[Print this page]

Vincent Toner







A) Early Atomic Theory

1) Democritus (400 B.C.) proposed the first Atomic Theory.


      a) Stated the world was made up of empty

    space and atoms.


b) His theory was not widely accepted


        2) Aristotle’s Theory was accepted although

            wrong. He believed matter was continuous, 

            and made up of one substance.


        3) Antoine Lavoisier showed that the total mass

            Of all the reactants always equaled the total

            Mass of all the products.


        àDeveloped the law of the conservation of

             Mass. ( Matter cannot be created or



        4) John Dalton looked at work from other

             Scientist and put together an atomic theory.


               a) All atoms of the same element are

                   identical. ( This is not correct) All

                    Atoms of the same elements have the

                   same number of protons. This is correct.


               b) Atoms of different elements are different.


               c) Atoms unite in definite ratios to form



        d) All matter is composed of  atoms.


B) Early Research on Atomic particles.


        1) A cathode ray tube was used to discover



        a) anode – the positive terminal.


        b) cathode – the negative terminal


2) J.J. Thompson discovered the rays in a

     cathode ray tube were made of electrons.


               àA negative ion is called an ANION

                    Because it is attracted to the positive

      terminal- the ANODE



àA positive ion is called a CATION

     because it is attracted to the negative   

     terminal – The CATHODE


               3) Robert Millikan determined the charge of

            an electron to be -1. The symbol for an

     electron is “e-1


4) J.J. Thompson discovered the proton.


        àThe proton has a charge of  +1 and is

              1836 times the mass of an electron.


5) James Chadwick discovered the Neutron.


        a) The Neutron has no Charge.


        b) The neutron and proton are about

    equal in mass.





6) Particles that make up the atom


 Particle               Charge             Location


      Protons                    +1          Makes up the nucleus


      Neutrons                    0          Makes up the nucleus


      Electrons                     -1         Orbits the nucleus


II) Isotopes and Atomic Number


      A) The atomic number is the number of protons

              in the nucleus of the atom. The symbol used

      is “ Z”


             THE ELEMENT.


B) Isotopes contain the same number of protons,

     But a different number of neutrons.


àThe number of neutrons determines the

      particular isotopes of the element.


C) Nucleons are particles that make up the nucleus of the atom can be either a proton or a neutron.


D) Mass Number or Atomic Mass – The sum of the protons and neutrons in an atom. The symbol is “A”.


E) Nuclide – is an atom with a specific number of protons and neutrons


àAn example is Uranium 235.  (Notice the    

     element with a number after it) This Isotope

     of Uranium contains 92 Protons and 143



F) How to find the number of neutrons in an atom.


# neutrons = Atomic Mass – Atomic Number


 Example Uranium 238. The atomic number is 92 for Uranium.


# neutrons =  238 – 92 = 146


     This isotope of Uranium has 146  neutrons.



















A) Molecular and Formula Mass

        1) The mass of the elements is compared to the  mass of carbon.

        2) The atom is too small to use grams. Therefore, the unit that measures the mass of an element is

               an “atomic mass unit”, the symbol is amu or U.

         3) The mass of carbon is 12 amu, the mass of hydrogen is 1 amu. Carbon is 12 times heavier than hydrogen.

         4) On the periodic table the atomic mass is always the larger of the two numbers.

5) Molecular mass or formula mass is the sum of the atomic masses of the atoms in a compound.


Example #1 Find the mass of  H2O ( Round the mass to the closest whole number.)


                   (Subscript)  x  (Mass)                      

        H2O       H =  2   x  1 amu =     2 amu

                      O =  1    x 16 amu = 16 amu

                       The mass of water =18 amu


Example #2   KH2PO4


    K =  1   x  39 amu =  39 amu

  H =  2   x   1 amu =      2 amu

   P =  1   x  31 amu =   31 amu

   O =  4  x   16 amu =  64 amu

                                    136 amu