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Atoms: The Building Blocks of Matter Ch. 3 Early atomic theory • Democritus (400 B.C.) ▫ Atoms are nature’s basic particles ▫ “atom” Greek word meaning “indivisible” • Aristotle ▫ Disagreed, thought matter was continuous ▫ Aristotle's theory prevailed for next 2000 yrs. 1700s • 1st experimentation • Definition of element (pure sub. made of 1 kind of atom) • Elements combine compounds Late 1700s • Quantitative studies of reactions • law of conservation of mass Dalton’s Atomic Theory • All matter composed of atoms Atoms… • of same element are identical; atoms of diff. elements are diff. • cannot be divided, created, destroyed • combine in simple whole # ratios to form compounds (Law of Definite Proportions) • are combined, separated or rearranged in chem. rxns. Modern Atomic Theory • Not all of Dalton’s ideas were correct ▫ Atoms can be divisible ▫ Atoms of same element can have diff. masses • Most still holds true Sec. 2 - Structure of the Atom • Atom ▫ nucleus – protons & neutrons ▫ electron cloud Discovery of the e- (1897) • Cathode ray tube – electric current passed through gases, found neg. charged particles • JJ Thomson ▫ Did more expts. and found particles identical (no matter which gas used) ▫ Named them electrons • Millikan ▫ Oil drop exp. ▫ Mass of e- extremely small, neg. charged ▫ Atom is neutral, therefore must contain + charge also Choc. chip cookie model - + + - + - - + - + + + - - + + Discovery of Nucleus • Rutherford ▫ 1900 ▫ Gold foil exp. ▫ Confirmed dense nucleus with + charge • Chadwick discovered neutrons ~30 yrs. Later Modern atomic model • nucleus: p+ and n0 ▫ All the mass, very little space • e- cloud ▫ No mass, lots of space Nuclear forces • Like forces repel, normally • BUT when protons are really close there is a strong attractive force • Same with neutrons • Called nuclear forces (energy, bombs) Size of atom • Tiny • Radius from center of nucleus to edge of ecloud • Measured in pm (10-12) • Atomic radii range from 40-270 pm Sec. 3 - Counting Atoms • Atomic # ▫ # of protons in nucleus ▫ Determines what element Ex: oxygen is 8 8 p+ and 8 e- • Mass # ▫ Total # of p+ and n0 in an isotope Calculate # of neutrons: Mass # - atomic # (p + n) – (p) = n • Isotopes ▫ Atoms of same element with diff. # neutrons ▫ Diff. masses Ex: hydrogen 1 1H protium 99.985% 2 H 1 deuterium 0.015% 3 1H tritium trace, synthetic *Don’t differ much in chem. props. • Isotope symbols ▫ Hyphen ▫ Nuclear symbol uranium-235 235 92 U Charges Atomic # Mass # Protons Neutrons electrons 80 35 Br 35 80 35 45 35 80 35 Br-1 35 80 35 45 36 Ca+2 20 40 20 20 18 40 20 • amu’s ▫ atomic mass units ▫ masses so small, came up with scale that is relative ▫ amu = mass of 1/12 mass of carbon-12 atom • Average atomic masses ▫ # on periodic table ▫ Decimals averages Light • The photoelectric effect refers to the emission of electrons from a metal when light shines on the metal. Atomic Energy • The lowest energy state of an atom is its ground state. • A state in which an atom has a higher potential energy than it has in its ground state is an excited state. Hydrogen’s line emission spectrum • Visual Bohr Model • According to the model, the electron can circle the nucleus only in allowed paths, or orbits. • The energy of the electron is higher when the electron is in orbitals that are successively farther from the nucleus. • When an electron falls to a lower energy level, a photon is emitted • Energy must be added to an atom in order to move an electron from a lower energy level to a higher energy level.