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Information About the Atom

History of the Atom

  • Atom
  • Elements are made out of atoms
  • The smallest particle that retains all of the properties of the element
  • Electrons are constantly moving
  • Greek Philosophers (5th century BC & 460-370 AD)
  • Leucippus and Democritus
  • atoms (atomos) small, indivisible particles
  • theory
  • John Dalton - Atomic Theory (1808) - Experiments
    • Each element is composed of tiny indestructible particles called atoms.
    • He believed that atoms were solid spheres
    • All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements.
    • He believed that all atoms of a given element were identical. This is wrong because that is actually not true because if they have a different number of neutrons, then the carbon would be an isotope and therefore they would have different masses. However, they all do have the same number of protons. Atoms from different atoms are different because they have different numbers of protons.
    • Atoms combine in simple, whole-number ratios to form compounds
    • (4) When a chemical reaction takes place, the atoms are rearranged
    • This proves the law of conservation of mass
  • JJ Thomson (1856 - 1940)
    • discovered the electron (he thought it was the smallest things but he was wrong)
    • electrons were first called cathode ray
    • electron - negative charge therefore it was attracted to the positively charged plate
    • proton
    • proposed the plum pudding model
    • You have plasma that has a positive charge and then you have these randomly embedded electrons
    • Protons were lated discovered via this same method (but the inverse of it) - opposite charges attract
    • Then neutrons were discovered
  • Ernest Rutherford (1909)
    • Gold foil experiment
    • He rolled the foil extremely thin - almost like one atom
    • Alpha particles are positive
    • It is a form of radioactive decay
    • Mad scientist came from scientists working with mercury which broke down their nervous systems
    • If a positively hits a positive, it bounces back
    • This means that there was a very small dense positively charged center of the atom
    • If a positive hits a negative, it goes straight through
    • Nuclear theory of the atom
    • Most of the atom's mass and all of its positive charge are contained in a small core called the nucleus
    • Most of the volume of the atoms is empty space through which the tiny, negatively charged electrons are dispersed.
    • There are as many negatively charged electrons outside the nucleus as there are positively charged particles (protons) inside the nucleus, so the atoms is electrically neutral.
  • Nucleus
    • protons and neutrons (these two things account for 99.9% of the mass of the atom)

Atoms

  • Atomic mass unit - 1/12 of a carbon 12 atom (the 12 represents the mass number - it is the number of protons plus the number of neutrons). On the periodic table, the mass number is not there, the atomic mass is there instead.
  • Atomic mass
    • the average of all of the naturally occurring isotopes of an element

an isotope is an element with a different number of neutrons

  • Calculate this:
    • change the percent natural abundance to a decimal
    • multiply the decimal by the mass number for that isotope in amus
    • add those two products (or however many products their are)
  • Products
    • Chlorine has two isotopes, chlorine-35 (atomic mass 34.97 amu, relative abundance 75.77%) and chlorine-37 (atomic mass 36.97 amu, relative abundance 24.345). Calculate the atomic mass of chlorine.
  • (34.97*.7577) + (36.97amu*.2434) = 26.496769 + 8.998 = 35.50 amu (typically we would round this to two decimal places**)

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