Atoms: Brainstorm

Zeno’s Paradox

Motion is Impossible

Is Matter Infinitely Divisible

• Is time infinitely divisible?

• Space?• Atoms?

Democritus (460B.C.-370B.C.)Atom: a = un, tomos = cuttable

Atoms are indivisible/indestructible

4.1

Aristotle: believed all things were made of 4 elements:

Dalton’s View of Atoms (1766-1844)

Dalton and Democritus

• What is wrong with Dalton’s atomic theory?

Billiard Ball Model

J.J. Thomson (1856–1940) Cathode Ray Tube Experiments

A Cathode Ray is Deflected by electrically charged plates

Inferring If a cathode ray is attracted to a positively charged plate, what can you infer about the charge of the particles that make up the cathode ray?

A Cathode Ray is Deflected by a magnet as well

Are electrons magnetic?

Lenz’s Law

• Electricity and Magnetism are the same force.

• A moving B (magnetic field) induces an electric current.

• That moving electric current can induce a magnetic field.

Because like charges repel and opposite charges attract: Thomson concluded that a cathode ray is a stream of negative particles now called electrons and that all atoms must have electrons.

Plum-Pudding Model of the Atom

• Protons and Neutrons were also discovered shortly after J.J. Thompson’s experiments. A new vision of the atom emerged.

• The atom was a positively charged sphere with negatively charged particles embedded in it.

Blueberry Muffin Model

Milikan & Fletcher 1909 (oil drop)• The experiment entailed balancing the

downward gravitational force with the upward drag and electric forces on tiny charged droplets of oil suspended between two metal electrodes. Since the density of the oil was known, the droplets' masses, and therefore their gravitational and buoyant forces, could be determined from their observed radii. Using a known electric field, Millikan and Fletcher could determine the charge on oil droplets in mechanical equilibrium. By repeating the experiment for many droplets, they confirmed that the charges were all multiples of some fundamental value, and calculated it to be 1.5924(17)×10−19 C, within 1% of the currently accepted value of 1.602176487(40)×10−19 C.

• They proposed that this was the charge of a single electron.

Rutherford’s Gold-Foil Experiment (1911)

Alpha particles scatter from the gold foil.

The Rutherford Atomic Model

• Rutherford concluded that the atom is mostly empty space. All the positive charge and almost all of the mass are concentrated in a small region called the nucleus.

• In the nuclear atom, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

Nuclear Atom

Rutherford’s atomic model could not explain why objects change

color when heated

Chemistry is such a Bohr (1913)

• Bohr devised the Planetary Model of the Atom based upon the work of Max Planck.

• Electrons orbit the Nucleus like planets orbit the sun.

• Electron energies are quantized (they exist at fixed levels).

Electron Orbits and Energy

• Orbitals are like swinging a paddleball.

• The more energy the further away from a nucleus an electron will be.

•When excited the electrons jumps up to another level.

Bohr’s Strange Laddder

• The higher the energy level occupied by an electron, the less energy it takes to move from that energy level to the next higher energy level.

Heisenberg Uncertainty Principle

• Werner Heisenberg in 1920 says we can’t know both the position and momentum of an electron in at atom.

• We can only know the probability of finding an electron in a given space.

1926 Schrödinger Wave-Quantum Mechanical Model of the Atom

• The probability of finding an electron within a certain volume of space surrounding the nucleus can be represented as a fuzzy cloud. The cloud is more dense where the probability of finding the electron is high.

• Electrons are quantized but they have very specific orbital shapes and configurations.

1932 Chadwick Discovers Neutron

What does a duck with an advanced degree in science say?

• Protons and Neutrons are made of quarks.

• Quarks STRINGS???

• Standard model physics• Leptons• Hadrons• Baryons• Mesons

Supercollider!

Atomic Mystery• Like charges repel • Opposite charges attract.

• Why don’t electrons rush into the nucleus?• Why doesn’t the nucleus pull itself apart?

Atomic history: Nature of ScienceFour hundred years before Christ, from the philosophical musings of Democritus came the solid, uniform, indivisible and indestructible atom. Over 2000 years later the English schoolteacher John Dalton published his atomic theory which said that atoms combine in simple whole number ratios and that atoms of one element are completely identical but different than atoms of another element. Atoms were still considered indivisible and uniform. JJ Thomson came along passing current through tubes of gas. Because opposite charges attract, he surmised via his Cathode Ray experiment that the atom had negative particles now called electrons. This led to the blueberry muffin model of the atom. A dilute positive sea of cake embedded with negative blueberries. The atom is no longer uniform or indivisible at this time. After Thomson came Rutherford with his famous gold foil experiment. He was shocked when a few of the positive alpha particles aimed at the foil came back in almost the opposite direction. Most of the particles did go straight through and this led him to conclude that there must be a small, dense, central region of positive charge in an atom. The nucleus was found and it was determined that most of the atom was completely empty with electrons whizzing around the nucleus. After Rutherford things get Bohring or more difficult to understand. Bohr came up with the planetary model of the atom. Electrons orbit the atom in fixed, quantized orbits just like the planets orbit the sun. If the electrons become excited, they can jump up an orbital and when they “hop back down”, light or “photons” thanks to Einstein's photoelectric effect, are emitted. In the last stage of atomic history we have the equations of Schrödinger and quantum mechanics telling us that electron energies are quantized as Bohr said, but that they exist in fuzzy clouds or probabilistic regions in an atom. Oppositely spinning electron pairs reside in these dumbbell, spherical and oddly shaped p,s,d, and f orbitals. No one can know exactly the location and momentum of an electron. We only know where it might be. Chadwick discovered the neutron in the 1930s and standard model physics has unearthed a whole host of other subatomic particles and we now know that baryons such as protons and neutrons are made up of quarks. The wheels of science keep turning and our models of the atom keeps getting refined.

Timeline of the Atom

Timeline of the Atom

Research Paper

• Format: APA • Length: 4 pages• Topic: How does the history of atomic theory relate

to the nature of science.

• You should explain the history of atomic theory in the paper and also what science is and how they relate. Remember in class we learned that science doesn’t “prove” things and the scientific method is cyclical.