History of the History of the Development of Atomic Development of Atomic

TheoryTheoryChemistryChemistry

Chapter 4Chapter 4

Mr. GilbertsonMr. Gilbertson

Models – help us visualize complex Models – help us visualize complex ideas and conceptsideas and concepts

• Cannot see atoms directly must view indirectly (STM)

• Physical Model – a representation of a very large or small object shown at a convenient size

• Conceptual Model – an explanation that treats what is being explained as a system.

• System – interactions between component parts are what is important

• Accuracy is determined by how well it predicts the behavior of the system.

STM ImagesSTM Images

Additional images

New technologyAtomic Force Microscopy (AFM)

Atomic ModelsAtomic Models

• Best described by conceptual models• Not a representation of what it would look like

but how it behaves.• Models must evolve as new information is

discovered which cannot be explained by the model.

• The most useful models are almost always mathematical.

• Early models are based on spectral analysis

Timeline

History of the Development of History of the Development of Atomic TheoryAtomic Theory

• Democritus proposed the name – “atmos” meaning “indivisible” hypothesized that matter could be cut into two parts until it reached this level

• John Dalton – formally proposed the atom, suggested that elements were made up of tiny solid spheres. They differ in mass and size. He believed atoms could combine and rearrange to form new substances

Important laws in development Important laws in development of Atomic Theoryof Atomic Theory

• Law of definite proportions – regardless of the amount, a compound is always composed of the same elements in the same proportions (ratio)

• Law of multiple proportions – When different compounds are formed by a combination of the same elements, different masses of one eloement combine with the relative mass of the other element in a ratio of small whole numbers.

Important laws in development Important laws in development of Atomic Theoryof Atomic Theory

• Law of definite proportions – regardless of the amount, a compound is always composed of the same elements in the same proportions (ratio)

• Law of multiple proportions – When different compounds are formed by a combination of the same elements, different masses of one eloement combine with the relative mass of the other element in a ratio of small whole numbers.

History of the Development of History of the Development of Atomic TheoryAtomic Theory

• Robert Brown – noticed that grains of pollen moved about continuously when floating in a drop of water. He suggested that this was due to bombardment by atoms (Brownian motion).

JJ ThomsonJJ Thomson

• At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. This is a tiny negatively charged particle that is much, much smaller than any

• He discovered the electron are tiny, negatively charged particles that orbit the nucleus of an atom in energy levels (or shells). electron, Thomson was experimenting by applying high voltages to gases at low pressure. He noticed an interesting effect.

JJ Thomson apparatus

Discovery of the electron

The Thomson ModelThe Thomson Model• Thomson proposed a different

model for the atom. He said that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread through a Christmas pudding – but with lots more space in between).

Mass of the Mass of the ElectronElectron

• Thompson was unable to determine the mass of an electron directly with the cathode ray tube.

• He was able to determine the ratio or mass of an electron to its charge

• By comparing this ratio for several known elements he concluded the mass must be much less than a Hydrogen atom.

• This meant that particles smaller than the atom existed.

Robert Millikan’s Oil drop Experiment

•Determined the charge on an electron•His value is within 1% of the currently measured value•Determined to be -1.6 x 10 -19 Coulombs•What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. •Found that it always required a small whole number multiple.

Once the charge was determined the masscould be calculated using the mass/charge ratio.

Ernest RutherfordErnest Rutherford

• The next development came about 10 years later. Two of Ernest Rutherford's students, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that Thomson's model described. However, they got a big surprise.

Rutherford’s Experiment

Video

The Rutherford ModelThe Rutherford Model

• In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. In this model, the electrons orbited around the dense nucleus (centre of the atom).

The Rutherford ModelThe Rutherford Model

Came to be known as the Planetary Model.Volume of the atom is 10,000 times the volume

of the nucleus where all mass is located.

Niels BohrNiels Bohr

• The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one.

Discovery of ProtonDiscovery of Proton• Rutherford proposed that the nucleus contained another

particle he called a proton• A proton is a particle carrying a charge equal to but

opposite that of an electron.• A proton has a positive charge (1+).

• Demonstrated with an anode ray tube filled with

hydrogen.

Discovery of the NeutronDiscovery of the Neutron• Rutherford predicted the existence of a

third particle which contributed mass but had no charge.

• James Chadwick demonstrated the existence of the Neutron.

Has the mass of a proton and no electrical charge, hence the name Neutron.

Atomic FactsAtomic Facts• Spherical in shape• Tiny dense nucleus of

positive charge• Surrounded by

negatively charged electrons held in place by attraction to positively charged nucleus (electron cloud

• Mostly empty space• Nucleus contains

protons and neutrons• Nucleus contains

99.97% of mass• Atom is electrically

neutral, having equal numbers of protons and electrons

How Atoms Differ• Atomic Number

– Defined as the number of protons (+) in the nucleus of an atom of an element

– All atoms of an element have the same number of protons

– Determines the position of elements on Periodic Table

– Since most atoms are neutral (no charge) also equals the number of electrons (-)

Mass Number

• Equals the total number of nucleons (protons and neutrons) within the nucleus of an atom.

• Determined for an element by taking average atomic mass and rounding off to nearest whole number (usually)

• Isotopes – atoms of an element with different mass numbers because of differences in the number of neutrons

Average Atomic Mass

• The mass shown on the periodic table

• The average mass of all of the different isotopes of an element based on the relative abundance of each

• Mass is measured using the Atomic Mass Unit (AMU) which equals 1/12th of the mass of a carbon-12 atom.

Properties of Subatomic Particles

Particle Name

Mass in AMU

Charge Location Symbol

PROTON 1 AMU1.67 x 10-24g

Positive

+ Nucleus P+

NEUTRON 1 AMU1.67 x 10-24g

Neutral

0 Nucleus No

ELECTRON 0 AMU1/1840 AMU

Negative

-

Electron Cloud e-

Determining Particle Numbers from Periodic Table

• # P+ = AN = # e-

• MN = # P+ + # No

• # No = MN - AN

RadioactivityRadioactivity

• The process by which certain elements emit particular forms of radiation.

• Three major forms of radiation– Alpha – combination of 2P+ and 2No – Beta – an electron which is ejected from the nucleus

as a neutron decays.– Gamma – extremely energetic form of

electromagnetic radiation (beyond x-ray) has no mass or charge but most damaging.

• All forms of radiation are dangerous.

RadiationRadiation

Alpha ParticlesAlpha Particles• Made up of a helium nucleus • Mass of 4AMU • +2 charge• Slow moving, do not easily penetrate solids due

to mass and charge• Have great kinetic energy (energy of motion)• Can do significant damage to surfaces

especially living tissue.• Easy to protect against damage.• Eventually captures electrons and becomes

harmless helium.

Beta ParticlesBeta Particles

• An electron which is ejected from the nucleus as a neutron decays

• Small mass but negative charge• Very fast moving particle not so easy to stop.• Can penetrate skin and cause damage to cells• Can be stopped by dense materials like

aluminum foil• Once stopped they become part of the material

that stopped them.

Gamma RaysGamma Rays

• Like light a form of electromagnetic wave• Much higher frequency than visible light so

much greater energy.• Has no mass or charge and much energy so it

can pass right through objects• Has the capacity to do great damage to living

tissue, potentially causing mutations or cancer.• Can also be very useful in the process of food

preservation.• Can be stopped by dense materials such as

lead but must be thick.

Radioactivity is a Natural Radioactivity is a Natural PhenomenonPhenomenon

• Has been around as long as there have been atoms.

• A certain fraction of the atoms (isotopes) of almost every element are radioactive.

• There are ways that the average dosage can be needlessly increased.

• Dosage is cumulative, which means it continues to accumulate.

• Cells can repair damage if not too severe.• Mutations can occur if damage is to DNA.

Natural Sources of RadiationNatural Sources of Radiation

Radioactivity can be used to Radioactivity can be used to determine age of materialsdetermine age of materials

• Radioactive materials have a half-life, the time needed for half of the material to decay to a stable non-radioactive form.

• Half-life is constant for a given material.– Carbon-14 becomes Nitrogen-14 – 5730 years– Uranium-238 becomes Lead-206 – 4.5 billion yrs.

• By comparing amounts of radioactive element and decay product we can determine the number of half-lives elapsed since formation.

The EndThe EndElement song

Nuclear FissionNuclear Fission• The splitting of the atom.• Uranium-235 nucleus when bombarded by

neutrons becomes unstable and separates into two smaller less massive atoms.

• A tremendous amount of energy is also released in the process.

• The process also releases additional neutrons, and gamma radiation.

• Neutrons released can go on to create a chain reaction.

• Before a chain reaction can occur there must be a certain “critical mass”.

Nuclear Fission as an Energy SourceNuclear Fission as an Energy Source

• By using the tremendous heat produced to boil water it is possible to generate electricity.

• 1kg of uranium fuel can produce the same energy as 30 freight car loads of coal.

• Without producing any CO2 emissions.

• Produces radioactive wastes (small amounts) no good way of disposal.

Nuclear ReactorsNuclear Reactors

E=mcE=mc22

• The energy from a nuclear reactor comes from the direct conversion of matter (mass) to energy.

• This equation proposes that matter and energy are two interchangeable quantities, two aspects of the same thing.

• There is a loss of mass in a nuclear reaction but there is an increase in energy.

EnergyEnergy

• Defined as the ability to do work or cause change to itself or its surroundings.

• Energy is calculated as the force times the distance over which it is applied.

• The amount of energy produced is directly proportional to the mass of the matter that is lost.

Nuclear FusionNuclear Fusion• Fusion involves taking small nuclei and

combining them to form heavier nucleus.• 4 hydrogen nuclei (protons) are combined (fuse)

to form a helium nucleus (42He).

• In the process a tremendous amount of energy is released. (reaction of the sun)

• Can only occur at very high temperatures (millions of degrees) called thermonuclear fusion.

• 657million tons of H is converted to 653 million tons of He every second on the sun. Lost mass is converted to energy.

Nuclear WeaponsNuclear Weapons

• Fission bombs are limited because of the need for critical mass.

• Fusion Bombs are not limited and are a thousand times more destructive than the bomb dropped on Hiroshima at the end of WWII. (hydrogen or thermonuclear bomb)

• The bomb is technology and as such can be seen as good or bad.

Controlled FusionControlled Fusion

• Requires temperatures in the millions of degrees, will melt any container.

• Must be confined by magnetic field, called containment (nonmaterial container)

• At this temperature hydrogen gas loses its electrons and becomes free protons, with a positive charge

• The superheated gas is called a plasma which has a positive charge which can be controlled by a magnetic field.

Fusion ProcessFusion Process

Controlled FusionControlled Fusion• Once the heated gas is confined it is

compressed by the magnetic field causing it to heat even more. (1 million degrees)

• Nuclei are moving so fast that they are able to overcome the natural repulsive forces.

• The reaction produces less energy than is consumed at this point. So there is no gain in energy.

• At 100 million degrees the reaction still does not produce enough energy to be self-sustaining

• It take 350 million degrees to make a reaction self- sustaining.

• Now all that is needed is a steady feed of nuclei.

Fusion Fusion ReactorReactor

High Energy LasersHigh Energy Lasers

• So far the magnetic confinement has been disappointing and produced instabilities in the plasma.

• A new approach uses lasers to implode a small vial of solidified hydrogen.

• Theoretically this will produce a dense plasma 20 times more dense than lead.

• Could produce hundreds of times more energy than is required to fire the lasers.

• The problem is development of stable, reliable high energy lasers.

Use of Use of High High

Energy Energy LasersLasers

Energy for the FutureEnergy for the Future• Fusion could provide a cheap continuous source

of energy for the future• It uses hydrogen for fuel (91% of the universe is

hydrogen) • It produced helium as a waste product, harmless

gas.• Future humans could theoretically fuse the

elements that they require and there would be no shortages of natural resources. They would also be producing large amounts of energy in the process.