He reasoned that the uranium compound was emitting some kind of radiation that passed through the cloth to expose the photographic plate. But in 1896, the French scientist Henri Becquerel found that a uranium compound placed near a photographic plate made an image on the plate, even if the compound was wrapped in black cloth. In 1932, Chadwick discovered the neutron and that the neutron helps stabilize the nucleus of the atom.Radioactivity and Nuclear Chemistry 3.1 Major Forms of Radioactivity Alpha Particle (α) Beta Particle (β) Gamma Radiation (γ) Positron Emission (β + decay) and Electron Capture Nuclear Fission 3.2 Radioactive Half Lives 3.3 Biological Effects of Radiation Exposure 3.4 Uses of Radioactive Isotopes 3.5 Chapter Summary 3.6 ReferencesĪtomic theory in the nineteenth century presumed that nuclei had fixed compositions. James Chadwick James Chadwick was an English physicist born in England on October 20, 1891. Finding one value (the energy or the location), means that there will be some uncertainty when finding the other value. The Uncertainty Principle states that the exact location and energy of an electron cannot be determined simultaneously. Heisenberg is known for his Uncertainty Principle in 1925. Werner Heisenberg Werner Heisenberg was a German theoretical physicist born in Germany on December 5, 1901. His work also led to the theory of different energy levels in atoms. In 1913, NielsBohr theorized that electrons travel around an atomic nucleus in a stationary orbit. NielsBohr NielsBohr was a Danish physicist born in Denmark on October 7, 1885. While experimenting with Rutherford, Soddy observed that radioactive elements decayed into other elements and produced alpha, beta, and gamma radiation in 1913. Using his oil drop experiment, he discovered the mass of electrons and positively charged atoms.įrederick Soddy Frederick Soddy was an English radio chemist born in England on September 2, 1877.
The mass of the droplets and the density of the oil were known, so the gravitational and buoyant forces could be calculated from the measured radii of the oil drops. In 1910, Millikandiscovered the downward gravitational force and the upward electrical and buoyant forces of charged oil droplets suspended between two metal plates. Robert Millikan Robert Millikan was an American physicist born in Illinois on March 22, 1868. Using his gold foil experiment, he discovered that something dense and positively charged was inside the atom, which was the nucleus. In 1907, Rutherford fired alpha particles at a thin gold foil with a cathode ray and observed how a particle was rebounded of the gold foil and back to the ray. In 1897, Thomson developed the planetary model of the atom, stating that these positive and negative particles revolved around the atom.Įrnest Rutherford Ernest Rutherford was a British chemist born in New Zealand on August 30, 1871. Thomson first suggested that atoms were made of positive and negative particles. Thomson was a British physicist born in the United Kingdom on December 18, 1856. Accidentally, Henri had discovered radioactivity. In 1896, Henri accidentally left a piece of uranium on a photographic plate and saw that it left an image even without light. Henri Becquerel Henri Becquerel was a French chemist born in France on December 15, 1852. He is also credited for discovering the proton. In 1885, Goldstein experimented with cathode rays and noticed that a second stream of particles were attracted to the negatively charged electrode and called them anodes. He arranged the first 63 previously discovered elements in order according to their atomic weight and chemical and physical properties.Įugene Goldstein Eugene Goldstein was a German physicist born in Poland in September 5, 1850. In 1869, Mendeleev developed the Periodic Table of Elements. His theory stated that atoms are indestructible and indivisible atoms of a given element have identical masses and properties compounds are a combination of two or more different atoms a chemical reaction is a rearrangement of atoms.ĭmitri Ivanovich Mendeleev Dmitri Mendeleev was a Russian chemist born in Siberia in 1834. In 1803, Dalton developed his own atomic theory.
John Dalton John Dalton was an English chemist in Eaglesfield, England on September 6, 1766. His theory was, however, repeatedly questioned due to the fact that he had not conducted any experiments to prove it. These atoms cannot be divisible or destroyed.
Democritus’ atomic theory stated that everything is composed of atoms. History of Atomic Theory By: Tiyan Singletaryĭemocritus Democritus was a Greek philosopher born in 460 B.C.