![]() ![]() ![]() Goldstein discovered canal rays, which have a positive charge equal to an electron.ġ896 Henri Becquerel discovered radiation by studying the effects of x-rays on photographic film.ġ897 J.J. Stoney theorized that electricity was comprised of negative particles he called electrons.ġ879 Sir William Crookes’ experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge.ġ886 E. Plucker built one of the first cathode-ray tubes.ġ869 Dmitri Mendeleev created the periodic table.ġ873 James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.ġ874 G.J. Democritus’ atomic theory posited that all matter is made up small indestructible units he called atoms.ġ704 Isaac Newton theorized a mechanical universe with small, solid masses in motion.ġ803 John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.ġ832 Michael Faraday developed the two laws of electrochemistry.ġ859 J. The element is said to "transmutate" into another element that is two z units smaller.400 B.C. The atom's atomic mass will decrease by four units (because two protons and two neutrons are lost) and the atomic number (z) will decrease by two units. An α particle contains two protons and two neutrons (and is similar to a He nucleus: ). There are three common types of radiation and nuclear changes:Īlpha Radiation ( α) is the emission of an alpha particle from an atom's nucleus. Unlike normal chemical reactions that form molecules, nuclear reactions result in the transmutation of one element into a different isotope or a different element altogether (remember that the number of protons in an atom defines the element, so a change in protons results in a change in the atom). All elements heavier than bismuth (Bi) (and some lighter) exhibit natural radioactivity and thus can "decay" into lighter elements. ![]() ![]() In 1902, Frederick Soddy proposed the theory that "radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element." Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a change in the atom itself. Marie Curie was the first woman to win a Nobel Prize and the first person to win two (the first, shared with her husband Pierre Curie and Henri Becquerel for discovering radioactivity the second for discovering the radioactive elements radium and polonium). Curie found that radiation was proportional to the amount of radioactive element present, and she proposed that radiation was a property of atoms (as opposed to a chemical property of a compound). Soon after Becquerel's discovery, Marie Sklodowska Curie began studying radioactivity and completed much of the pioneering work on nuclear changes. In 1896, Henri Becquerel expanded the field of chemistry to include nuclear changes when he discovered that uranium emitted radiation. Traditional chemical reactions occur as a result of the interaction between valence electrons around an atom's nucleus (see our Chemical Reactions module for more information). ![]()
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