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Models of the Atom

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Review of Ancient Models of MatterLeucippus and Democritus -

the idea of the “atom” – the indivisible particle

Empedocles and Aristotle - the 4-element model

Aristotle, Aquinas and The Church – the 4-element model persists for over 2000 years.

The Alchemists – develop new chemical lab equipment and techniques but do not apply the Scientific Method

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Enter the ScientistsThe great breakthroughs made by Galileo,

Copernicus and Isaac Newton made way for a revolution in pure science in Europe.

Great scientists such as Boyle, Lavoisier, Priestly and Scheele began to make strides in the field of Chemistry.

Scientists such as Francis Bacon and Rene Descartes developed the process we know as the Scientific Method.

The first scientist to apply scientific methodology to the study of matter and atoms was John Dalton

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John DaltonDalton was a schoolteacher and later

a professor of Mathematics and Natural philosophy (Science)

His strongest interests were in the field of Meteorology (Dalton’s Law of Partial Pressures) but he also made contributions in Chemistry

He began to make measurements of the masses of elements and created a simple table of elements.

He then went on to conduct experiments with chemical reactions and determined the masses and ratios of elements in the newly formed compounds.

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Dalton’s Chemical Formulae

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Dalton’s Atomic TheoryIn 1803, Dalton suggested an atom

is a solid featureless sphere.Billiard Ball Model of the AtomDalton made two assertions about

atoms:1) atoms of each element are all

identical to one another but different from the atoms of all other elements

2) atoms of different elements can combine to form more complex substances.

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Five main points of Dalton's atomic theory1) Elements are made of extremely small particles

called atoms.2) Atoms of a given element are identical in size,

mass, and other properties; atoms of different elements differ in size, mass, and other properties.

3) Atoms cannot be subdivided, created, or destroyed.

4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

5) In chemical reactions, atoms are combined, separated, or rearranged.

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Problems with Dalton’s ModelAtoms exhibit electrical properties – charged atoms, static electricity, etc…

The true nature of atoms could not be determined since atoms were extremely small and no known methods of testing individual atoms was known

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J.J. ThomsonIn 1897, J.J. Thomson conducted

experiments using a cathode ray tube.Thomson’s was able to create a stream of

particles that flowed through the tube and fluoresced when they hit a detectoe screen

Using magnets Thomson was able to bend the flow of particles.

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JJ Thomson’s Model of the Atom The particles were called “electrons” because

of their electrical propertiesElectrons were determined to have very

small masses and a negative charge (they were repelled by the negative force of a magnet)

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J.J. Thomson’s Model of the AtomAtoms contain particles called electronsElectrons have a small mass and a negative

chargeThe rest of the atom is a sphere of positive

chargeElectrons are embedded in this sphere, so

that the resulting atoms are neutral or uncharged

Thomson compared the (negative) electrons to plums embedded in a positively charged pudding

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J.J. Thomson’s Model of the AtomHence the Plum Pudding Model or

Raisin Bun Model

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Ernest RutherfordIn 1895, the German scientist Roentgen

discovered X-rays and started a rush of work to study the electromagnetic radiation emitted by radioactive elements.

In the early part of the 20th century, many scientists such as the Curies, Becquerel, Geiger, and Rutherford studied these invisible rays.

Scientists began using these minute waves to study the properties of atoms.

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Ernest RutherfordRutherford decided to test the model of

his mentor, Thomson.In 1911, Ernest Rutherford conducted his

famous gold foil experiment.He aimed alpha particles (positively

charged particles) at a thin sheet of gold foil

PREDICTION: Particles would pass straight through the gold foil

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The Gold Foil Experiment

OBSERVATION: A small number of alpha particles bounced almost straight back from the gold foil

PREDICTED

OBSERVED

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Ernest Rutherford’s Model of the AtomAn atom has a tiny, dense,

positive core called the nucleus (which deflected the alpha particles and contains protons)

The nucleus is surrounded mostly by empty space, containing rapidly moving negative electrons (through which the alpha particles passed unhindered)

Ernest Rutherford

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Ernest Rutherford’s Model of the Atom

Rutherford’s atom had a tiny, dense, positive nucleus orbited by electrons. Most of the atom’s volume is empty space

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James ChadwickIn 1932, Chadwick

conducted experiments that suggested that at least half of the mass of the nucleus was unaccounted for.

Chadwick’s research suggested the existence of a neutral particle in the nucleus with similar mass to the proton.

James Chadwick – the real Jimmy Neutron

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Updated Model of the Atom

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Modern Development of the Model of the Atom

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A Problem with the Planetary ModelClassical Newtonian Physics suggest that

any object travelling in a curved path (even if it is moving at a constant speed) is said to be accelerating.

An electron accelerating around the nucleus would continuously emit electromagnetic radiation and lose energy

Therefore, it would eventually fall into the nucleus and the atom would collapse

However, not consistent with real-world observations – atoms are stable

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Learning CheckpointAssigned Questions

p. 15 UC # 1, 2, 3, 4, 5, 6