Atomic Theory Timeline

Leucippus and Democritus

~400 B.C.E

Leucippus and Democritus

-Greek philosophers who were among the first to suggest the existence of atoms

-Reasoned that matter is discontinuous and atoms were indivisible and indestructible

-Lacked experimental support

(http://chemed.chem.purdue.edu/genchem/history/leucippus.html)

John Dalton

1766-1844

John Dalton

-Used experimental methods to transform Leucippus and Democritus's ideas into a scientific theory

-Studied the ratios in which elements combine in chemical reactions

Formed Dalton's atomic theory:

1. All elements are composed of tiny indivisible particles called atoms

2. Atoms of the same element are identical

3. Atoms of different elements can phycially mix together or can chemically combine to form compounds

4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed.

(Wilbraham, et. al "Chemistry" Pearson)

J.J. Thomson

1897

J.J. Thompson

- Discovered the electron

-Used experiments that passed electric current through gases at low pressure in a glass tubes with electrodes at both ends

-Resulted in a cathode ray that traveled from the positively charged anode to the negatively charged cathode

-Thomson knew that opposite charges attract and like charges repel so he hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed and were later named electrons

-Used additional tests to measure the ratio of an electron's charge to its mass

(Wilbraham, et. al "Chemistry" Pearson)

Robert A. Millikan

1868-1953

Robert Millikan

- A U.S. physicist who found the quantity of an electron's charge

-Used oil-drop experiments and suspended negatively charged oil droplets between two charged plates. He then changed the voltage on the plates to see how it affected the rate of fall.

-He found that the charge on each oil droplet was a multiple of 1.60E-19 which was the charge of an electron

-Millikan was able to calculate the electron's mass and is similar to those accepted today

(Wilbraham, et. al "Chemistry" Pearson)

Ernest Rutherford

1911

Ernest Rutherford

-Wanted to test the existing plum-pudding model by the gold-foil experiment

-Used alpha particles directed at a very thin sheet of gold foil where the particles went straight through the gold foil or were slightly deflected at very large angles

-From the plum-pudding model, the particles should have passed easily with a slight deflection

-Suggested a new theory of the atom that is is mostly empty space because of the lack of deflection from the alpha particles.

-Concluded that all the positive charge and almost all the mass are concentrated in a small region called the nucleus that has enough positive charge to account for the great deflection of some of the alpha particles

(Wilbraham, et. al "Chemistry" Pearson)

Neils Bohr

1913

Niels Bohr

-Developed a new model of the atom by proposing that electrons are arranged in concentric circular orbits around the nucleus.

-Proposed theory based on quantum theory that energy is transferred only in certain well defined quantities; therefore, electrons should move around the nucleus but only in orbits

(https://www.nobelprize.org/prizes/physics/1922/bohr/facts/)

Erwin Schrödinger

1926

Erwin Schrodinger

-An Austrian physicist who used mathematics to further advance the Bohr atom model

-Used equations to describe the likelihood of finding an electron in a certain position so his quantum mechanical model of the atom does not define the exact path of an electron but predicts the locations of the electron.

-Model introduced the concept of sub-energy levels

(http://www.abcte.org/files/previews/chemistry/s1_p6.html)

James Chadwick

1932

James Chadwick

-An English physicist who confirmed the existence of the neutron

-Bombarded beryllium atoms with alpha particles and produced an unknown radiation. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton

(http://www.abcte.org/files/previews/chemistry/s1_p6.html)