History of Atomic Theory

History of Atomic Theory

Mr. Hendley - Chemistry

What is Matter made of?

• Since ancient times, scientists have pondered this question.

• Democritus, a Greek philosopher around 400 BC, proposed matter was made of basic units that could not be cut apart.

• He called these atomon (Greek for "uncuttable").

A Differing View

• Aristotle disagreed with Democritus's idea.

• He argued that matter made of particles would fall apart, like sand.

• Aristotle proposed matter was infinitely divisible.

• He also proposed the idea of basic elements: fire, earth, air, water and aether (the stuff of the heavens)

So, who was right?

• Neither idea could be proven correct because there was no experimental support.

• So both ideas reigned for approx. 2,000 years...

Multiple Choice

Who proposed the idea of atoms?

• Democritus

• Aristotle

• Einstein

• Newton

Years go by...

Scientists experiment and determine things over the years.

As scientists do more and more experiments and as measurement and science emerge, certain laws appear to those paying attention.

The Law of Conservation of Mass

• States: "For any chemical reaction, mass is neither created nor destroyed in the chemical reaction."

• The mass of the reactants must equal the mass of the products.

The Law of Definite Proportions

• Also known as the Law of Constant Composition.

• States: "A chemical compound always contains the same elements in exactly the same proportions by mass.

• For example, glucose (C₆H₁₂O₆) is always 42.1% by mass carbon, 51.4% by mass oxygen and 6.5% by mass hydrogen

Law of Definite Proportions

• Consider the image to the right:

• The "magic ratio" is 10.00 g of lead for 1.56 g of sulfur to make 11.56 g of lead sulfide.

• However, if you increase the sulfur (2nd example) or the lead (3rd example), it does not change the amount of lead sulfide that can be made.

Law of Definite Proportions

• However, if viewed from the perspective of an atom, this makes more sense.

• Lead sulfide is one lead atom and one sulfur atom. Adding more of either does not allow more of the compound to be made.

• To make more lead sulfide, you must add BOTH.

Law of Multiple Proportions

• States: "Whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small, whole numbers."

• For example, water is H₂O and Hydrogen peroxide is H₂O₂. Both contain just hydrogen and oxygen.

Law of Multiple Proportions

• Consider Carbon and Oxygen

• 1.333 grams of oxygen combine with 1 gram of carbon to form carbon monoxide.

• However, 2.666 grams of oxygen combine with 1 gram of carbon to form carbon dioxide.

• The ratio between 1.333 and 2.666 is a whole number ratio: 1-to-2.

Multiple Choice

Which law would best explain the scenario pictured?

• law of conservation of mass

• law of definite proportions

• law of multiple proportions

• law of gravity

John Dalton

• 1803

• Postulated Atomic Theory to explain the 3 previous laws.

Multiple Select

Which laws was John Dalton trying to explain? (Check all that apply)

• Law of conservation of mass

• Law of conservation of energy

• Law of definite proportions

• Law of multiple proportions

Dalton's Atomic Theory

• 1) All matter is composed of extremely small particles called atoms.

• 2) Atoms of a given element are identical in size, mass and other properties.

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

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

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

Dalton's Theory Exceptions

• We do know there are some exceptions to his original theory today, 200 years later...

• Isotopes are atoms of the same element with different masses.

• Atoms can be subdivided (protons, neutrons, electrons)

• Atoms can be created (fusion) or destroyed (E=mc²)

Dalton's Atomic Model

Dalton's atomic model was simply wooden spheres with holes to model bonding.

Multiple Choice

Dalton said "Atoms cannot be subdivided, created or destroyed." Which law did this help explain?

• law of conservation of mass

• law of definite proportions

• law of multiple proportions

Multiple Choice

Dalton said "Atoms of different elements can combine in simple, whole-numbered ratios to form chemical compounds." Which law is this least related to?

• law of conservation of mass

• law of definite proportions

• law of multiple proportions

Sir Joseph John Thomson

• 1897

• Worked with Cathode Ray Tubes (CRTs)

• Credited with discovering the electron

Thomson and the Electron

• CRTs had two electrodes that emit a beam when hooked up to an electric power source.

• It moved away from a negative chaarge or towards a positive charge.

• Because opposites attract (and like charges repel), the beam must have a NEGATIVE charge!

Thomson and the Electron

• By using an electric field, Thomson was able to demonstrate the amount of deflection experienced by the cathode ray.

• This allowed him to calculate the mass to charge ratio of the electron.

Electrons are matter

• A paddlewheel was placed in the CRT

• When the beam was turned on, the wheel moved down the CRT

• Energy, like light, would not make the wheel move.

• Thus, electrons have mass and are matter (not energy).

Thomson's Atomic Model

• The atom is neutral, so the atom is a big positive sphere with negative electrons throughout.

• Same in all directions

• Called the "Plum-Pudding" model because electrons are like raisins in a plum pudding.

Robert Millikan

• 1908-1917

• Oil-drop experiment

• Calculated charge and mass of the electron

Millikan's Oil-Drop Experiment

• Oil droplets were sprayed and hit with an X-ray to charge them.

• The charged plates were adjusted to have the charged oil droplet hover.

• The change in charge let him determine the charge of each electron.

• Using Thomson's ratio, he calculated the electron's mass.

Multiple Choice

What was Dalton's contribution?

• The idea of the atom

• Atomic Theory

• Discovered the electron

• Calculated the mass of the electron

Lord Ernest Rutherford

• 1911

• Gold-foil experiment

• Discovered the nucleus of the atom

Gold-Foil Experiment: The Hypothesis

• Gold foil, only a few atoms thick, was hit with positively-charged alpha particles from a radioactive piece of polonium (Po).

• Testing Thomson's plum-pudding model.

• Hypothesis: The alpha particles should go straight through the gold foil, with only some small deflection.

Gold-Foil Experiment: The Result

• Rutherford's hypothesis was mostly correct: most of the particles went straight through.

• However, some deflected more than expected. (only about 1 in 80,000 bounced back)

• Ultimate Conclusion: The atom has a positively-charged nucleus.

Conclusion: Nucleus is massive

• Alpha particles have a hefty mass

• Whatever they were bouncing off of must also be massive!

• Conclusion: The nucleus contains most of the atom's mass.

Conclusion: Nucleus is positive

• Alpha particles have a positive charge

• Whatever they were bouncing off of must also have a positive charge!

• Conclusion: The nucleus has a positive charge.

Conclusion: Nucleus is small

• Most of the alpha particles went straight through the atom.

• Conclusion: The nucleus is very, very small. (Rutherford compared it to a grape hanging in a cathedral.)

Conclusion: The atom is mostly empty space

• Most of the alpha particles went straight through the atom.

• Conclusion: The atom is mostly made of empty space. (Pretty close to original Rutherford hypothesis.)

Rutherford's Atomic Model: The Nuclear Atom

• Atoms have a dense, positive nucleus where most of the mass of the atom is concentrated.

• Electrons occupy the empty space.

James Chadwick

• 1932

• Beryllium-wax experiment

• Discovered the neutron

The neutron

• In an experiments, Chadwick discovered a particle that did not have a charge.

• This neutral charged particle was named a neutron.

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Multiple Choice

Rutherford's hypothesis was mostly correct.

• True

• False

Multiple Choice

Which subatomic particles have no charge?

• proton

• neutron

• electron

• nucleus

Multiple Choice

Which scientist carried out the gold-foil experiment?

• John Dalton

• J. J. Thomson

• Ernest Rutherford

• James Chadwick

Multiple Choice

In the image of Dalton's list of elements, what element is no longer considered an element in the modern periodic table?

• hydrogen

• lime

• iron

• zinc

Multiple Select

Which scientist(s) proposed an atomic model? (mark all that apply)

• Democritus

• John Dalton

• J.J. Thomson

• Ernest Rutherford

• James Chadwick