Notes about scientists

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 AP Chemistry Name ______________________________________

Period ___ Date ___/___/___

2  Atoms and Elements


Use this grid to fill in information about scientists. Some are our “signpost” scientists who made major contributions toward our understanding of the atom. Others contributed smaller important ideas that help with the overall story.


Their “Experiment”

What they added to the atomic theory

Name of their model


No real experiment. He had the “thought experiment” about whether matter was continuous or discontinuous. He stated that matter was discontinuous; as you look closer and closer, you eventually get to a tiny indivisible particle, “atomos”

If we need to look back to the first person who said matter was made up of small, discrete particles, we come to Democritus.

No actual model, but he had the idea of “atomos” which means indivisible.

John Dalton

Dalton was exploring the solubility of gases.

As part of his work, Dalton came up with the idea that there were atoms (atoms) and compound atoms (molecules). Some substances are made of their own atoms, but many substances were made of different combinations of the same atoms.

He thought of the atom as a hard, little sphere. We will call this the “billiard ball” model.

J.J. Thomson

J.J. was exploring gas discharge tubes. He showed that the beam of electrons (cathode rays) had a negative charge. He also was able to bend the beam using electric and magnetic fields leading to calculating the charge-to-mass ratio.

He discovered the electron. He determined the charge to mass ratio of the electron. He didn’t know the charge or the mass, but he knew the ratio. With Millikan’s work on the charge of the electron, we now know the charge and mass of the electron.

He thought of the atom as a positive mass with embedded electrons, like a “plum pudding”.

In modern terms, I call this the chocolate chip cookie dough model.

Ernest Rutherford

Using the new technology of Bequerel Rays, Rutherford shot alpha particles at a thin gold sheet. A few of the particles bounced back.

This shows that most of the mass is concentrated in a tiny, dense, positively-charged nucleus.

Rutherford described his atom as similar to a small solar system with the electrons circling a tiny, dense, nucleus.

Some of these scientists are named and described in the video, some will come from your textbook.


Notes: (What did they do that helped develop the atomic theory?) Refer to Question 8 in the Ch 2 Study Questions.

Michael Faraday

He worked with electrolysis and showed that matter and electricity were closely related. Electrical forces hold atoms together.

Joseph Proust

Matter has a definite composition. H2O is always 11% H and 89% O (by mass). This is obvious now, but back then, this was a surprising finding. Analogy, if we broke down a cookie, we could get many different compositions, but if you break down a particular compounds, you always get the same composition. Dalton explained this with atoms.

Antoine Lavoisier

Lavoisier (who was beheaded in the French Revolution) was one of the first people to carefully measure the masses of reactants and products. In a closed system, he was able to show that when a reaction occurs, mass is conserved.

Robert Millikan

With his famous oil-drop experiment (Physics Laser Disk?) he was able to determine the charge of an electron.

Benjamin Franklin

He found that lightning was electricity and defined positive and negative electrical charges.

Henri Becquerel

Discovered radioactivity (although he did not name it).

Marie Curie

Studied radioactivity and figured out that the Becquerel rays were coming from atoms that were disintegrating. She coined the term, radioactivity.

James Chadwick

Discovered the neutron.

Henry Moseley

His work helped define the number of protons as the atomic number and explained some of the anomalies of Mendeleev’s periodic table because he showed that periodicity depended on atomic number, not atomic mass.

Dmitri Mendeleev

Developed the first periodic table of the elements based on chemical properties of the substances such as oxygen combination. He predicted undiscovered elements and did not strictly follow the rule that elements in each period had to increase in atomic mass.

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