes light displays particle-like behavior, and sometimes it acts like a wave; it all depends and sometimes it acts like a wave; it all depends on what sort of experiment you're doing. This is on what sort of experiment you're doing. This is known as wave/particle duality. known as wave/particle duality.

If we begin to think of electrons as waves, we'll If we begin to think of electrons as waves, we'll have to change our whole concept of what an have to change our whole concept of what an "orbit" is. Instead of having a little particle "orbit" is. Instead of having a little particle whizzing around the nucleus in a circular path, whizzing around the nucleus in a circular path, we'd have a wave sort of strung out around the we'd have a wave sort of strung out around the whole circle. whole circle.

Remember…Remember…

ScientistsScientists DaltonDalton Thomson – Cathode Thomson – Cathode

Ray Tube Ray Tube Millikan – Oil dropper Millikan – Oil dropper

Apparatus Apparatus Rutherford – Gold Foil Rutherford – Gold Foil

Alpha ScatteringAlpha Scattering BohrBohr PlanckPlanck EinsteinEinstein

ModelsModels Thomson ModelThomson Model Rutherford ModelRutherford Model Bohr ModelBohr Model Quantum MechanicalQuantum Mechanical

Sub-Atomic ParticlesSub-Atomic Particles

Protons – positive charge, found in the Protons – positive charge, found in the nucleus.nucleus.

Neutrons – no charge, found in the Neutrons – no charge, found in the nucleus.nucleus.

Electrons – negatively charged, found Electrons – negatively charged, found around the nucleus (electron cloud).around the nucleus (electron cloud).

Atomic StructureAtomic Structure Isotopes – same elements but different number Isotopes – same elements but different number

of neutrons.of neutrons. The mass number will INDIRECTLY give you the The mass number will INDIRECTLY give you the

number of neutrons.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 The gaining of electrons gives the atom a negative

charge.charge. The charge goes in the top right hand corner in a The charge goes in the top right hand corner in a

chemical symbol.chemical symbol.