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)


  • 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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