sci

• Atom


  The basic unit of matter
• Democritus


  • Proposed that all matter was composed of small indestructible particles, which he called "atoms"
  • The word "atomos" means "indivisible"
• Aristotle


  • Strongly opposed Democritus's idea of the atom
  • Believed that all things can be divided infinitely
  • Claimed that all matter consisted of four elements: earth, fire, water, and air
• Dalton's Atomic Theory


  • Matter is composed of small indivisible particles called atoms
  • Atoms of the same element are identical
  • Compounds contain atoms of more than one element
  • Chemical reactions involve only the separation, combination, or rearrangement of atoms
• Dalton formulated his atomic theory based on Lavoisier's Law of Conservation of Mass and Proust's Law of Definite Proportion
• Law of Conservation of Mass


  The total mass of substances before and after a reaction are equal (matter can neither be created nor destroyed)
• Law of Definite Proportion


  Different samples of the same compound always contain constituent elements in the same proportion by mass
• Law of Multiple Proportion


  If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers
• Thomson's Atomic Model


  • Described the atom as a positively charged sphere with negatively charged electrons embedded in it (the "plum-pudding" or "bread-raisin" model)
• Rutherford's Gold Foil Experiment

  • Most alpha particles passed through the gold foil undeflected
  • Some alpha particles were deflected at large angles
  • A small number of alpha particles bounced back
• Rutherford's Atomic Model


  • Described the atom as mostly empty space with a small, dense, positively charged nucleus and electrons orbiting the nucleus
• Bohr's Atomic Model


  • Electrons occupy specific energy levels and move in definite orbits around the nucleus
• Proton


  Positively charged subatomic particle in the nucleus
• Neutron

  Neutral subatomic particle in the nucleus
• Electron Cloud Model


  The most recent model of the atom, which describes the atom as having a small, positively charged nucleus surrounded by a large region occupied by a cloud of electrons
• Subatomic Particles


  • Proton
  • Electron
  • Neutron
• Element


  A pure substance made of only one kind of atom
• Types of Elements


  • Metals
  • Nonmetals
  • Metalloids
• Atomic Number (Z)


  The number of protons in the nucleus of an atom, which defines the identity of the element
• Mass Number (A)


  The total number of protons and neutrons in an atom
• Ion


  An atom that has gained or lost one or more electrons, making it electrically charged
• Anion


  A negatively charged ion formed when an atom gains one or more electrons
• Cation


  A positively charged ion formed when an atom loses one or more electrons
• Isotope


  Atoms of the same element that have different numbers of neutrons
• Anion


  Negatively charged atom
• Cation


  Positively charged atom formed when an atom loses one or more electrons
• Cation formation


  • Neutral sodium atom (Z=11) loses one electron to become a cation with a 1+ charge
  • Neutral oxygen atom (Z=8) gains two electrons to become an anion with a 2- charge
• Metal atoms tend to lose electrons to form cations and nonmetal atoms tend to gain electrons to form anions
• Ionic compounds tend to be composed of metals bonded with nonmetals, as in NaCl
• Isotopes


  Atoms of the same element that have different numbers of neutrons
• Isotopes of carbon


  • Carbon-12
  • Carbon-13
  • Carbon-14
• Isotope symbol


  Mass number as a superscript to the left of the element symbol, atomic number sometimes written as a subscript preceding the symbol
• Isotope symbols


  • 24Mg
  • 25Mg
  • 26Mg
• All magnesium atoms have 12 protons in their nucleus, they differ only in the number of neutrons
• Atomic symbol


  Indicates the element via its usual two-letter symbol, the mass number as a left superscript, the atomic number as a left subscript (sometimes omitted), and the charge as a right superscript
• The modern periodic table arranges the elements by their atomic numbers and periodic properties
• Several scientists worked over almost a century to assemble the elements into the periodic table format
• Lavoisier's attempt at classifying elements

  • Grouped elements as metals and nonmetals
• Dobereiner's triads


  • Arranged elements in groups of three in increasing order of atomic weight, with the middle element's properties approximating the average of the other two
• The first international conference of chemistry in 1860 concluded that hydrogen would be assigned the atomic weight of 1 and the atomic weight of other elements would be decided by comparison with hydrogen