History of Atomic Theory

Democritus

404 BCE

Greek Philosopher born around 460 B.C. and lived till 370 B.C. formed the idea of the atom and believed they were indestructible. Democritus also posed that atoms must be moving or vibrating and were brought together to form larger bodies or worlds.

John Dalton

1803

(1766-1844) Dalton discovered that gas behaves independently because like atoms in gas repel each other instead of attracting as atoms usually would believing that elements had their own kind of atom and that the atoms in different elements vary in size and mass. He the created law of multiple proportions which is when two elements form more than one compound, masses from one element that combine with a fixed mass of the other are in a ratio of small numbers.

Cathode Ray Tube Experiment

1897

J.J. Thomson used cathode tubes to discoer electrons by vacuum sealing them with elements in them and a cathode and anode on the inner side of them. Because the cathode is negatively charged and the anode is positively charged, the electrons would flow towards the anode where there was a small hole that allowed them to create a beam of electrons. The other side of the tube had a coating which made the electrons glow when passed, allowing them to become visible.

J.J. Thomson

1897

Used a cathode tube rays to display that the rays are negatively charged, which led to his discovery of the electron. With this, he found that all matter is made of particles smaller than atoms called corpuscles, later called electrons. Later, he discovered isotopes in stable elements through his study of positively charged ions. He won a Nobel Prize in Physics. Thomson was born in Cheetham Hill, England on December 18, 1856, lived in and would die in Cambridge August 30, 1940

Oil Drop Experiment

1909

Millikan had a closed chamber with two metal plates that had a negative or positive charge when an electrical current is applied. He would spray oil drop in the upper part of the chamber; with gravity and air resistance, some droplets would fall through a hole in the top plate. Space between the plates is ionized with radiation, which causes electrons to attach to oil droplets and have a negative charge and can be seen with light. Millikan would then calculate the mass by the speed it falls.

Robert Millikan

1909

Used the Oil Drop Experiment to discover the value for electron charge,e. Millikan measured the charge of oil drops in an electric field, originally used water but it was not precise. He was born on March 22, 1868 in Morison Illinois, and would live in Chicago as a professor until moving to San Marino California to become the director of The Norman Bridge Laboratory and the California Institute of Technology and die on December 19, 1953.

Ernest Rutherford

1911

The first to discover atoms have a small charged nucleus, known as the Planetary or Rutherford Model, with the Gold Foil Experiment which deflected alpha particles through gold foil, displaying the nuclear nature of atoms. Rutherford also won a Nobel Prize in Chemistry in 1908. Born August 1871 in Brightwater New Zeland and died October 19, 1937.

Gold Foil Experiment

1911

Ernest Rutherford described the structure of an atom along with the nucleus through the gold foil experiment. He did this by having a radioactive source emit alpha particles similar to a helium nucleus placed inside a lead shield with the radiation stream going through a slit in the lead. Then a piece of gold foil and a coat of zinc sulfide were placed in front of the slit to detect the alpha particles and the particles would then stick to the screen with light, which determined alpha particles.

Niels Bohr

1913

The Danish Physicist published articles in "The Philosophical Magazine" that electrons only occupy certain orbits by the quantum action and electromagnetic radiation from atoms when the electrons jumped to lower orbits of energy. The Bohr model describes the atomic electrons and their possible values when they jump between states, which absorbs or emits radiation. Bohr would win a Nobel Prize in 1922 for his contribution to Physics (1885-1962).

Erwin Schrodinger

1926

The Austrian scientist lived from 1887 to 1961 and received a Nobel Prize for his work in Physics. Schrodinger used the wave equation with Bohr's Model to create the quantum mechanical model of the atom, which describes the probability of finding an electron in a specific position.

James Chadwick

1932

(1891-1974) Chadwick is a British Physicist who is responsible for the discovery of the neutron. Chadwick found that when alpha particles went with Beryllium and emitted a stream of neutral radiation while finding evidence of Rutherford's tightly bound "proton-electron pair". In 1935 Chadwick won a Pece Prize for his discoveries.