Early Ideas about Matter atom • The smallest piece of the element with all the chemical properties of the element • an old and new idea Greeks • Discontinuists –Democritus • Continuists –Aristotle Discontinuists • Democritus • Believed there was a smallest piece • “atomos” = indivisible Continuists • Continuists thought there was no “smallest piece” • they believed a bar of gold, for example, could be cut into smaller pieces forever Aristotle’s ideas • There are four elements –earth, air, fire, and water • all matter is composed of these four elements in different proportions The Alchemists • Tried to turn lead into gold –put more fire into it –the “medicine stone” would “cure” the Pb • developed many lab techniques and apparatus The Renaissance • A new approach to science • experiment to test if the hypothesis is true • many of Aristotle’s ideas were shown to be wrong Law of Conservation of Mass • Lavoisier> France, 1790’s • Matter (or mass) is neither created nor destroyed in an ordinary chemical reaction Lavoisier did three important things: 1. brought in an entire new language for chemistry 2. established the correct identity of the elements and distinguished them from compounds, showing that those are combinations of elements 3. introduced the art of precise measurements. • He turned chemistry into a physical science and can properly regarded as the father of modern chemistry. Law of Definite Proportions • In a given compound, elements are always combined in the same proportion by mass. • Ex: H2O is always 11% H, 89% O by mass. Dalton’s Atomic Theory • All matter is composed of atoms –atoms are indestructible –“billiard ball” model • In a chemical reaction, are rearranged –conservation of mass atoms • All atoms of the same element are identical • Atoms of one element are different from the atoms of any other element –elements could be identified and distinguished from each other • Atoms combine to form compounds in small whole number ratios. –Ex: could be XY, X2Y, XY2, etc, but never X11/2Y What came next? • 49 new elements were discovered between 1801 - 1900 • new particles were discovered –alpha, beta, gamma - radiation –particles smaller than atoms • 1897: JJ Thomson discovered electrons – smaller than atoms – negatively charged • Robert Millikan – problem with Dalton’s model • 1914 Rutherford’s Gold Foil Experiment – nuclear model of atom • http://www.shsu.edu/~chm_tgc/sounds/pushmovies/ Gold Foil experiment setup Gold Foil Experiment - results online model Early Ideas about Matter Gold Foil Experiment - results online model • The repulsive force between the positively charged nucleus and positive alpha particles caused the deflections. Modern atom • Nucleus – in geometric center of the atom – very small and dense – nearly all of the atom’s mass – gives the atom its identity – protons (+) and neutrons – Nucleus held together by the “strong force” • Electron cloud –gives the atom its size –mostly empty space –electrons –electrons do not “orbit” like planets; more like bees/beehive Summary of the subatomic particles Summary of the subatomic particles particle where? Mass who? Proton + p Nucleus 1.007amu Rutherford o Neutron n Nucleus 1.008amu Chadwick - Electron e Electron 0.00055 cloud amu Thomson Atomic Number (Z) • the number of protons in the nucleus of an atom • gives the atom it’s identity • also, equal to the number of electrons in a neutral atom Mass Number (A) • Equal to the number of protons plus the number of neutrons in the nucleus of an atom • #no = mass # - atomic # • #no = A - Z Important note! • The atomic number of an element is on the periodic table, but the mass number is usually not! • The location of the numbers on the periodic table is at the printer’s discretion • The good news: –all atoms of the same element have the same atomic number • The bad news: –not all atoms of the same element have the same mass number • What is different? –The number of neutrons isotopes • Atoms of the same element with different mass numbers – different numbers of neutrons • different versions of the same element • no difference in chemical properties • only fundamental difference is in mass Isotope symbols • The elements symbol, plus... • the mass # (A) is written in the upper left corner • the atomic # (Z) is written in the lower left corner 82 36 Kr Examples of isotope symbols • Compare carbon-12, carbon-13, and carbon-14 • each have the same number of protons, but a different number of neutrons Examples of isotope symbols 12 6 13 6 14 6 C + 6p o 6n C + 6p o 7n C + 6p o 8n Isotopes and Mass Number Early Ideas about Matter Ions • An atom that is no longer neutral – #p+ ≠ #e- • No change in the nucleus, so the same element, but now with a + or - charge How does an atom become an ion? • The atom either gains or loses electrons • The nucleus is not directly involved in ionization • The number of protons does not change How does an atom become an ion? • If the atom gains electrons, it becomes negatively charged • Ex: If a sulfur atom, with it’s 16 protons and 16 electrons gains two electrons… • It now has a charge of (+16) + (-18) = -2 How does an atom become an ion? • If the atom loses electrons, it becomes positively charged • Ex: If an aluminum atom, with it’s 13 protons and 13 electrons loses three electrons… • It now has a charge of (+13) + (-10) = +3 anions • Atoms that have gained one or more electrons • Become negative (-) ions • Usually nonmetals cations • Atoms that have lost one or more electrons • Become positive (+) ions • Usually the metals

Early Atomic Theory

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